Voltage Drop Formulas

The size of conductor for any voltage drop can be determined readily by using mathematical formulas which calculate the voltage drop for given wires sizes, lengths, and types under load. These formulas may be used to determine any one of the four factors affecting voltage drop if the other three factors are known. Keep in mind there are separate formulas for single and three phase, and for copper and aluminum.

Formulas

For Copper Single Phase Circuits:

Where: For Copper Three Phase Circuits:
Where 13 = average three phase line current in amperes.

For sizes of aluminum conductors, these formulas may be used and the results multiplied by 1.6 or the formulas may be modified as follows:

Find the size of copper wire to carry a load of 40 amperes at 240 volts a distance of 500 feet with 2% voltage drop. Use the formula:

Example 1
Referring to Table showing "Data on Sizes & Weights of Conductors," it will be found that this size lies between No. 1 and No. 0, so No. 0 would be the wire size selected. For aluminum, multiply 104,167 x 1.6 = 166,667 circular mils. This lies between No. 00 and No. 000, so No. 000 would be selected.

Example 2
How far can No. 6 copper wire be used to carry a load of 30 amperes at 240 volts and keep within 1% voltage drop?