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)). Both assume 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.
Ground wire for 40 amps
| Ground | Copper | Aluminum |
|---|---|---|
| Equipment ground (bare / green) (Table 250.122) | 10 AWG | 8 AWG |
The equipment ground is sized by the breaker, not the load: a 40A breaker falls in Table 250.122's 60 A row. Upsize the hots for voltage drop and the ground must grow in proportion (NEC 250.122(B)). Every size, with a worked upsizing example, is in the ground wire size chart.
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.
Parts for a 40 A circuit
A plug-in EV charger on a 40 A circuit can draw up to 32 A for hours a night: the NEC treats it as a continuous load at 80% of the breaker. Use a heavy-duty 14-50 receptacle built for that duty, not a residential-grade range outlet. Both picks take copper conductors only, so an aluminum run needs a device listed for aluminum.
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240 V receptacle
Leviton 1450R EV-Rated NEMA 14-50 Receptacle
Best for: The receptacle behind a plug-in EV charger
Non-contact voltage tester
Klein Tools NCVT-3P
Best for: Confirming a circuit is dead before you open a device or box
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