TABLE OF CONTENTS GUIDE NO. 4

[DISTRIBUTION SYSTEMS][697] to 720

Classification—series system—danger in series arclight system—constant current system—parallel system—arrangementof feeder and mains in parallel system—series-parallelsystem—center of distribution—Edisonthree wire system—evolution of the three wire system—balancedthree wire system—balancing of three wiresystem—copper economy in three wire systems—Dobrowolskythree wire system—modifications of threewire system—three wire storage battery system—threewire double dynamo system—three wire bridge system—threewire three brush dynamo system—Dobrowolsky threewire system—three wire auxiliary dynamo system—threewire compensator system—extension of the three wireprinciple—five wire system—dynamotor—connections ofbalancing set—balancing coils—distribution by dynamomotor sets—boosters and their uses—auxiliary bus bar.

[WIRES AND WIRE CALCULATION][721] to 764

Preliminary considerations—various wires—copper wire—ironwire—German silver wire—standard of copperwire resistance—relative conductivity of different metalsand alloys—conductors—cable for elevator annunciators—coveredconductors—rubber covered conductors—rubber insulatedtelephone and telegraph wires—weather proof conductors—twistedweather proof wires—precautions in usingweather proof conductors—slow burning wire; where used—slowburning weather proof wire; where used; how installed—miscellaneousinsulated conductors—safe carrying capacityof wire—pothead wires—circular mils—squaremils—mil foot—lamp foot—ampere foot—center ofdistribution—wire gauges—B. & S. standard wiregauge—micrometer screw gauge—calculating gauge—tableof various wire gauges—table of lamp feet forrubber covered wires—lamp table for weather proofwires—symmetrical and unsymmetrical distribution—wiringtable for light and power circuits—table of wireequivalents; how to use—table of cable capacities—incandescentlamps on 660 watt circuits—"tree" and"modified tree" system of wiring—distribution with sub-feeders—wrongand right methods of loop wiring—table,of amperes per motor; of amperes per dynamo—calculationsfor three wire circuit—three wire circuit panelboard—size of the neutral wire.

[INSIDE WIRING][765] to 798

The term "wiring"—open or exposed wiring—selectionof wires—installation—disadvantages of open wiring—splicing—pitchof wires—crossing of wires—wiring acrosspipes—practical points relating to exposed wiring—methodsof carrying wires, through floors; on walls—protectingexposed wiring on low ceilings—various porcelain knobsand cleats—wires used in mouldings—standard woodenmoulding—kick box—usual character of moulding work—practicalpoints relating to wiring in mouldings—tappingoutlets—arc light wiring—arc lamps on low pressureservice—circular fixture block—concealed knob andtube wiring; objections; method of installation—arrangementof switch and receptacle outlet in knob and tubewiring—switch boxes—rigid conduit wiring; advantage—typesof rigid conduit; requirements—-conduit box—disadvantagesof unlined conduit—flexible conduit wiring—Greenfieldflexible steel conduit—"fishing"—insulatingpoint—canopy insulator—fish plug and method of insertion—methodof installing conduits in fireproof buildings—serviceentrance to rubber conduit system—conduletoutlet to arc lamp—hickey—methods of bending large conduits—armouredcable wiring; features; installation.

[OUTSIDE WIRING][799] to 824

Materials for outside conductors—tensile strength ofcopper wire—pole lines—pole constructing tools—woodenpoles—preservation of wooden poles—preservation processes—methodsof setting wooden poles in unsuitable soil—reinforcedconcrete poles—cross arms—lineman's portableplatform—poles for light and power wires—spacing of poles—erectingthe poles—guy anchors—method of raisinga pole—method of pulling anchor into place—guys forpoles—head and foot guying—guying corner poles—guystubs and anchor logs—climbers—wiring the line—payout reels—method of stringing wires—"come alongs"—tensionon wires—sag table—lineman's block and fallwith "come alongs"—attaching wire to insulator—splicing;American wire joint; McIntire sleeve and sleeve joint—transpositions—insulators—overheadcable construction—petticoatinsulator—Clark's "antihum"—service connectionsand loops—method of making series, and parallelservice connections—joint pole crossing—service wires.

[UNDERGROUND WIRING][825] to 844

City conditions—underground systems—various conduits—vitrifiedclay pipe conduit—vitrified clay orearthenware trough conduit—joints in multiple-duct vitrifiedclay conduit—concrete duct conduits—methods oflaying conduit—method of laying single duct vitrified clayconduit—method of laying multiple duct clay conduit—woodenduct conduits—objection to use of wood;remedy; adaptation wooden built-in conduits; method ofinstallation—wrought iron or steel pipe conduits; methodof installation—porcelain bridgework or carriers—cast ironpipe and trough conduit; advantages—fibre conduits; joints—Edisontube system—underground cables—metalsheaths on underground cables—pot heads—General Electricmanhole junction box—pot head connections.

[WIRING OF BUILDINGS][845] to 864

Preliminary considerations—electrician's instructions—locationof receptacles—ceiling buttons—hallway wiring—selectionof wiring system—three wire convertiblesystem—method of wiring—location of panel boards—currentrequired on each floor of building—arrangementof feeders—installation of motors—largest size of feederpermissible—method of cutting pocket in floor—outletbaseboard—completed pocket—how to examine partitioninteriors—house plan of conduit wiring—attachment ofmains to knobs—precautions in making joints—wiringfor heating appliances—wiring with combination ofmoulding, flexible tubing or conduit in non-fire proofbuilding—feeder system for large hotel.

[SIGN FLASHERS][865] to 884

Classification—Carbon flashers—wiring diagram for Dull'scarbon flashers—brush flashers—knife flashers—flashertransmission gearing—simple on and off flashers—flash systemof gas lighting—high speed flashers—lighting flashers—wiringdiagram for flags—diagram showing method ofwiring for high speed effects on single lines—method ofwiring for a torch—wiring diagram for high speeds—Dull'slightning—script breakers—chaser flashers—thermoflashers—carriage calls—monogram for carriage calls—wiringdiagrams for sign illumination—-National carriagecall operating keyboard—clock monogram—Bett's clockmechanism for operating electric monogram time flasher—talkingsigns—two way thermal flasher.

[LIGHTNING PROTECTION][885] to 892

Lightning rods; why sharp points are used; erection—diagramshowing principle of air gap arrestor—variablegap arc breaker—location of lightning arrestor—carbonarresters with fuses for telephone lines—ground connection.

[STORAGE BATTERIES][893] to 968

Early experiments—theory—description of storage cell—electrolyte—effectof current passing through electrolyte—typesof storage batteries—Plant cells—Willard plates;wood separator—parts of "Autex" automobile cells—Faureor pasted type—comparison of Plant and Faureplates—the electrolyte; kind generally used; preparation;test; mixing acid and water; kind of vessel used;—specificgravity table—effect of deep vessel—density of electrolyte—hydrometersyringe—impurities in electrolyte—testsfor impurities; chlorine, nitrates, acetic acid, iron, copper,mercury, platinum—old electrolyte—voltage of asecondary cell—charging—connections for charging—charging;first time; period; regulation of voltage—Edisoncell data—frequency of charging—cadmium test—emergencyconnections for weak ignition battery—portabletesting instruments—charge indications—two methods ofcharging from a direct current lighting system—colors of theplates—how best results are obtained in charging—chargevoltage—two ways of charging—diagram of chargingconnections—how to keep charging current constant—testswhile charging; after charging—charge indications—behaviorof electrolyte during discharge—lead burningoutfit—"boiling"; causes—hydrogen gas generatorfor lead burning—quick charging—charging through thenight—charging period for new battery—Willard underhungbattery box for automobiles—high charging rates—"National"instructions for taking voltage readings—mercuryarc rectifier—capacity—table of capacity variation fordifferent discharge rates—how to increase the capacity—discharging;too rapidly rating; maximum rate—Edisonalternating current rectifier—attention after discharging—thebattery room—battery attendants—points oncare and management—placement of cells—how to avoidleakage—precautions when unpacking cells—assemblingcells—connections—precaution in joining terminals—batterytroubles—short circuiting; indication; location—overdischarge;buckling—sulphation of plates—data onNational cells; American cells; Autex cells—action in idlecell—lack of capacity—how to prevent lead poisoning—lowspecific gravity without short circuits; causes—treatmentof weak cells—pole testing paper—disconnectingcells—sulphuric acid specific gravity table—how totake a battery out of commission—Witham chargingboard—putting batteries into commission—cleaning jars—tableof voltage charge as affected by discharge rate—condensedrules for the proper care of batteries.

[STORAGE BATTERY SYSTEMS][969] to 996

Uses of storage batteries; their importance in powerplants—load curve showing use of storage battery as aidto the generating machinery—parallel connection of batteryand dynamo—"floating the battery on the line"—diagramshowing effect of battery in regulating dynamoload—connections and circuit control apparatus—diagramshowing action of battery as a reservoir of reservepower—three wire system with battery and dynamo—methodsof control for storage batteries—diagram of connectionsfor ignition outfit—variable resistance—end cellswitches—diagram of connection of battery equipment forresidential lighting plant—end cell switch diagram—featuresof end cell switch construction—end cell switch control—circuitdiagram for charging battery in two parallelgroups and discharging in series—reverse pressure cells; regulation—Holzer-Cabotdynamotor—boosters—applicationof series booster system—Bijur's battery system—loaddiagram—characteristics of series booster—shunt boosters;with battery—Entz' carbon pile booster system—applicationof shunt booster—circuit diagram for non-reversibleshunt booster and battery system—compound boosters;their connections—method of charging battery at onevoltage and supplying lights at a different voltage—connectionsof one form of differential booster—differentialboosters; with compensating coil; adaptation.

