The “transformer” is a device for reducing the current from one potential or tension to another, and consists of a ring of many turns of insulated wire; the high tension wire is then wound around and through this ring several turns, and the low tension wire carrying the current into the building is wound around the opposite side of the ring in like manner. The difference in pressure is controlled by the number of turns around the ring on either side, as shown.
All the wires being insulated, the reduction is made by “induction” only. The size of the wire makes no difference in the voltage or pressure, but it makes all the difference in the “amperes” or quantity used to supply the building.
The electric light company will supply a sufficiently large wire from transformer to switch; the firm does the remainder of the wiring.
FEEDERS.
From this large double-pole switch (which is the main cut-off) radiate “feeder” wires to the various parts of the building, to supply the lamp circuits. These feeders have to be carefully gauged according to the number of lamps, or rather, amperes, they are required to carry, in order to avoid resistance in the wire; or in other words, must not crowd the carrying capacity of the wire, or the resistance will make the wire hot and might cause fire.
A “feeder” to the circuits for fifty incandescent 16-candle power lamps should be of somewhat higher capacity, say a No. 10 Brown & Thorp Gauge (B. & S. G.) which has a safe carrying capacity of thirty-two amperes or sixty-four 16-candle power lamps, allowing one-half ampere to each lamp; a No. 12 B. & S. G. wire is allowed to carry twenty-three amperes, or about forty-five 16-candle power lamps; No. 13 B. & S. G. wire is allowed to carry sixteen amperes, or about thirty 16-candle power amps.
The “feeder” should have considerably more carrying capacity than is required in order to permit the addition of extra lights, as occasion may require, and to provide for the addition of extra window lights, if there be not a special feeder for the purpose.
The feeder wires are tapped for lamp circuits by a branch block carrying a fuse for the protection of the circuit. The construction and operation of the branch block, or cut-out, will be understood at once on inspection. The fuse here is the important part; it must be of smaller carrying capacity than the wire it is to protect, and not depend upon the amount of current in the feeder; thus, a No. 10 B. & S. G. might be protected with a 20-ampere fuse (forty lamps, sixteen candle power), or with a 25-ampere fuse, leaving a margin of seven amperes, the wire being allowed to carry thirty-two amperes. It will be understood that “open work” is meant in every case, and wires that are concealed are not allowed to carry so much. The No. 12 wire B. & S. G. is allowed twenty-three amperes, and would be protected by fuse of fifteen to twenty amperes capacity.
The lamp circuits from feeders are in many cities restricted to a certain number of amperes, usually six to ten, or say from ten to twenty lamps (sixteen candle power). Each of these circuits must be protected by its own fuse and have an independent switch. All fuse boxes must have their porcelain cover kept on, and no inflammable material allowed near them. “Twin wires,” or two wires encased, are objectionable.
Drop cords are not allowed except for drop lamps, and must be protected by one-ampere fuse in rosette or cut-out. All wires where joined must be soldered—to prevent an arc forming by making the path of the current continuous or tight.