| amperes × feet × 21.6 | |
| circular mils = | ———————————— |
| % loss × imp. pressure | |
| ———————— | |
| 100 | |
amperes × feet × 2,160 | |
| = | ———————————— (3) |
| % loss × imp. pressure |
Equation (3) is modified for calculation in terms of watts as follows: The power in watts is equal to the applied voltage multiplied by the current, that is to say, the power is equal to the volts at the consumer's end of the circuit multiplied by the current, or simply
watts = volts × amperes
from which
| watts | ||
| amperes = | ——— | (4) |
| volts | ||
Figs. 2,699 to 2,703.—Stranded copper cables. For conductors of large areas and in the smaller sizes where extra flexibility is required it becomes necessary to employ stranded cables made by grouping a number of wires together in either concentric or rope form. The concentric cable as here illustrated is formed by grouping six wires around a central wire thereby forming a seven wire cable. The next step is the application in a reverse direction of another layer of 12 wires and a nineteen wire cable is produced. This is again increased by a third layer of eighteen wires for a 37 wire cable and a fourth layer of 24 wires for a 61 wire cable. Successive layers, each containing 6 more wires than that preceding, may be applied until the desired capacity is obtained. The cuts show sectional views of concentric cables each formed from No. 10 B. & S. gauge wires.
Substituting this value for the current in equation (3) and remembering that the pressure taken is the volts at the consumer's end of the line