Classification—series system—danger in series arc light system—constant current system—parallel system—arrangement of feeder and mains in parallel system—series-parallel system—center of distribution—Edison three wire system—evolution of the three wire system—balanced three wire system—balancing of three wire system—copper economy in three wire systems—Dobrowolsky three wire system—modifications of three wire system—three wire storage battery system—three wire double dynamo system—three wire bridge system—three wire three brush dynamo system—Dobrowolsky three wire system—three wire auxiliary dynamo system—three wire compensator system—extension of the three wire principle—five wire system—dynamotor—connections of balancing set—balancing coils—distribution by dynamo motor sets—boosters and their uses—auxiliary bus bar.

Preliminary considerations—various wires—copper wire—iron wire—German silver wire—standard of copper wire resistance—relative conductivity of different metals and alloys—conductors—cable for elevator annunciators—covered conductors—rubber covered conductors—rubber insulated telephone and telegraph wires—weather proof conductors—twisted weather proof wires—precautions in using weather proof conductors—slow burning wire; where used—slow burning weather proof wire; where used; how installed—miscellaneous insulated conductors—safe carrying capacity of wire—pothead wires—circular mils—square mils—mil foot—lamp foot—ampere foot—center of distribution—wire gauges—B. & S. standard wire gauge—micrometer screw gauge—calculating gauge—table of various wire gauges—table of lamp feet for rubber covered wires—lamp table for weather proof wires—symmetrical and unsymmetrical distribution—wiring table for light and power circuits—table of wire equivalents; how to use—table of cable capacities—incandescent lamps on 660 watt circuits—"tree" and "modified tree" system of wiring—distribution with sub-feeders—wrong and right methods of loop wiring—table, of amperes per motor; of amperes per dynamo—calculations for three wire circuit—three wire circuit panel board—size of the neutral wire.

The term "wiring"—open or exposed wiring—selection of wires—installation—disadvantages of open wiring—splicing—pitch of wires—crossing of wires—wiring across pipes—practical points relating to exposed wiring—methods of carrying wires, through floors; on walls—protecting exposed wiring on low ceilings—various porcelain knobs and cleats—wires used in mouldings—standard wooden moulding—kick box—usual character of moulding work—practical points relating to wiring in mouldings—tapping outlets—arc light wiring—arc lamps on low pressure service—circular fixture block—concealed knob and tube wiring; objections; method of installation—arrangement of switch and receptacle outlet in knob and tube wiring—switch boxes—rigid conduit wiring; advantage—types of rigid conduit; requirements—-conduit box—disadvantages of unlined conduit—flexible conduit wiring—Greenfield flexible steel conduit—"fishing"—insulating point—canopy insulator—fish plug and method of insertion—method of installing conduits in fireproof buildings—service entrance to rubber conduit system—condulet outlet to arc lamp—hickey—methods of bending large conduits—armoured cable wiring; features; installation.

Materials for outside conductors—tensile strength of copper wire—pole lines—pole constructing tools—wooden poles—preservation of wooden poles—preservation processes—methods of setting wooden poles in unsuitable soil—reinforced concrete poles—cross arms—lineman's portable platform—poles for light and power wires—spacing of poles—erecting the poles—guy anchors—method of raising a pole—method of pulling anchor into place—guys for poles—head and foot guying—guying corner poles—guy stubs and anchor logs—climbers—wiring the line—pay out reels—method of stringing wires—"come alongs"—tension on wires—sag table—lineman's block and fall with "come alongs"—attaching wire to insulator—splicing; American wire joint; McIntire sleeve and sleeve joint—transpositions—insulators—overhead cable construction—petticoat insulator—Clark's "antihum"—service connections and loops—method of making series, and parallel service connections—joint pole crossing—service wires.

City conditions—underground systems—various conduits—vitrified clay pipe conduit—vitrified clay or earthenware trough conduit—joints in multiple-duct vitrified clay conduit—concrete duct conduits—methods of laying conduit—method of laying single duct vitrified clay conduit—method of laying multiple duct clay conduit—wooden duct conduits—objection to use of wood; remedy; adaptation wooden built-in conduits; method of installation—wrought iron or steel pipe conduits; method of installation—porcelain bridgework or carriers—cast iron pipe and trough conduit; advantages—fibre conduits; joints—Edison tube system—underground cables—metal sheaths on underground cables—pot heads—General Electric manhole junction box—pot head connections.

Preliminary considerations—electrician's instructions—location of receptacles—ceiling buttons—hallway wiring—selection of wiring system—three wire convertible system—method of wiring—location of panel boards—current required on each floor of building—arrangement of feeders—installation of motors—largest size of feeder permissible—method of cutting pocket in floor—outlet baseboard—completed pocket—how to examine partition interiors—house plan of conduit wiring—attachment of mains to knobs—precautions in making joints—wiring for heating appliances—wiring with combination of moulding, flexible tubing or conduit in non-fire proof building—feeder system for large hotel.

Classification—Carbon flashers—wiring diagram for Dull's carbon flashers—brush flashers—knife flashers—flasher transmission gearing—simple on and off flashers—flash system of gas lighting—high speed flashers—lighting flashers—wiring diagram for flags—diagram showing method of wiring for high speed effects on single lines—method of wiring for a torch—wiring diagram for high speeds—Dull's lightning—script breakers—chaser flashers—thermo flashers—carriage calls—monogram for carriage calls—wiring diagrams for sign illumination—-National carriage call operating keyboard—clock monogram—Bett's clock mechanism for operating electric monogram time flasher—talking signs—two way thermal flasher.

Lightning rods; why sharp points are used; erection—diagram showing principle of air gap arrestor—variable gap arc breaker—location of lightning arrestor—carbon arresters with fuses for telephone lines—ground connection.

Early experiments—theory—description of storage cell—electrolyte—effect of current passing through electrolyte—types of storage batteries—Plant cells—Willard plates; wood separator—parts of "Autex" automobile cells—Faure or pasted type—comparison of Plant and Faure plates—the electrolyte; kind generally used; preparation; test; mixing acid and water; kind of vessel used;—specific gravity table—effect of deep vessel—density of electrolyte—hydrometer syringe—impurities in electrolyte—tests for impurities; chlorine, nitrates, acetic acid, iron, copper, mercury, platinum—old electrolyte—voltage of a secondary cell—charging—connections for charging—charging; first time; period; regulation of voltage—Edison cell data—frequency of charging—cadmium test—emergency connections for weak ignition battery—portable testing instruments—charge indications—two methods of charging from a direct current lighting system—colors of the plates—how best results are obtained in charging—charge voltage—two ways of charging—diagram of charging connections—how to keep charging current constant—tests while charging; after charging—charge indications—behavior of electrolyte during discharge—lead burning outfit—"boiling"; causes—hydrogen gas generator for lead burning—quick charging—charging through the night—charging period for new battery—Willard underhung battery box for automobiles—high charging rates—"National" instructions for taking voltage readings—mercury arc rectifier—capacity—table of capacity variation for different discharge rates—how to increase the capacity—discharging; too rapidly rating; maximum rate—Edison alternating current rectifier—attention after discharging—the battery room—battery attendants—points on care and management—placement of cells—how to avoid leakage—precautions when unpacking cells—assembling cells—connections—precaution in joining terminals—battery troubles—short circuiting; indication; location—overdischarge; buckling—sulphation of plates—data on National cells; American cells; Autex cells—action in idle cell—lack of capacity—how to prevent lead poisoning—low specific gravity without short circuits; causes—treatment of weak cells—pole testing paper—disconnecting cells—sulphuric acid specific gravity table—how to take a battery out of commission—Witham charging board—putting batteries into commission—cleaning jars—table of voltage charge as affected by discharge rate—condensed rules for the proper care of batteries.

Uses of storage batteries; their importance in power plants—load curve showing use of storage battery as aid to the generating machinery—parallel connection of battery and dynamo—"floating the battery on the line"—diagram showing effect of battery in regulating dynamo load—connections and circuit control apparatus—diagram showing action of battery as a reservoir of reserve power—three wire system with battery and dynamo—methods of control for storage batteries—diagram of connections for ignition outfit—variable resistance—end cell switches—diagram of connection of battery equipment for residential lighting plant—end cell switch diagram—features of end cell switch construction—end cell switch control—circuit diagram for charging battery in two parallel groups and discharging in series—reverse pressure cells; regulation—Holzer-Cabot dynamotor—boosters—application of series booster system—Bijur's battery system—load diagram—characteristics of series booster—shunt boosters; with battery—Entz' carbon pile booster system—application of shunt booster—circuit diagram for non-reversible shunt booster and battery system—compound boosters; their connections—method of charging battery at one voltage and supplying lights at a different voltage—connections of one form of differential booster—differential boosters; with compensating coil; adaptation.

CHAPTER XXXVI
DISTRIBUTION SYSTEMS

The selection of the system of transmission and distribution of electric energy from the generating plant to lamps, motors, and other devices, is governed mainly by the cost of the metallic conductors, which in many electrical installations, is a larger item than the cost of the generating plant itself. This is especially true in case of long distance transmission, while in those of the lighting plants, the cost of wiring is usually more expensive than that of the boilers, engines, and generators combined.

The principal distribution systems, are classed as:

1. Series;
2. Parallel;
3. Series-parallel;
4. Parallel-series.

Ques. What is the characteristic feature of each class?

Ans. In the series systems the current is constant, but the voltage varies. In the parallel systems, the voltage is constant, but the current varies.

Series System of Distribution.—A series system affords the simplest arrangement of lamps, motors, or other devices supplied with electric energy. The connections of such a system are shown in [fig. 783]. The current from the terminal of the dynamo passes through the lamps, L, L, L, L, one after the other and finally returns to the terminal. The current remains practically constant, but the voltage falls throughout the circuit in direct proportion to the resistance, and the difference in pressure between any two points in the circuit is equal to the current in amperes multiplied by the resistance in ohms included between them.

