circular mils = amperes × feet × 21.6 drop × 4
but if an over all voltage, (that is, the voltage between the outer wires), of 220 volts be used, the two wire formula does not require any modification.
The proper size of wire may also be calculated by means of the formula
drop 2 × distance × amperes = resistance per foot . . . . (1)
Example.—If in calculating a three wire feeder, the over all voltage be 220 volts, the drop = 4.4 volts, twice the distance = 400 feet, and the current = 20.5 amperes, then,
4.4 volts 400 feet × 20.5 amperes = .0005365 ohms per foot.
In the table of the properties of copper wire which gives the resistance of various sizes of wire, it will be noted that at all of the given temperatures No. 8 wire has a resistance greater than the value just calculated, therefore, No. 6 B. & S. gauge wire should be used for the outer wires of the feeder. In the table the resistance is given per 1,000 feet, hence it is only necessary to move the decimal point to obtain the resistance per foot.
If the calculation be based on the lamp voltage, 110 volts, the formula (1) must be modified to
drop × 4 2 × distance × amperes = resistance . . . . (2)
In this case, drop = 2.2 volts, 2 × distance = 400 feet, and current = 41 amperes, then,
2.2 volts × 4 400 feet × 41 amp. = 8.8 16,400 = .005364 ohms.
Size of the Neutral Wire.—In three wire circuits, the size of the neutral wire will depend to a great extent upon operating conditions. In the case of installations which occasionally have to be worked as two wire systems, the cross section of the neutral wire should be equal to the combined cross section of the two outer wires.