What Size Wire for a 40 Amp Breaker?
A 40 A breaker needs 8 AWG copper or 8 AWG aluminum with 75°C terminations (NEC Table 310.16). Longer runs may need a size up for voltage drop — distances below.
Minimum wire size for 40 amps
| Application | Copper (75°C) | Aluminum (75°C) |
|---|---|---|
| Branch circuit / feeder (Table 310.16) | 8 AWG | 8 AWG |
Assumes 75°C-rated terminations (NEC 110.14(C)) and no more than three current-carrying conductors in a raceway at 30°C ambient. Derating, continuous loads, and local amendments can change the answer — confirm with a licensed electrician.
How far can you run it? (voltage drop)
Keeping voltage drop under the recommended 3% at a 32 A load (80% of the breaker), 8 AWG copper is good for roughly:
| System voltage | Max one-way run (8 AWG Cu) |
|---|---|
| 120 V | ≈ 71 ft |
| 240 V | ≈ 143 ft |
Past these distances, step up one gauge. Run your exact length and load through the voltage drop calculator.
What runs on 40 amps
- Electric range
- 32 A EV charger
- Larger AC condensers
Common receptacle at this rating: NEMA 14-50.
Frequently asked questions
8 AWG copper with 75°C terminations (NEC Table 310.16). In aluminum, use 8 AWG. Longer runs, high ambient temperatures, or bundling can require upsizing.
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