For example. Each open arc lamp requires about 50 volts. In the system shown in [fig. 783], the pressure measured across the brushes of the dynamo is assumed to be 1,000 volts. As this current flows through the circuit 45 volts will be actually lost in each lamp, and as the drop on the line wire is usually about 10 per cent. of the total voltage, there will be a drop of 5 volts on the conductor between any two lamps. In the circuit shown, there are twenty lamps, therefore, the difference in pressure between either terminal of the dynamo and middle point A of the circuit will be 10 lamps × 50 volts = 500 volts. Likewise, the difference in pressure between any two points on the circuit will be equal to 50 volts multiplied by the number of lamps included between them.

Fig. 783.—Series system of distribution. This is a constant current system, so called because the current remains practically constant. It is used chiefly for arc lighting.

Ques. Describe the danger in a series arc light system?

Ans. Since the total voltage of the system is equal to the sum of the volts consumed in all of the lamps, it is high enough to be dangerous to personal safety.

This is illustrated in [fig. 783]. If the line be grounded at B owing to defective insulation, the pressure of the circuit at that point will be zero, and therefore, a man standing on the ground could touch that point without receiving a shock, but if he should touch the line at the point C, he will receive a slight shock of 150 volts, as there are three lamps between the point C, and the ground connection B. Therefore, the danger of touching the circuit increases directly with the resistance between the point touched and the ground connection, so that if a man touch the circuit at the point D, he will receive a dangerous shock of 16 × 50 = 800 volts. In practice, sixty lamps are often placed on a single arc lighting circuit, so that its total pressure is about 3,000 volts, thus greatly increasing the danger of the system.

Ques. What is a constant current system?

Ans. The series system is a constant current system, and is so called because the current remains practically constant, while the voltage falls throughout the circuit in direct proportion to the resistance.

Ques. What are the principal applications of the series system?

Ans. For arc lighting, and telegraphic circuits.

Ques. What are the advantages of the series system?

Ans. In the case of telegraphic circuits only one wire is required, and for lighting and power transmission and distribution, only two wires; therefore, it is simpler and cheaper than any other system.

Ques. What is the disadvantage of the series system?

Ans. The danger due to the high voltage in installations such as arc lighting circuits.

Parallel System.—Parallel or multiple systems are usually more complicated than series systems, but since the voltage can be maintained nearly constant by various methods, practically all incandescent lamps, electric motors, and a large proportion of arc lamps are supplied by parallel systems.

The general principle of the parallel system is shown in [fig. 784]. With six lamps on the circuit, each requiring one-half ampere of current, at 110 volts, the dynamo will have to supply a current of 3 amperes at a pressure of 112 volts, and this current will flow through the circuit and distribute itself as shown on account of the lesser resistance of the wire relatively to that of the lamps. At the first lamp, the 3 amperes will divide, ½ ampere flowing through the lamp and the remaining 2½ amperes passing on to the next lamp and so on through the entire circuit. The reduction of pressure from 112 volts across the brushes to 110 volts at the last lamp is due to the resistance of the conducting wires.

Ques. What three effects are due to this drop in pressure?

Ans. 1, All the lamps or motors in the circuit receive a lower voltage than that at the dynamo, 2, some lamps or motors may receive a lower voltage than the others, and 3, the voltage at some lamps or motors may vary when the others are turned on or off.

Fig. 784.—Parallel system of distribution. This is a constant voltage system and is used principally for incandescent lighting and electric motor circuits.

The first is the least harmful and may be counteracted by running the dynamo at a little higher voltage; but the second and third are very objectionable and difficult to overcome. They are counteracted successfully in practice, however, by various methods of regulation, the use of boosters, and the operation of dynamos in parallel.

Ques. What are the principal applications of parallel or constant pressure systems?

Ans. They are used on practically all incandescent lamp and electric motor circuits, and on some arc lamp circuits.

Ques. Why is it specially applicable to incandescent lamp circuits?

Ans. Incandescent lamps cannot be made to stand a pressure much over 220 volts, and therefore have to be operated on low voltage systems.

Ques. What is the principal disadvantage of a parallel system as compared with a series system?

Ans. The greater cost of the copper conductors.

Fig. 785.—Arrangement of feeder and mains in parallel system. By locating the feeder at the electrical center, less copper is required for the mains. The cut does not show the fuses which in practice are placed at the junction of feeder and main.

Ques. What is the usual arrangement of parallel systems?

Ans. Conductors known as a feeder run out from the station, and connected to these are other conductors known as a main to which in turn the lamps or other devices are connected as shown in [fig. 785].

Ques. In what two ways may feeders be connected?

Ans. They may be connected at the same end of the mains, known as parallel feeding, or they may be connected at the opposite end of the main, called anti-parallel feeding.

The main may be of uniform cross section throughout, or it may change in size so as to keep the current density approximately constant. The above condition gives rise to four possible combinations:

1. Cylindrical conductors parallel feeding, [fig. 786];
2. Tapering conductors, parallel feeding, [fig. 787];
3. Cylindrical conductors, anti-parallel feeding, [fig. 788];
4. Tapering conductors, anti-parallel feeding, [fig. 789].

Figs. 786 to 789.—Various parallel systems. Fig. 786, cylindrical conductors parallel feeding; [fig. 787], tapering conductors parallel feeding; [fig. 788], cylindrical conductors anti-parallel feeding; [fig. 789], tapering conductors anti-parallel feeding. The term "tapering" is here used to denote a conductor made up of lengths of wire, each length smaller than the preceding length, the object of such arrangement being to avoid a waste of copper by progressively diminishing the size of wire so that the relation between circular rails and amperes is kept approximately constant. In an anti-parallel system, the current is fed to the lamp from opposite ends of the system.

Series-Parallel System.—This is a combination of the series and parallel systems, and is arranged as indicated in [fig. 790]. Several lamps are arranged in parallel to form a group, and a number of such sets are connected in series, as shown. It is not necessary for the groups to be identical, provided they are all adapted to take the same current in amperes, which should be kept constant, and provided the lamps of each set agree in voltage. For example, on the ordinary 10-ampere arc circuit, one group might consist of 5 lamps, each requiring 2 amperes at 50 volts; the next might be composed of 10 lamps, each taking 1 ampere at 100 volts, and so on.

Fig. 790.—Series-parallel system of distribution. It consists of groups of parallel connected receptive devices, the groups being arranged in the circuit in series.

Parallel-Series System.—In this method of connection, one or more groups of lamp are connected in series and the groups in parallel as shown in [fig. 791].

Fig. 791.—Parallel-series system of distribution. It consists of groups of series connected receptive devices, the groups being arranged in the circuit in parallel.

Ques. When is a parallel-series system used?

Ans. When it is desired to operate a number of lamps or motors on a line where voltage is several times that required to operate a single lamp or motor.

The parallel-series system is employed chiefly in the lighting circuit on electric traction lines; here, usually five 110 volt lamps are connected to the source of supply which has a pressure of 550 volts.

Center of Distribution.—It is important to determine the point at which the feeders should be attached to the mains in order to minimize the amount of copper required. The method employed is similar to that used in determining the best location of a power plant as regards amount of copper required. The center of distribution may be called the electrical center of gravity of the system, and is found by separately obtaining the center of gravity of straight sections and then determining the total resultant and point of application of this resultant of the straight sections.

Feeders (feeding cables or conductors) are run from the source of supply to the distributing centers, and, as these feeders are in many cases of considerable length, a substantial loss of pressure generally occurs in them. The pressure at the source of supply, however, is so regulated as to compensate for the drop in the feeders, and the pressure at the distributing centers is thus kept constant; or the same result is obtained by the use of regulating devices in the feeders. The essential condition in most systems is that the pressure at the distributing centers shall be kept practically constant, irrespective of the load.

Edison Three Wire System.—In electric lighting systems used up to about 1897, it was not considered practicable to use incandescent lamps requiring a pressure exceeding 120 volts. This limited the operating voltage of parallel systems, and necessitated the use of conductors of large size and weight, especially where the current had to be transmitted a considerable distance.

The effect of this limiting voltage is more apparent when it is clearly understood that the size of wire required to carry a current depends upon the amperes and not upon the volts.

A wire capable of carrying a current of 10 amperes at 20 volts, can carry 10 amperes at 20,000 volts or any other voltage. Therefore, since the amount of electric energy or power transmitted through a conductor is equal to the amperes multiplied by the volts, it is clear that by increasing the voltage, the power transmitting capacity of a current can be almost indefinitely increased without increasing the size of the conducting wire. This is the reason why considerations of economy dictate the use of the highest voltages possible in long distance transmissions. The voltage of the current is determined, however, by the requirements of the apparatus to be operated.

Incandescent lamps usually require a pressure of 110 volts, and the current required by a 16 candle power lamp at that voltage is about ½ ampere. Therefore if the lamp be designed for a pressure of 220 volts, the current will be reduced to ¼ ampere, and the same size of wire could be used to feed twice as many lamps.

Figs. 792 and 793.—Evolution of the three wire system. Fig. 792 shows two dynamos supplying two independent circuits. These may be connected in series as in [fig. 793], thus operating the two circuits of [fig. 792] with two wires instead of four. To balance the system in case of unequal loading, a third or neutral wire is used as shown in [fig. 794].

The saving of copper is the sole merit of the three wire system, and the object which led to its invention was to effect this economy with the use of 110 volt lamps.

Principle of the Three Wire System.—In [fig. 792], two dynamos A and B are shown supplying two independent incandescent lighting circuits, each circuit receiving 3 amperes of current at a pressure of 110 volts. It is evident that the dynamos could be connected with each other in series, and the lamps connected in series with two each, as shown in [fig. 793], thus making the two wires K and L of the two independent circuits unnecessary, as the pressure will be increased to 220 volts while the current will remain at 3 amperes, and each lamp will require ¼ ampere.

Fig. 794.—Balanced three wire system. The middle conductor, known as the neutral wire, keeps the system balanced in case of unequal loading, that is, a current will flow through it, to or from the dynamos, according to the preponderance of lamps on the one side or the other. These current conditions are shown in [fig. 797].

