Ans. It should be continuous from outlet to outlet, without being spliced and installed on the loop system. Outlet boxes should be installed at all outlets, although, where this is impossible, outlet plates may be used under certain conditions. Clamps should be provided at all outlets, switch boxes, junction boxes, etc., to hold the cable in place, and also to serve as a means of grounding the steel sheathing.

Ques. Is armoured cable wiring expensive?

Ans. It is less expensive than the rigid conduit or the flexible steel conduit, but more expensive than cleat wiring or knob and tube wiring, and is strongly recommended in preference to the latter.

CHAPTER XXXIX
OUTSIDE WIRING

In the equipment of lighting and power plants, the cost of the outside wiring represents a considerable proportion of the total investment, sometimes costing more than the engines, boilers and dynamos.

A thorough knowledge of outside wiring is therefore necessary to properly proportion and install the wires so that the system will prove economical and safe.

Materials for Outside Conductors.—Copper wire is now considered to be the most suitable material not only for the transmission of current for electric light and power purposes, but also for telegraph and telephone lines, in place of the iron wire formerly employed.

Hard drawn copper wire is used in outside construction, because its tensile strength ranges from 60,000 to 70,000 pounds or about twice that of soft copper. This is desirable to withstand the stresses to which the wire is subjected which, in the case of long spans, are considerable.

The table on the next page gives the tensile strength, in pounds per square inch of cross section, hard drawn copper wire of various sizes B. & S. gauge.

The metal aluminum possesses certain advantages as a material for overhead wires. Its conductivity is about .6 that of copper. The specific gravity of aluminum is about 2.7, while that of copper is 8.89, so that a given volume of copper will weigh 3.3 times more than an equal volume of aluminum, and copper wire of given length and resistance would be about twice as heavy as an aluminum wire of equal length and resistance.

There are several disadvantages, such as, low tensile strength, high electro-positive quality of the metal, higher electrostatic capacity, etc.