Voltage Drop Calculator

Calculate voltage drop in electrical wiring based on wire gauge, length, and load current.

Result

Fill in the fields above, then press Calculate.

A voltage drop calculator determines the voltage loss across electrical wiring based on wire gauge, length, and current draw to ensure safe and efficient installations.

Enter the source voltage, load current in amps, one-way wire length, wire gauge, and material. The calculator determines the voltage drop across the wire run and whether it falls within acceptable limits.

Examples

20A load on 100 ft of 12 AWG copper

Voltage drop = 20 x 1.98 x 200 / 1000 = 7.92 V, or 6.6% on a 120 V circuit. This exceeds the 3% recommendation; consider upsizing to 10 AWG.

Frequently Asked Questions

What is an acceptable voltage drop?
The NEC recommends a maximum 3% voltage drop for branch circuits and 5% for the entire system (feeder + branch). Excessive drop leads to dimming lights, overheating motors, and wasted energy.
Why does wire length double in the formula?
Current flows out to the load and back to the source, so the total conductor length is twice the one-way distance.
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Quick Tips

  • •NEC recommends no more than 3% voltage drop for branch circuits and 5% total for feeder + branch.
  • •Use a larger wire gauge (lower AWG number) to reduce voltage drop on long runs.
  • •Always verify calculations against local electrical codes before installation.