The amount of copper saved will be 100 per cent., but this arrangement is open to the objection, that when one of the lamps is turned off, or burned out, its companion will also go out. This difficulty is avoided in the three wire system by running a third wire N, from the junction O, between the two dynamos, as shown in [fig. 794], thus providing a supply or return conductor to any one of the lamps, and permitting any number of lamps to be disconnected without affecting those which remain. If the system be exactly balanced, no current will flow through the wire N, because the pressure toward the - terminal of the dynamo A, will be equal to the pressure from the + terminal of dynamo B, thus neutralizing the pressure in the wire. For this reason the middle wire of a three wire system is called the neutral wire, and is usually indicated by the symbol O or ± the latter meaning that it is positive to the first wire and negative to the second. If the system be unbalanced, a current will flow through the neutral wire, to or from the dynamos, according to the preponderance of lamps in the upper or lower sets. When the number in the lower set is the greater, the current in the neutral wire will flow from the dynamos as shown in [fig. 797], and toward the dynamos under the reverse condition.

In the case represented in [fig. 797], there are five lamps in circuit, requiring 2½ amperes of current at a pressure of 110 volts. The two lamps in the upper set will require 1 ampere, and the three lamps in the lower set, 1½ amperes. Since a pressure of 110 volts can force only a current of one ampere through resistance of the two lamps in the upper set, it is evident, that the additional ½ ampere required by the three lamps in the lower set will have to be supplied through the neutral wire, as shown.

Balancing of Three Wire System.—In practice it is impossible to obtain an exactly balanced system, as the turning on and off of lamps as required results in a preponderance of lamps in the upper or lower sets, and furthermore, even when the number of lamps in the two sets are equal, they may be located irregularly, thereby causing the currents to flow for short distances in the neutral line. Therefore, the larger the number of lamps in the circuit, the easier it will be to keep the system in a balanced condition.

Copper Economy in Three Wire Systems.—Theoretically, the size of the neutral wire has to be only sufficient to carry the largest current that will pass through it. A large margin of safety, however, is allowed in practice so that its cross section ranges from about one-third that of the outside line, in large central station systems, to the same as that of each outside line in small isolated systems.

If the neutral wire be made one-half the size of the outside conductor, as is usually the case in feeders, the amount of copper required is 5/16 of that necessary for the two wire system. For mains it is customary to make all three conductors the same size increasing the amount of copper to ⅜ of that required for the two wire system.

Fig. 795.—Dobrowolsky three wire system with self-induction coil. It consists of an ordinary direct current dynamo, the armature A and pole pieces N and S of which are shown. A self-induction coil D, is connected to two diametrically opposite points of the winding of the armature A. The coil D may be carried by and revolve with the armature; but in the construction represented, it is stationary, being connected to the armature winding through the brushes CC, rings and wires JJ. The middle point of the self-induction coil D, is connected to the neutral conductor O of the three wire system, the outside conductors + and - being supplied from the brushes BB in the usual manner. The pressure at the terminals of the coil D is alternating; hence the latter, on account of its self-induction, does not act as a short circuit to the armature. Furthermore, the inductances of the two halves of the coil D being equal, the pressure of the neutral wire O is kept midway between the pressures of the outside wires + and -. When the two sides of the system are unbalanced in load, the difference in current carried in one direction or the other by the neutral wire passes freely through the coil D, since the current is steady, or varies slowly, and is therefore unimpeded by the self-induction. It is evident that the ohmic resistance of D should be as low and its self-induction as high as possible, in order that the loss of energy and the difference in voltage on the two sides of the system shall be as small as possible under all conditions.

Modifications of the Three Wire System.—By the employment of suitable arrangements, it is possible to operate a three wire system with only one dynamo. Some of the various arrangements which have been used or proposed in this connection may be briefly mentioned as follows:

Three Wire Storage Battery System, in which a storage battery is connected between the two outside wires, and the pressure of the neutral wire varied to balance the system by shifting the point at which it is connected to the battery.

Three Wire Double Dynamo System, in which a double dynamo having two armature windings upon the same core, connected to two separate commutators, is used in the same manner as two separate dynamos connected in series.

Three Wire Bridge System, in which a resistance is connected across the two outside wires, and the neutral wire is brought to a point on the resistance through a movable switch. The pressures on the two sides of the circuit are equalized by adjusting the arm of the switch for any change of load.

Fig. 796.—Three wire compensator system. A and B are the compensators or equalizers. They consist of auxiliary dynamos coupled together and connected to the system as shown. D is the main dynamo, and E, a booster.

Three Wire Three Brush Dynamo System, in which the neutral wire is connected to a third brush on the dynamo.

Dobrowolsky Three Wire System, in which a self-induction coil is connected to two diametrically opposite points of the armature of an ordinary direct current dynamo. The principle of this system is illustrated in [fig. 795].

Three Wire Auxiliary Dynamo System, in which the neutral wire is connected to an auxiliary dynamo which supplies a pressure one-half as great as that of the main dynamo. The auxiliary dynamo is usually belt driven by the main dynamo, and acts as a dynamo when the load is greater on the negative side of the circuit, and as a motor when the excess of load is on the positive side.

Three Wire Compensator System, in which two auxiliary dynamos A and B called compensators or equalizers, are coupled together and connected to the system as shown in [fig. 796]. Each compensator generates one-half as much pressure as the main dynamo D, and serves to equalize the pressure and the load, the compensator on the lightly loaded side operating as a motor and driving the other as a dynamo. When the system is exactly balanced, both compensators run as motors under no load, therefore, consume very little energy. In this arrangement only one booster E, is required for both sides of the system, as the compensators are connected to the outside wires at a point beyond the boosters, and therefore, sub-divide the increased difference of pressure equally between the two sides of the system.

Fig. 797.—Three wire double dynamo system having two separate windings on the same core and separate commutators A and B as shown.

Extension of the Three Wire Principle.—In order to attain still greater economy in copper, the principles of the three wire system may be extended to include four, five, six, and seven wire systems. The comparative weights of copper required by such systems are as follows:

Twowiresystem1.000
Three""all wires of equal size.370
Three""neutral wire one-half size.313
Four""all wires of equal size.222
Five""""""".156
Seven""""""".096

The four wire system requires about two-ninths as much copper, and the seven wire system about one-tenth as much copper, as an equivalent two wire system; but neither is desirable, as their operation involves too much inconvenience, too many unavoidable complications, and create a possibility of accident, which more than offsets the saving in copper.

Fig. 798.—Diagram showing dynamotor connections when used as an equalizer in the three wire system. DM, dynamotor; G, generator side; M, motor side.

The Five Wire System.—This system is employed advantageously in many places in England and Europe, but has not as yet been introduced to any extent in America. It is very probable that in the future the three wire 440 volt system will be selected in preference to the five wire system.

Dynamotor.—This is a combination of dynamo and motor on the same shaft, one receiving current and the other delivering current, usually of different voltage, the motor being employed to drive the dynamo with a pressure either higher or lower than that received at the motor terminals.

The dynamotor in the direct current circuit corresponds to the transformer in the alternating current circuit.

Fig. 799.—Diagram showing connections of balancing set in three wire one dynamo system. The set consists of a motor and dynamo connected, and its operation is practically the same as a dynamotor.

Ques. How is the dynamotor used as an equalizer in the three wire system?

Ans. When thus used, the machine is connected as in [fig. 798]. When both sides of the system are balanced, there will be no current in the neutral lead N, and a small current will pass through the two armature windings of the dynamotor in series, both armatures acting as motors. If the load on one side of the system become larger than the load on the other side, there will be a greater drop in the leads connected to the overloaded side and consequently a lower voltage will exist over the larger load than exists over the smaller load. The armature winding of the dynamotor connected to the higher voltage will act as a dynamo, whose pressure will tend to raise the voltage of the more heavily loaded side.

The direction of the currents in an unbalanced three wire system that is being supplied with energy from a main dynamo is shown in the figure. The commutator at G is connected to the dynamo winding of the dynamotor and is supplying current to the upper or larger load, and the lower commutator is connected to the motor winding of the dynamotor and is taking current from the lightly loaded side.

Motor-Dynamo or Balancing Set.—A balancing set or balancer consists of a motor mechanically connected to a dynamo used to balance a three wire system. The operation of such a combination is practically the same as the dynamotor just described. The balancer is connected as shown in [fig. 799].

Fig. 800.—Holzer-Cabot type M motor-dynamo set. This combination is known as a booster, and is used to raise or lower the voltage on feeders. The motor is series wound and connected in series with one leg of the feeder. Thus, the voltage which the booster will add to the line will be directly in proportion to the current flowing in the feeder. The regulation is therefore automatic.

When an unbalanced load comes on, the voltage on the lightly loaded side rises and on the heavily loaded side drops. The machine on the light side then takes power from the line and runs as a motor driving the machine on the heavy side as a dynamo, supplying the extra current for that side. This action tends to bring the voltage back to normal and gives good regulation.

In some cases the field of each machine is connected to the opposite side of the system which gives a quicker action. This regulation is automatic and the set takes care of unbalanced loads in either direction without adjustment.

Balancing Coils.—Another method of balancing a three wire system which does away with any additional rotating machines makes use of balance coils.

Ques. Describe the type of dynamo used with balancing coils?

Fig. 801.—Diagram showing connections of balancing coil system. The dynamo used in this system is provided with both commutator and collector rings.

Ans. The regular two wire dynamo is used supplying power to the outside wires, but there are collector rings connected to the armature. These rings are much lighter than they would be for a converter as they carry only about ⅛ of the dynamo load. These rings being light are usually placed at the end of the commutator and are connected directly to the commutator bars.

Ques. How are the balancing coils constructed?

Ans. They are built of standard transformer parts, and are placed in cases similar to those of ordinary small transformers.

The coil has a straight continuous winding, both ends and a connection from the middle point of the winding being brought out of the case.

