What is an acceptable voltage drop?

The NEC recommends no more than 3% on a branch circuit and 5% total from service to load.

Why does voltage drop hurt motors?

Current rises as voltage falls to hold the same power, so windings run hotter, starting torque drops with the square of voltage, and contactors chatter.

Does aluminum wire need to be upsized?

Yes. Aluminum has about 61% of copper's conductivity, so it is usually run one to two sizes larger for the same drop.

Voltage Drop Calculator — Motor & Air Compressor Wiring

All calculators

Formula

Vd = (√3 × K × I × L) / CM

Unit system

Current

One-way length

Source voltage

Material

Wire gauge

Voltage drop

9.04V

Drop percentage

1.88%

Within NEC 3% recommendation.

How to use this calculator

Check conductor size against length, current and phase for acceptable voltage drop.

Enter conductor size, material and one-way circuit length.
Enter load current and system voltage, and select single- or three-phase.
Read voltage drop in volts and percent.
Increase conductor size if the drop exceeds 3% on the branch circuit.

Frequently asked questions

What is an acceptable voltage drop?: The NEC recommends no more than 3% on a branch circuit and 5% total from service to load.
Why does voltage drop hurt motors?: Current rises as voltage falls to hold the same power, so windings run hotter, starting torque drops with the square of voltage, and contactors chatter.
Does aluminum wire need to be upsized?: Yes. Aluminum has about 61% of copper's conductivity, so it is usually run one to two sizes larger for the same drop.