What Size Wire for a 45 Amp Breaker?
A 45 A breaker needs 6 AWG copper in NM-B cable (“Romex”), or 8 AWG copper in conduit with 75°C terminations. NEC 334.80 caps NM cable at the 60°C column, which is why the two differ — most homes are cable, so 6 AWG is the safe default.
Minimum wire size for 45 amps
| Application | Copper | Aluminum |
|---|---|---|
| NM-B cable / “Romex” (334.80, 60°C) | 6 AWG | not typical |
| Conduit / SER, 75°C terminations (310.16) | 8 AWG | 6 AWG |
The NM-B row applies the 60°C column per NEC 334.80; the conduit row 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.
How far can you run it? (voltage drop)
Keeping voltage drop under the recommended 3% at a 36 A load (80% of the breaker), 6 AWG copper is good for roughly:
| System voltage | Max one-way run (6 AWG Cu) |
|---|---|
| 120 V | ≈ 101 ft |
| 240 V | ≈ 203 ft |
Past these distances, step up one gauge. Run your exact length and load through the voltage drop calculator.
What runs on 45 amps
- Heat pump with strip heat
- Large AC condensers
Frequently asked questions
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