Figs 802 and 803.—Distribution by dynamo-motor sets. Fig. 802, sets in parallel; [fig. 803], sets in series. In [fig. 802], current produced by the main dynamo G, is carried to the machines by the conductors A and B to which the motor portions M are connected in parallel. These motors are provided with shunt wound field coils which may be connected to the primary or to the secondary circuit, consequently the machines run at a practically constant speed. The dynamo portions D of the transformers are connected to the secondary circuits which supply the lamps, etc., L, as indicated. The field magnets of these dynamos may also be fed by the main circuit AB, or they may be self-excited by shunt or compound winding. In [fig. 803], the motors M are all connected in series with the main dynamo G, and the dynamo elements D of the transformers, connected to the lamps, etc., L. If the current be kept constant (the dynamo G having a regulator like a series arc dynamo), and the motors M are simple series wound machines, they will exert a certain torque, or turning effort, which will be constant. It follows, therefore, that if the dynamos D be also series wound, each will generate a certain current which will be constant. If lamps or other devices, designed for that particular current, be connected in series on the secondary circuits, the dynamos D will always maintain that current, no matter how many lamps there may be. When lamps are added, the resistance of the local circuit is raised, and the current in it decreases, so that the dynamo increases its speed until it generates sufficient pressure to produce practically the same current as before. Hence this constitutes a system which is self-regulating, when lamps, etc., are cut in or out of the secondary circuits. No harm results even if the secondary be short circuited, since only the normal current can be generated. But if the secondary circuit be opened, then the machine will race, and probably injure itself by centrifugal force, because the torque of the motor M has its full value, and there is no load upon the dynamos D. To guard against this danger, some automatic device should be provided to short circuit the field or armature of the motor when its speed or reverse voltage rises above a certain point.

Ques. How are the coils connected to the dynamo?

Ans. Two coils are used and are connected to the collector rings as shown in [fig. 801], one coil across each phase. The connections from the middle points of the coils are connected together and to the neutral wire of the system.

Fig. 804.—Diagram to show correctness of balancing coil connection. In the figure, AE, BF, CG, and DH represent the balance coil and its connection for different positions of the armature of a bipolar machine.

Ques. What is the action of the coils in equalizing the load?

Ans. On balanced load, the coils take a small alternating exciting current from the collector rings as any transformer does when connected to an alternating current line with its secondary open. When an unbalanced load comes on, the current in the neutral divides, half going to each coil. This enters the coil at the middle point and half flows each way through the coil and the slip rings into the armature winding. The unbalanced current is thus fed back directly into the dynamo armature continuously.

The coils are small and can be placed back of the switchboard or below the floor, as they require no attention. The current flowing to each slip ring is 25% of the direct current in the neutral wire with the small exciting current taken by the coil added.

The coils are usually built to take care of current in the neutral equal to 25% of full load current of the dynamo with a voltage regulation not to exceed 2 per cent.

Ques. Upon what does the operation of the balancing coil system depend?

Ans. It depends on the following points: First, the impedance[1] of the coils keeps the exciting current which they take from the collector rings down to a small value as it is alternating current. At the same time the current from the neutral wire flows through the four half coils in parallel, and being direct current is impeded only by the ohmic resistance of the coils, which is low, giving only a slight loss in the coils. The common point to which the neutral wire is connected must at all times be neutral to the - and + direct current brushes.

That this common point is at all times neutral is readily shown. Referring to [fig. 804], let AE, BF, CG and DH represent the balance coil and its connection for different positions of the armature of a bipolar machine. Let O be the tap to the middle point of the winding.

Take the instant when the balance coil taps are directly under the direct current brushes as shown at position AE. It is evident that since the point O is the middle point of the coil, it is neutral between A and E. When the armature turns so that the balance coils take the position BF, the voltage drop between A and E may be divided into 4 parts, AB, BO, OF and FE. As in the first instance, O is neutral between the ends of the coil, and the voltage drop over OF equals that over OB.

Since the space AB includes the same number of armature coils as space FE and they are in fields of equal strength, the voltages across the two spaces will be equal, and the voltage over AB equals that over FE. Then adding equals: AB + BO = FE + FO and O is neutral between A and E as in the first case.

In the same way it can be shown that O is neutral between the direct current brushes for any position of the balance coil taps. One coil will operate the system, but two coils, giving four points spaced 90 electrical degrees apart, give better distribution of the current to the armature winding and better regulation of the voltage.

Boosters.—A booster may be defined as, a dynamo inserted in a circuit at a point when it is necessary to change the voltage. A booster is generally driven by a motor, the two armatures being directly coupled, although boosters are sometimes driven from the engine or line shaft.

Fig. 805.—Crocker Wheeler motor-dynamo set. There are numerous cases where such a combination is useful for furnishing a circuit with a voltage different from that of the main plant or with a voltage that can be varied independently. For storage battery charging and electrolytic work, where constant current is desirable, it forms a simple means of voltage regulation. Where a circuit of special voltage is required, the set not only supplies current at the desired pressure, but insulates the special circuit, which may be subject to more severe requirements than the main system. The advantage of the three wire distribution can be obtained from any two wire dynamo by means of a small rotary balancer or balancing transformer, which consists of two direct current machines of the same voltage, mechanically connected together with their armatures in series. Multiple voltage systems for speed regulation can also be obtained by a similar arrangement.

Ques. Explain the use of a booster?

Ans. When a number of feeders run out from a station, the longest and those carrying the heaviest loads will have so much drop on the line that the pressure at distant points is too low. It is therefore necessary to raise the pressure to compensate for the drop and this is done by inserting a booster in the circuit.

It would not be economical to raise the voltage on all the lines by supplying current from the main dynamo at higher pressure, hence the voltage is raised only on the lines which need it by means of the booster working in series with the main dynamo.

Fig. 806.—Diagram showing use of auxiliary bus bar. In order to avoid the necessity for boosters, some stations have an extra bus bar, which is kept at a higher pressure than the main bus, and to this are connected the feeders that have an extra large drop.

Ques. For what other service are boosters employed?

Ans. They are used in connection with storage battery plants for the purpose of raising the voltage of the bus bars to the pressure necessary for charging storage batteries.

Ques. What is an auxiliary bus bar?

Ans. An extra bus bar which is kept at higher pressure than the main bar.

Ques. What is the object of an auxiliary bus bar?

Ans. It is used in place of a booster as shown in [fig. 806]. One or more dynamos maintain the pressure between the auxiliary bar and the common negative bar. The feeders which need boosting are connected to the common negative bar and the auxiliary bar as shown.

CHAPTER XXXVII
WIRES AND WIRE CALCULATIONS

The wireman who is called upon to plan and install a system of wiring will find it necessary first to have a knowledge of the various kinds of wire so as to select the one best suited for the work, and to be able to make simple calculations in order to determine the proper sizes of wire for the various circuits.

Wires are generally made of circular cross section. The process of manufacture consists in drawing the material through steel dies, when its properties permit this treatment. In the case of some substances, as for instance, tin and lead, difficulties arise in the drawing process, and these are therefore "squirted."

The metals most extensively used for wires are copper and iron; German silver, tin and lead are also employed, but only at points where it is desirable to have a comparatively high resistance in the circuit.

Copper Wire.—Copper is used in nearly all cases of wiring because it combines high electrical conductivity with good mechanical qualities and reasonable price. In conductivity it is only surpassed by silver, but the cost of the latter of course prohibits its use for wiring purposes.

Copper wire is used for electric light and power lines, for most telephone and some telegraph lines, and for all cases where low resistance is required at moderate cost.

Hard drawn copper wire is ductile, and has a high tensile strength; these properties allow it to be bent around corners and drawn through tubes without injury.

Pure annealed copper has a specific gravity of 8.89 at 60° Fahr. One cubic inch weighs .32 pound; its melting point is about 2,100° Fahr.

Good hard drawn copper has a tensile strength of about three times its own weight per mile length. Thus, a number 10 B. & S. gauge copper wire, weighing 166 lbs. per mile, will have a breaking strength equal to approximately 3 × 166 = 498 lbs.

Iron Wire.—This kind of wire is largely used for telegraph and telephone lines, although it is rapidly being replaced by copper in long lines.

There are three grades of iron wire:

1. Extra best best (E. B. B.) which has the highest conductivity and is the nearest to being uniform, in quality, being both tough and pliable;

2. Best best (B. B.), which varies more in quality, is not so tough, and is lower in conductivity. It is frequently sold as E. B. B.;

3. Best (B.), which is the poorest grade made, being more brittle, and lowest in conductivity. Iron wire should be well galvanized.

German Silver Wire.—German silver is an alloy consisting of 18 to 30% nickel, and the balance about four parts copper to one part zinc. It is very largely used as a resistance material in making resistance coils, and is sold in the form of wire, and strip. The resistance of this wire varies with its composition.

The resistance of the 18% alloy at 25° C. is 18 times that of copper, and of the 30% alloy about 28 times that of copper.

The safe carrying capacity of the wire in spirals in open air for continuous duty is such that the circular mils per ampere varies from about 1,500 in No. 10 wire to about 475 in No. 30. For intermittent duty the capacity is twice as great.

Standard of Copper Wire Resistance.—Matthiessen's standard for resistance of copper wire is as follows: A hard drawn copper wire one meter long, weighing one gramme, has a resistance of .1469 B. A. unit at 32° Fahr. Relative conducting power: silver, 100; hard or un-annealed copper, 99.95; soft or annealed copper, 102.21.

A committee of the Am. Inst. Electrical Engineers recommends the following form of Matthiessen's standard, taking 8.89 as the specific gravity of pure copper: A soft copper wire one meter long and one millimeter in diameter has an electrical resistance of .02057 B. A. unit at 0°C.[2] From this the resistance of a soft copper wire one foot long and .001 in. in diameter (mil-foot) is 9.72 B. A. units at 0°C.

For every degree Fahr., the resistance of copper wire increases .2222%. Thus a piece of copper wire having a resistance of 10 ohms at 32°, would have a resistance of 11.11 ohms at 82°.

Relative Conductivity of Different Metals and Alloys.
(According to Lazare Weiler.)

