This is obtained directly from the formula (1) by solving the equation for circular mils, thus:
circular mils = length of wire in ft. × 10.79 resistance in ohms . . . . (2)
EXAMPLE. What is the circular mil area of a wire 1,500 feet long and having a resistance of 1.559 ohms?
Substituting the values in equation (2)
circular mils = 1,500 × 10.79 1.559 = 10,381
Figs. 818 and 819.—Diagrams illustrating the meaning of the term lamp foot, and how to apply it in calculating a circuit. As defined, one 16 candle power lamp at a distance of one foot from the fuse block or point of supply is called a lamp foot; this is equivalent to one 8 candle power lamp at a distance of 2 feet, or one 32 candle power lamp one-half foot from the fuse block. In [fig. 819], there are four 8 candle power lamps, and the distance to center of distribution is 10 feet. The circuit then contains 4 ÷ 2 × 10 = 20 lamp feet.
Lamp Foot.—This unit facilitates laying out wiring and calculating the drop. A lamp foot is defined as one 16 candle power lamp at a distance of one foot from the point of supply. Accordingly the number of lamp feet in any circuit is equal to the number of 16 candle power lamps (or equivalent in other sizes) in the circuit multiplied by the distance in feet from the fuse block to the center of distribution.
When no point is specified, the feet are always measured from the supply point to the center of distribution. When other than 16 c.p. lamps are in the circuit they must be reduced to 16 c.p. lamps. Thus two 8 c.p. lamps would be counted one 16 c.p. lamp, one 32 c.p. lamp would be counted two 16 c.p. lamps, etc.
Ampere Foot.—From the foregoing explanation of lamp foot, the significance of ampere foot is easily understood—the two terms are in fact self-defining.