What Size Wire for 40 Amps at 100 Feet?

Answer
8 AWG copper

At 100 ft one way on a 240 V circuit, 8 AWG copper keeps voltage drop at 2.55%, under the recommended 3%. Ampacity alone would allow 8 AWG, but voltage drop decides the size at this distance. In aluminum, use 6 AWG.

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A
%
3% branch, 5% total (NEC note)
Phase
Conductor
Recommended wire
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How we calculated this

    For estimating and educational purposes only. Results are approximate and are not a substitute for a licensed professional, the manufacturer's instructions or your local code. Disclaimer

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    Voltage drop by wire size (40 A, 100 ft, 240 V)

    Copper wireDrop (V)Drop (%)Within 3%
    8 AWG6.11 V2.55%✓
    6 AWG3.93 V1.64%✓
    4 AWG2.46 V1.03%✓
    3 AWG1.96 V0.82%✓
    2 AWG1.55 V0.65%✓

    Voltage drop = 2 × 100 ft × 40 A × resistance ÷ 1000. Keeping it under 3% on branch circuits (5% total) is the NEC recommendation in the informational notes to 210.19 and 215.2.

    Frequently asked questions

    Is 8 AWG enough for 40 amps at 100 feet?

    It carries 40 A, but at 100 ft the voltage drop is 2.5%, which is acceptable.

    Does the distance include the return wire?

    No. Enter the one-way distance; the formula already counts both conductors.

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