What Size Wire for 40 Amps at 150 Feet?

Answer
6 AWG copper

At 150 ft one way on a 240 V circuit, 6 AWG copper keeps voltage drop at 2.46%, under the recommended 3%. Ampacity alone would allow 8 AWG, but voltage drop decides the size at this distance. In aluminum, use 4 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, 150 ft, 240 V)

    Copper wireDrop (V)Drop (%)Within 3%
    8 AWG9.17 V3.82%✗
    6 AWG5.89 V2.46%✓
    4 AWG3.70 V1.54%✓
    3 AWG2.94 V1.23%✓
    2 AWG2.33 V0.97%✓

    Voltage drop = 2 × 150 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 150 feet?

    It carries 40 A, but at 150 ft the voltage drop is 3.8%. That is over 3%, so the next size up is recommended.

    Does the distance include the return wire?

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

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