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502 INSTALLATION OF ARC LAMPS [Cn. XIII
§ 696. Selection of material for installing the arc lamp. — After determining the maximum amount of current needed for the arc lamp, then the wire of proper size and quality and insulation to conform with the National Electrical Code should be obtained. The simplest way to do this is to go to some reliable dealer in electrical supplies and get the standard material.
Standard switches, etc., are all marked plainly so that there is no difficulty in selecting the correct sizes. In America, wire is more often designated by some standard wire gauge, e. g., that of Brown & Sharp, than by the actual diameter in millimeters or inches. In the above table the sizes in millimeters and inches corresponding with the B & S gauge numbers are given, also the area measured in circular mils.
One must not forget that everything that is used wears out, and when any piece of apparatus or the wire becomes deteriorated by use it should be replaced.
WIRING FOR THE ARC LAMP, THE RHEOSTAT OR OTHER BALANCING DEVICE, AND THE LAMP SWITCH
§ 697. Connection with the electric supply. — It is assumed that the electric supply has been properly installed by an electric company, or from a private dynamo, to within a short distance of the arc lamp. This supply will be in a proper outlet box, with fuses and switches in accordance with the National Electrical Code. In case the outlet box is on the wall close to the arc lamp, the simplest and most convenient connection between the lamp switch and the supply in the outlet box is by means of a separable attachment of the proper capacity for the maximum current. (See the table of flexible cables, § 695.) If the current is direct, then it is a convenience to have this attachment irreversible, or polarized so that
For No. 1 8 wire, as before, the circular mils are 1624 and the relative carrying
capacity is assumed to be — — = — — , whence X = .116 amperes. If there
3 X
are 40 wires in each cord then each cord should carry . 1 16 x 40 = 4.64 amperes, or in round numbers 5 amperes. If the double cord were used for each conductor to the lamp, then in like manner twice as much could be carried, as there are 80 wires: .116 x 80 = 9.28 amperes or 10 amperes in round numbers.