Pure silver100
Pure copper100
Alloy, ½ copper, ½ silver86.65
Telephonic siliceous bronze35
Pure zinc29.9
Brass with 35% zinc21.5
Swedish iron16
Pure platinum10.6
Copper with 10% nickel10.6
Pure lead8.88
Pure nickel7.89
Phosphor-bronze, 10% tin3.88

Conductors.—Copper is used more than any other metal for transmitting electrical energy, and for interior wiring it is used exclusively. Copper conductors should be of the highest commercial conductivity, not less than 97%.

For conductors up to sizes as large as No. 8 B. & S. gauge, single conductors may be used, but for larger sizes the necessary conductivity should be obtained by conductors made up of strands of smaller wires. The size of these strands depend upon the size of the conductors and the conditions under which they are to be used.

Where conductors are very large (as for instance dynamo leads), and where it is essential that they should be as flexible as possible, strands as small as No. 20 or 22 B. & S. gauge may be used.

Conductors for flexible cords, pendants, fixtures, etc., should also consist of very fine strands, so that they may be perfectly pliable and flexible.

The individual strands for instance, for a No. 16 B. & S. gauge flexible cord should be as fine as No. 30.

Fig. 807.—Elevator cable for annunciators. This type of cable is designed for connecting the movable elevator car with the signal buttons upon the different floors, and is constructed so as to secure strength and flexibility.

Covered Conductors.—For most conditions of service, wires are protected with an insulating covering. Wires used in interior circuits should have a covering which shall act both as an electrical insulator and as a mechanical protection. In some instances, however, the insulating qualities are of secondary importance.

The various forms of covering now in use commercially for wires are:

1. Rubber;
2. Weather proof;
3. Slow burning;
4. Slow burning weather proof;
5. Armoured.

Rubber Covered Conductors.—This class of conductor consists of a tinned copper wire with a rubber covering, protected by an outside braiding of cotton saturated with a preservative compound.

Ques. What are the advantages of rubber insulation for conductors?

Ans. It is waterproof, flexible, fairly strong, and has high insulating qualities.

Fig. 808.—Rubber insulated telephone and telegraph wires. The inner coat of rubber should be free from sulphur or other substances liable to corrode the copper.

Ques. What are the disadvantages of rubber insulation?

Ans. It deteriorates more or less rapidly and is quickly injured by temperatures above 140° Fahr.

Ques. For what service are rubber covered conductors adapted?

Ans. For interior wiring.

Ques. Is pure rubber used?

Ans. No. The covering should be made from a compound containing from 20 to 35 per cent. of pure rubber.

It would be difficult to place pure rubber on a wire, and moreover a covering made of pure rubber would not be durable and would deteriorate very rapidly, particularly at temperatures above 120° Fahr. Accordingly, it is mixed with other materials, such as French chalk, silicate of magnesia, sulphur, red lead, etc.

Weather Proof Conductors.—In this class of conductor, the wire is protected from the weather by a waterproof covering, consisting usually of braided cotton of two or three thicknesses saturated with a moisture resisting insulating compound.

Ques. Where are weather proof conductors used?

Ans. In places subject to dampness, such as cellars, tunnels, open sheds, breweries, etc.

Fig. 809.—Twisted weather proof wires. The insulation consists of two or three thicknesses of braided cotton saturated with a moisture resisting insulating compound.

Ques. What are the advantages of weather proof conductors?

Ans. The insulation is cheap, very durable, and does not deteriorate unless exposed to high temperatures such as will melt the compound.

Ques. State the disadvantages.

Ans. The covering is more or less inflammable and is not very efficient as an insulator.

Ques. What precaution should be taken in using weather proof conductors?

Ans. On account of the inflammable character of the covering, care should be taken in wiring at points where any considerable number of conductors are brought together, or where there is much woodwork or other combustible material.

Ques. For what use are weather proof conductors especially adapted?

Ans. For outside wiring where moisture is certain and where fireproof quality is not necessary.

Obviously conductors of this class should not be used in conduits, nor in fact, in any way except exposed on glass or porcelain insulators.

Slow Burning Wire.—This class of conductor is defined as: one that will not carry fire. The covering consists of layers of cotton or other thread, all the interstices of which are filled with the fireproofing compound, or of material having equivalent fire resisting and insulating properties. The outer layer is braided and specially designed to withstand abrasion. The thickness of insulation must not be less than that required for slow burning weather proof wire and the outer surface must be finished smooth and hard.—Underwriters' requirements.

Fig. 810.—Slow burning wire, formerly known as Underwriter's Wire. The insulation is triple braided, saturated with a white fireproof compound. Solid conductor.

Ques. Where should slow burning wires be used?

Ans. In hot dry places, where ordinary insulations would be injured, and where wires are bunched, as on the back of a large switchboard or in a wire tower.

A slow burning covering is considered good enough when the wires are entirely on insulating supports. Its main object is to prevent the copper conductors coming into contact with each other or anything else.

Ques. What must be done before using weather proof wire?

Ans. Permission to use the wire must first be obtained from the local Inspection Department.

Slow Burning Weather Proof Wire.—The covering of this type wire is a combination of the underwriters and weather proof insulations. The fireproof coating comprises a little more than half of the total covering. When the fireproof coating is placed on the outside, the wire is called "slow burning weather proof."

Fig. 811.—Slow burning weather proof wire. The insulation is composed of two braids thoroughly saturated with a fire proof composition, over which is a highly polished weather proof third braid. This wire was formerly known as "fire and weather proof" wire.

Ques. How does slow burning weather proof wire compare with weather proof wire?

Ans. It is less inflammable and less subject to softening under heat.

Ques. Where should slow burning weather proof wire be used?

Ans. In places where the wires are to be run exposed and where moisture resisting quality is desired, also where at the same time it is desirable to avoid an excess of inflammable covering.

Ques. How should it be installed?

Ans. It should be set on glass or porcelain insulators.

Miscellaneous Insulated Conductors

Fig. 812.—Armoured submarine cable. This type of cable is insulated with a rubber compound containing not less than 30% of pure Para rubber. The following specifications have been adapted by various telegraph companies and the United States Government for general use.

No. of
Conductors		Gauge of
Conductors
(B. & S.)		No. of
Armour
Wires		Gauge of
Armour Wires
B. W. G.		Outside
Diameter
(inch)		Weight
per 1,000
Feet
114128⅞1,150
214168131/321,675
3141461¼2,400
414166 15/162,750
5141961⅜3,100
6142161½3,500
7142161½3,600
10142241⅞4,600

NOTE.—The above specifications refer only to river and harbor cables. Ocean cables are of an entirely different character, and consist of "shore end," "intermediate" and "deep sea" types.

Fig. 813.—Gas engine ignition cable. This is a special cable made to stand the hard service necessary on automobiles. The conductor is composed of 36 strands of No. 27 tinned copper wire, equal to No. 14 in capacity, which gives it necessary flexibility. About this conductor are woven two layers of cotton thread. Over this are woven, in opposite directions, several layers of specially prepared tape which has been given two coatings of fine insulating varnish. Two strong braids of cotton form the outside covering, and each of these different braids is passed through a bath of insulating liquid and baked in a steam heated oven. With three layers of tape the cable will stand a test of 18,000 to 20,000 volts, and with five layers, 30,000 volts.

Fig. 814.—Paper insulated lead encased telephone cable.

Ques. For what service is slow burning weather proof wire not suited?

Ans. It is not adapted to outside work.

Safe Carrying Capacity of Wire.—All wires will heat when a current of electricity passes through them. The greater the current or the smaller the wire, the greater will be the heating effect. Large wires are heated comparatively more than small wires because the latter have a relatively greater radiating surface.

Fig. 815.—Pothead wires. The standard wire for pothead work is either No. 19, 20 or 22 B. & S. gauge, either single conductor or twisted pair, insulated to a diameter of 3/32 inch over rubber, without any outer braid or protection. In the case of twisted pairs one conductor is usually made of a differently colored rubber than the other, so as to distinguish between them.

The temperature of a wire increases approximately as the square of the current, and inversely as the cube of the diameter of the wire.

The elevation in temperature of a wire carrying a current represents so much lost energy.

From these considerations it must be clear that it is important not to overload conductors in order to secure efficient working, and to avoid risk of fire on inside installations.

The Board of Underwriters specifies that the carrying capacity of a conductor is safe when the wire will conduct a certain current without becoming painfully hot.

In the following table of carrying capacity, prepared by the underwriters, a wire is assumed to have a safe carrying capacity when its temperature is not increased by the given current over 30° Fahr. above that of the surrounding air.

SAFE CARRYING CAPACITIES OF WIRES
(Maximum amperes allowed by the Underwriters.)

Brown
and
Sharpe
Gauge		Circular milsRubber
insulation
———
Amperes		Other
insulations
———
Amperes
181,62435
162,58368
144,1071216
126,5301723
1010,3802432
816,5103346
626,2504665
533,1005477
441,7406592
352,63076110
266,37090131
183,690107156
0105,500127185
00133,100150220
000167,800177262
0000211,600210312
200,000200300
300,000270400
400,000330500
500,000390590
600,000450680
700,000500760
800,000550840
900,000600920
1,000,0006501,000
1,100,0006901,080
1,200,0007301,150
1,300,0007701,220
1,400,0008101,290
1,500,0008501,360
1,600,0008901,430
1,700,0009301,490
1,800,0009701,550
1,900,0001,0101,610
2,000,0001,0501,670

The lower limit is specified for rubber covered wires to prevent gradual deterioration of the high insulations by the heat of the wires, but not from fear of igniting the insulation. The question of drop is not taken into consideration in the table on [page 731].

The carrying capacity of Nos. 16 and 18 B. & S. gauge wire is given, but no smaller than No. 14 is to be used, except as allowed under rules for fixture wiring.—Underwriters' Rules.

Circular Mils.—The unit of measurement in measuring the cross sectional area of wires is the circular mil; it is the area of a circle one mil (.001 in.) in diameter.

The area of a wire in circular mils is equal to the square of the diameter in mils.

