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CHAPTER IV. ELECTRICAL CONDUCTORS
( 1 ) Electric conductors usually are selected with certain basic ends in view, namely: (a) degree of conductivity, (b) tensile strength, (c) ductility, (d) freedom from corrosion, and (e) low cost.
(2) Conductors employed in projection have two widely different purposes. They are meant to convey current with the least possible resistance, or to offer comparatively high, non-fluctuating resistance.
(3) In the first case copper is employed almost exclusively for the reason that it best combines the basic requirements of low cost, low resistance, tensile strength, ductility and freedom from corrosion.
In the second case a metal that offers high resistance is used, since rheostatic resistance forms an integral part of some projection circuits.
Wire Capacities
(4) Wire capacities are fixed by the National Board of Fire Underwriters, (5) whose instructions it is dangerous to ignore or violate, since such violation would automatically void all insurance on a building, or even prevent the owner from obtaining insurance. The Board of Underwriters has adopted the wire capacity rating recommended by the American Institute of Electrical Engineers and it is now part of the National Electrical Code. Table No. 2 on page 63 gives the allowable continuous current-carrying capacities of copper wires and cables of 98% conductivity and (6) is based upon wire diameters measured with a Brown & Sharpe wire gauge. It takes in wires up to 0000 size only, which serves all projection room requirements.
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