| amperes × feet × 21.6 | ||
| circular mils = | —————————— | (1) |
| drop |
The quantity 21.6, is twice the resistance (10.8) of a foot of copper wire one mil in diameter (mil foot). This resistance (10.8) is multiplied by 2, giving the quantity 21.6, because the length of a circuit, or feet in the formula, is given as the "run" or distance one way, that is, one-half the total length of wire in the circuit, must be multiplied by 2 to get the total drop, viz.:
| amperes × feet × 10.8 × 2 | amperes × feet × 21.6 | |
| circular mils = | ———————————— = | —————————— |
| drop | drop |
It is sometimes however convenient to make the calculation in terms of watts. Formula (1) may be modified for such calculation.
In modifying the formula, the "drop" should be expressed in percentage instead of actual volts lost, that is, instead of the difference in pressure between the beginning and the end of the circuit.
In any circuit the loss in percentage, or
| drop | ||
| % loss = | ———————— | × 100 |
| impressed pressure |
from which
| % loss × impressed pressure | ||
| drop = | ———————————— | (2) |
| 100 |
Substituting equation (2) in equation (1)