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.


For Copper Single Phase Circuits:


CM = Area of conductor in circular mills
1 = Single Phase line current in Amperes
L = Length (one-way) of circuit in feet
V = Voltage Drop (Volts)

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
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?