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