What Size Wire for 40 Amps?

Answer
8 AWG copper (THHN, 75°C)

For a 40 A circuit, the smallest copper wire is 8 AWG in conduit (75°C rating) or 8 AWG if you use NM-B (Romex) cable, which is limited to the 60°C column. Aluminum needs 8 AWG. That holds up to about 100 ft at 240 V before voltage drop passes 3%.

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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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    Wire size for 40 amps by distance (240 V, 3% drop)

    One-way distanceCopperDropAluminum
    25 ft8 AWG0.64%8 AWG
    50 ft8 AWG1.27%8 AWG
    75 ft8 AWG1.91%6 AWG
    100 ft8 AWG2.55%6 AWG
    150 ft6 AWG2.46%4 AWG
    200 ft4 AWG2.05%3 AWG
    250 ft4 AWG2.57%2 AWG
    300 ft3 AWG2.45%1 AWG

    Single-phase 240 V, 75°C terminals, non-continuous load. For continuous loads (3+ hours) size for 50 A.

    Common 40 A circuits

    40 amp circuits typically feed electric ranges and 32 A EV chargers. The breaker protects the wire, so never pair a 40 A breaker with wire rated below 40 A.

    Frequently asked questions

    Can I use 8 AWG wire on a 40 amp breaker?

    Yes. 8 AWG copper is rated at least 40 A even in the 60°C column, so it works for NM-B cable and THHN alike, as long as voltage drop is acceptable.

    How far can I run 8 AWG wire at 40 amps?

    At 240 V with a 3% voltage-drop limit, about 100 ft one way. Beyond that, go up one size.

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