What Gauge Wire for a 40-Amp Circuit?
Use 8 AWG copper for 40-amp circuits. 8 AWG THHN: 55A at 90°C, 50A at 75°C, 40A at 60°C — adequate margin for the 40A breaker.
8 AWG copper is the standard wire gauge for 40-amp circuits. 8 AWG THHN delivers 55A at 90°C, dropping to 50A at 75°C terminals and exactly 40A at 60°C terminals — meets the 40A breaker requirement at all common termination ratings.
NEC reference
| Rule | Effect |
|---|---|
| NEC Table 310.16 | 8 AWG copper @ 90°C = 55A; @ 75°C = 50A; @ 60°C = 40A |
| NEC 110.14(C)(1)(a) | 100A or smaller circuits use 60°C column at most terminations |
| NEC 240.4 | No small-conductor rule above 10 AWG — protection follows ampacity |
Common 40-amp circuits
- Electric range (50A more common, but smaller ranges use 40A)
- Tankless electric water heater (smaller units)
- Level 2 EV charger at 32A continuous (40A breaker per NEC 625.42)
- Heat pump outdoor unit (varies by model)
- Pool pump motor larger sizes
Conductor count + conduit
| Circuit | Conductors | Min EMT |
|---|---|---|
| 40A 240V (range without neutral): 2 hots + EGC | 3 | 1/2" (16.5% fill) |
| 40A 240V with neutral: 2 hots + 1 N + 1 EGC | 4 | 1/2" (22% fill) |
| Two 40A circuits sharing EGC | 5 | 3/4" (34.3% fill — at limit per 8 AWG / 3/4 calc) |
Ampacity derating
With 4–6 CCCs: 55A × 0.80 = 44A — above 40A breaker, OK.
With 7–9 CCCs: 55A × 0.70 = 38.5A — below 40A breaker. Either reduce CCCs or upsize to 6 AWG (75A × 0.70 = 52.5A).
Voltage drop at 40A
8 AWG copper: 0.628 Ω/1000 ft. At 40A on 100 ft one-way (240V):
V drop = 200 × 40 × (0.628 / 1000) = 5.0V (2.1% of 240V)
Within 3% — 8 AWG is fine to ~140 ft for 40A circuits.
Quick reference
- Wire: 8 AWG copper THHN/THWN-2 (or 6 AWG aluminum)
- Breaker: 40A 2-pole
- Receptacle: NEMA 14-50 / 6-50 (often used for 50A — for 40A, no dedicated receptacle, hardwired)
- Conduit: 1/2" EMT fits one circuit comfortably
FAQ
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This page's figures are transcribed from NEC 2023 (NFPA 70) Chapter 9, Tables 1, 4 and 5, and checked by a version-controlled automated test suite — they are not signed off by a licensed professional. WireFillChart is an independent reference, not affiliated with or endorsed by NFPA. Always verify against the code edition adopted by your local authority having jurisdiction (AHJ).