Fig. 816.—Diagram illustrating circular mils. The circular mil is used as a unit of cross sectional area in measuring wires. It is equal to the area of a circle .001 in. diameter; its value is .0000007854 square inch. In the figure the sum of the areas of the nine small circles equals the area of the large circle. This is evident from the following: Take the diameter of the small circles as unity, then the diameter of the large circle is three. Hence, the sum of the area of the small circles × (¼ π × 12) × 9 = .7854 × 9 = 7.0686; area of the large circle = ¼ π × 32 = .7854 × 9 = 7.0686. Therefore since the area of the large circle equals the sum of the areas of the small circles, the area of a wire in circular mils is equal to the square of its diameter expressed in mils.

Thus a wire 2 mils in diameter (.002 in.) has a cross sectional area of 2 × 2 = circular mils. Accordingly to obtain the area of a wire in circular mils, measure its diameter with a micrometer which reads directly in mils or thousandths of an inch, and square the reading.

The circular mil (abbreviated C.M.) applies to all round conductors, and has a value of .7854 times that of the square mil, that is, 1 circular mil = .7854 square mil. If the diameter be expressed as a fraction of an inch, as for instance 1/3 in., the circular mil area may be found as follows: Reduce the fraction 1/3 to the decimal of an inch, multiply the result by 1,000 to express the diameter in mils, and square the diameter so expressed, thus: 1/3 = 1,000 ÷ 3 = .333. .333 × 1,000 = 333 mils; 333 × 333 = 110,889 circular mils.

The diameter of any wire may be found when its circular mil area is known by extracting the square root of the circular mil area.

Square Mils.—For measuring conductors of square or rectangular cross section, such as bus bars, copper ribbon, etc., the square mil is used. A square mil is the area of a square whose sides are one mil (.001 in. long) and is equal to .001 × .001 = .000001 square inch.

Fig. 817.—Diagram illustrating square mils. A square mil is a unit of area employed in measuring the areas of cross sections of square or rectangular conductors. It is equal to .000001 square inch. One square mil equals 1.2732 circular mils. The figure shows an area of nine square mils; this is equal to 9 × 1.2732 = 11.4588 circular mils.

EXAMPLE.—A copper ribbon for a field coil measures ⅝ inch by ⅛ inch. What is its area in square mils? What is its area in circular mils?

⅝ = .625 in., or 625 mils; ⅛ = .125 in., or 125 mils.

Area in square mils = 625 × 125 = 78,125.

Area in circular mils={78,125 ÷ .7854 }{or 78.125 × 1.2732} = 99,469.

Mil Foot.—This unit is used as a basis for computing the resistance of any given wire. A mil foot means a volume one mil in diameter and one foot long.

The resistance of a wire of commercially pure copper one mil in diameter and one foot long is taken as a standard in calculating the resistance of wires, and has been found to be equal to 10.79 ohms at 75° Fahr.

The calculation is made according to the following rule:

The resistance of a copper wire is equal to its length in feet, multiplied by the resistance of one mil foot (10.79 ohms) and divided by the number of circular mils, or the square of its diameter.

Expressed as a formula:

resistance in ohms = length of wire in ft. × 10.79 circular mils . . . . (1)

EXAMPLE. What is the resistance of a copper wire 1,500 feet long and having a transverse area of 10,381 circular mils?

Substituting these values in formula (1)

resistance= 1,500 × 10.79 10,381 =1.559 ohms.

The transverse area of a copper wire is found by multiplying the resistance of a mil foot (10.79) by its length in feet and dividing the result by its resistance in ohms.

This is obtained directly from the formula (1) by solving the equation for circular mils, thus:

circular mils = length of wire in ft. × 10.79 resistance in ohms . . . . (2)

EXAMPLE. What is the circular mil area of a wire 1,500 feet long and having a resistance of 1.559 ohms?

Substituting the values in equation (2)

circular mils = 1,500 × 10.79 1.559 = 10,381

Figs. 818 and 819.—Diagrams illustrating the meaning of the term lamp foot, and how to apply it in calculating a circuit. As defined, one 16 candle power lamp at a distance of one foot from the fuse block or point of supply is called a lamp foot; this is equivalent to one 8 candle power lamp at a distance of 2 feet, or one 32 candle power lamp one-half foot from the fuse block. In [fig. 819], there are four 8 candle power lamps, and the distance to center of distribution is 10 feet. The circuit then contains 4 ÷ 2 × 10 = 20 lamp feet.

Lamp Foot.—This unit facilitates laying out wiring and calculating the drop. A lamp foot is defined as one 16 candle power lamp at a distance of one foot from the point of supply. Accordingly the number of lamp feet in any circuit is equal to the number of 16 candle power lamps (or equivalent in other sizes) in the circuit multiplied by the distance in feet from the fuse block to the center of distribution.

When no point is specified, the feet are always measured from the supply point to the center of distribution. When other than 16 c.p. lamps are in the circuit they must be reduced to 16 c.p. lamps. Thus two 8 c.p. lamps would be counted one 16 c.p. lamp, one 32 c.p. lamp would be counted two 16 c.p. lamps, etc.

Ampere Foot.—From the foregoing explanation of lamp foot, the significance of ampere foot is easily understood—the two terms are in fact self-defining.

An ampere foot may be defined as the product of one ampere multiplied by one foot.

The unit ampere foot is used in figuring motor circuits or currents designed to carry a mixed load.

Fig. 820.—The center of distribution of a circuit coincides with the geometrical center of the group of lamps when the lamps are of uniform size and spaced equal distances apart. The center of distribution is here indicated by the dotted line A B.

The ampere feet of a main are found by multiplying the maximum load in amperes by the distance from the fuse block to the electrical center of the load.

Thus if the center of distribution be 50 feet from the fuse block and the maximum load is 9 amperes, the number of ampere feet is equal to 9 × 50 = 450.

Electrical Center of Distribution.—The electrical center of a circuit depends upon the distances between the lamps and the fuse block; also the relative sizes of the lamps.

It may be defined as the sum of the lamp feet for each section divided by the number of 16 candle power lamps in the circuit.

If the lamps be of uniform capacity, and placed at equal distances apart, the center of distribution will coincide with the geometrical center of the group of lamps. However, if the lamps vary in size, and be irregularly spaced, the electrical center will not coincide with the geometrical center unless the lamps be symmetrically arranged so as to compensate for the difference in sizes and spacing.

Fig. 821.—Diagram of an irregular circuit illustrating method of finding the center of distribution. Rule: Divide the sum of the lamp feet for each section by the number of 16 candle power lamps or equivalent in the circuit; the quotient gives the distance in feet from the fuse block to the center of distribution.

In such cases, as shown in [fig. 821], the electrical center can be determined by adding together the lamp feet of the several sections A, B, C, etc., of the main and dividing the result by the 16 c.p. units. Thus the lamp feet of

Section A = 10 lamps × 10 feet = 100
" B = 9 " × 5 " = 45
" C = 7 " × 6 " = 42
" D = 6 " × 4 " = 24
" E = 5 " × 5 " = 25
" F = 4 " × 10 " = 40
" G = 2 " × 5 " = 10
which added together gives a total of 286 lamp feet.

This when divided by the ten 16 c.p. units comprising four 16 c.p. lamps and three 32 c.p. lamps, gives a little over 28½ feet as the distance from the fuse block to the center of distribution, the position of which is shown by the line M N in [fig. 821], while that of the geometrical center is shown by the line K L.

SectionA = 10lamps× 10feet = 100
"B = 9"× 5" = 45
"C = 7"× 6" = 42
"D = 6"× 4" = 24
"E = 5"× 5" = 25
"F = 4"× 10" = 40
"G = 2"× 5" = 10
which added together gives a total of286lamp feet.

When the center of distribution is at a considerable distance from the supply circuit, and it becomes advisable to divide the wiring into two distinct elements—a feeder and one or more mains, the junction of the feeder and the mains should be located at the electrical center of the mains whenever possible. When this is done, it is obvious that the wire size of only one half the main needs to be calculated, as both halves of the main are identical.

Fig. 822.—Brown and Sharpe (B. & S.), or American Standard wire gauge. This gauge was adopted by the brass manufacturers Jan., 1858. The cut is full size, and therefore, shows the actual sizes corresponding to the gauge numbers.

Wire Gauges.—For the purpose of facilitating the measurement of wire, a number of gauges have been designed by various wire manufacturing concerns. The principal gauges used in the United States are the American or Brown & Sharp's gauge; the English standard or Birmingham gauge; Washburn & Moen's standard gauge; Imperial wire gauge; Stubs' steel wire gauge, and the U. S. Standard wire gauge.

The several gauges are here given with explanation of their use.

The American Standard or Brown and Sharp's Gauge.—This gauge is commonly designated as A. W. G. or B. & S., and has been adopted by brass manufacturers and is used mostly in measuring brass, copper, silver, German silver, and gold in both wire and plate.

Birmingham or Stub's Wire Gauge (B. W. G.).—Old English Standard and Iron Wire Gauge. Birmingham or Stubs' Iron Wire Gauge is not the same as Stubs' Steel Wire Gauge. A table of Stub's Steel Wire Gauge is given on [page 741].

Fig. 823.—Micrometer screw gauge. It consists essentially of a screw whose thread is accurately turned to a pitch of some convenient fraction of an inch or centimetre. When the screw is screwed home, the surfaces of A and B meet, and the instrument should then read zero on both the straight and the circular scale. If this be not so, there is a zero error which must be either allowed for, or corrected by means of the screw provided for that purpose. If the former course be adopted, the reading of the instrument is taken when the faces A and B are in contact, and this number added to or subtracted from the final reading according to whether the error makes the wire apparently smaller or greater than its real size. The surfaces A and B are now screwed apart and then, after the wire to be measured (which should be clean and straight) has been introduced between them, they are screwed together to lightly grip the wire. If the gauge be screwed up too tightly the value of the measurement is destroyed, since a copper wire can easily be crushed, and in addition the accurate screw may be permanently damaged. To avoid the possibility of this happening, screw gauges are provided with a ratchet which prevents an excessive force being applied to the screw. If the pitch of the screw in the gauge be 1/20th of an inch, and the circular scale consist of 50 divisions, then for each revolution of the screw, the surface B will travel a distance equal to the pitch, that is 1/20th of an inch. The graduations on an instrument of this kind are generally 1/10th of an inch on the straight scale, with shorter lines to mark the half divisions. The thickness of a wire on the straight scale can therefore be read to the nearest 1/20th inch. Each division of the circular scale represents 1/50th of a revolution of the screw, which corresponds to a change in distance between A and B, of 1/50 of 1/20 = 1/1,000 in. If then the reading on the straight scale be 1 and on the circular scale 35, the distance between A and B is .1 + .035 = .135 inch.

