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Voltage Drop Calculator

Calculate voltage drop over a wire run from wire size (AWG or mm²), material, current, voltage and length. Single or three phase. Free calculator.

Voltage drop
6.32 V (5.27%)
Voltage at the load
113.7 V
Wire resistance
1.9755 Ω per 1,000 ft
Power lost in the wire
101 W

Over 5%: use a thicker wire or shorten the run.

Why voltage drops along a wire

Every wire has resistance, so some voltage is lost along the run. The drop is 2 × K × I × L ÷ CM for single phase (√3 instead of 2 for three phase), where K is the conductor's resistivity (12.9 for copper, 21.2 for aluminum at 75 °C), I the current, L the one-way length in feet and CM the wire's area in circular mils. A 12 AWG copper run of 100 ft carrying 16 A at 120 V drops 6.3 V, or 5.3%. The NEC recommends no more than 3% on a branch circuit and 5% in total.

How to use it

  1. 1

    Choose the wire

    Copper or aluminum, and its size in AWG or mm².

  2. 2

    Enter the circuit

    Voltage, phase and the load current.

  3. 3

    Add the length

    One-way, in feet or meters.

Frequently asked questions

What is an acceptable voltage drop?

3% on a branch circuit and 5% for feeder plus branch together, as the NEC recommends.

How do I reduce voltage drop?

Use a thicker wire, shorten the run, or use a higher voltage.

Is length one-way or round trip?

One-way. The formula already counts both conductors.

Is this a substitute for an electrician?

No. Check local code and have electrical work done by a qualified electrician.

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