Washburn and Moen's Standard Wire Gauge.—Commonly designated as W. & M. G. has been adopted by the U. S. Steel Corporation in making their wire.

New British Standard (N. B. S.).—British Imperial English Legal Standard and Standard Wire Gauge, and is variously abbreviated by S. W. G. and I. W. G.

Roebling Gauge.—Washburn Moen, American Steel & Wire Co.'s Iron Wire Gauge.

Figs. 824 and 825.—U. S. wireman's calculating gauge; views showing both sides. On the side shown in [fig. 824], set the required number of feet on the small circle opposite the required number of amperes on the large circle, then set the small pointer at the required voltage and loss. Then on the other side ([fig. 825]) the large pointer will indicate the required size of wire in B. & S. gauge, and will also indicate the safe carrying capacity, while the wire may be gauged by slot A ([fig. 824]).

U. S. Standard Wire Gauge.—This gauge is used for measuring sheet and plate iron, and steel, by the U. S. Government in assessing duties, and in making requisitions for supplies.

Old English Standard Wire Gauge.—The old English gauge is the same as the Birmingham or Stubs' standard gauge, commonly designated as B. W. G. It is used chiefly for measuring sheet iron and steel, also soft steel and iron wire.

London Gauge.—Old English (not Old English Standard).

From the foregoing it is seen that great confusion exists with such a multiplicity of gauges and emphasizes the importance of specifying the gauge and of knowing what gauge to use.

In using the gauges known as Stubs' Gauges, there should be constantly borne in mind the difference between the Stubs' Iron Wire Gauge and the Stubs' Steel Wire Gauge. The Stubs' Iron Wire Gauge is the one commonly known as the English Standard Wire, or Birmingham Gauge and designates the Stubs' soft wire sizes. The Stubs' Steel Wire Gauge is the one that is used in measuring drawn steel wire or drill rods of Stubs' make and is also used by many makers of American drill rods.

Letter.Size of
Letter
in
Decimals.		No. of
Wire
Gauge.		Size of
Number
in
Decimals.		No. of
Wire
Gauge.		Size of
Number
in
Decimals.		No. of
Wire
Gauge.		Size of
Number
in
Decimals.
Z.4131.22728.13955.050
Y.4042.21929.13456.045
X.3973.21230.12757.042
W.3864.20731.12058.041
V.3775.20432.11559.040
U.3686.20133.11260.039
T.3587.19934.11061.038
S.3488.19735.10862.037
R.3399.19436.10663.036
Q.33210.19137.10364.035
P.32311.18838.10165.033
O.31612.18539.09966.032
N.30213.18240.09767.031
M.29514.18041.09568.030
L.29015.17842.09269.029
K.28116.17543.08870.027
J.27717.17244.08571.026
I.27218.16845.08172.024
H.26619.16446.07973.023
G.26120.16147.07774.022
F.25721.15748.07575.020
E.25022.15549.07276.018
D.24623.15350.06977.016
C.24224.15151.06678.015
B.23825.14852.06379.014
A.23426.14653.05880.013

The following table gives the diameters, in decimal parts of an inch, of the various sizes of wire corresponding to the number of gauge numbers of the different standard wire gauges used in the United States.

Number
of Wire
Gauge		American,
or Brown
& Sharpe
(B.&S.)		Birmingham,
or Brown &
Sharpe
(B. W. G.)		Washburn
& Moen
Mfg. Co.,
Worcester,
Mass.		Trenton
Iron Co.,
Trenton,
N. J.		G. W.
Prentiss,
Holyoke,
Mass.		Old
English,
From Brass
Mfrs' List		British
Standard
(S. W. G.)
0000000.500
000000.460.464
00000.430.450.432
0000.46000.454.393.400.400
000.40964.425.362.360.3586.372
00.36480.380.331.330.3282.348
0.32486.340.307.305.2994.324
1.28930.300.283.285.2777.300
2.25763.284.263.265.2591.276
3.22942.259.244.245.2401.252
4.20431.238.225.225.2230.232
5.18194.220.207.205.2047.212
6.16202.203.192.190.1885.192
7.14428.180.177.175.1758.176
8.12849.165.162.160.1605.160
9.11443.148.148.145.1471.144
10.10189.134.135.130.1351.128
11.090742.120.120.1175.1205.116
12.080808.109.105.1050.1065.104
13.071961.095.0920.0925.0928.0920
14.064084.083.0800.0800.0816.08300.0800
15.057068.072.0720.0700.0726.07200.0720
16.050820.065.0630.0610.0627.06500.0640
17.045257.058.0540.0525.0546.05800.0560
18.040303.049.0470.0450.0478.04900.0480
19.035890.042.0410.0400.0411.04000.0400
20.031961.035.0350.0350.0351.03500.0360
21.028462.032.0320.0310.0321.03150.0320
22.025347.028.0280.0280.0290.02950.0280
23.022571.025.0250.0250.0261.02700.0240
24.020100.022.0230.0225.0231.02500.0220
25.017900.020.0200.0200.0212.02300.0200
26.015940.018.0180.0180.0194.02050.0180
27.014195.016.0170.0170.0182.01875.0164
28.012641.014.0160.0160.0170.01650.0148
29.011257.013.0150.0150.0163.01550.0136
30.010025.012.0140.0140.0156.01375.0124
31.008928.010.0130.0130.0146.01225.0116
32.007950.009.0120.0120.0136.01125.0108
33.007080.008.0110.0110.0130.01025.0100
34.006305.007.0100.0100.0118.00950.0092
35.005615.005.0095.0095.0109.00900.0084
36.005000.004.0090.0090.0100.00750.0076
37.004453.0085.0085.0095.00650.0068
38.003965.0080.0080.0090.00575.0066
39.003531.0075.0075.0083.00500.0052
40.003145.0070.0070.0078.00450.0048
41 .0044
42.0040

NOTE.—The sizes of wire are ordinarily expressed by an arbitrary series of numbers. Unfortunately there are several independent numbering methods, so that it is always necessary to specify the method or wire gauge used. The above table gives the numbers and diameters in decimal parts of an inch for the various wire gauges in general use.

Wiring Terms.—The various members of a complex wiring installation are designated feeders, sub-feeders, mains, branches, and taps.

A feeder is a stretch of wiring to which no connection is made except at its two ends.

A sub-feeder is of the same class as a feeder, but is distinguished either by being one of two or more connecting links between the end of a single feeder and several distributing mains, or by constituting an extension of a feeder.

Fig. 826. Circuit diagram illustrating names of the various parts. A circuit may consist of the following parts as defined in the accompanying text: 1, feeder, 2, sub-feeders, 3, mains, 4, branches, 5, taps. It is well to clearly distinguish between these divisions because the terms are constantly used in wiring.

A main is a stretch of wiring supplied from one or more feeders or sub-feeders and distributing current to a number of taps, or else to a number of branches.

A branch distributes current among a number of lamps, etc.

A tap almost invariably delivers current to a single lamp or other device.

Reference to [fig. 826] will make these definitions clearer. This diagram is intended merely to illustrate the above definitions and does not represent any special plan of wiring.

Figs. 827 and 828. Simplest forms of circuit, consisting of a main with one or more lamps at the end. The smallest size wire allowed (No. 14 B.&S. gauge) will generally be found amply large for such circuits. Note carefully the difference between a main and a branch by comparison with [fig. 826]. A main begins from a fuse block, while a branch is an offset from a main without any fuse block.

The simplest possible wiring installation is one in which a single lamp or compact cluster of lamps is located at the end of a main, as shown in figs. 827 and 828. In such cases calculations are almost always unnecessary, for the reason that No. 14 wire, the smallest size allowed by the underwriters, will supply several lamps at a long distance (as interior wiring goes) with a very moderate drop. For example, if the three lamps shown at the end of the main in [fig. 828], be of 16 candle power each, and the voltage of the supply circuit be 110 volts, a main of No. 14 wire would supply the lamps at a distance of 135 feet from the fuse block with a drop of only 1 per cent.

When the lamps are strung along the main, however, as in [fig. 826], it is sometimes necessary to choose the size of wire with regard to the drop, and in order to do this the main must be measured for either "ampere feet" or "lamp feet."

Wire Calculations.—The problem of calculating the size of wire will be presented here in as simple a form as possible, with explanation of the various steps so that any one can understand how the formula is derived.

In determining the size of wire, there are four known factors which enter into the calculation, viz.:

1. Length of circuit in feet;
2. Maximum current in amperes;
3. Drop or volts lost in the circuit, in % of the impressed voltage;
4. Heating effect of the current.

The calculation is based on the mil foot, which as previously explained, is a foot of copper wire one mil in diameter and whose resistance is equal to 10.79 ohms at 75° Fahr.

Fig. 829.—Wiring for lights requiring unusually long feeders.

The first step is to find an expression for the resistance of the wire which may be later substituted in Ohm's law formula. Accordingly, the resistance of any conductor is equal to its length in feet multiplied by its resistance per mil foot and the product divided by its area in circular mils, thus:

resistance in ohms = length in feet × resistance per mil foot circular mils

or

ohms = feet × 10.8 circular mils . . . . (1)

(calling the resistance per mil foot 10.8 instead of 10.79 to facilitate calculation).