What Size Wire for a 125 Amp Service?
For a 125 A single-phase dwelling service, the NEC allows 2 AWG copper or 1/0 AWG aluminum (310.12 83% rule). A general 125 A feeder sized from Table 310.16 needs 1 AWG copper / 2/0 AWG aluminum.
Minimum wire size for 125 amps
| Application | Copper (75°C) | Aluminum (75°C) |
|---|---|---|
| Branch circuit / feeder (Table 310.16) | 1 AWG | 2/0 AWG |
| Dwelling service / main feeder (310.12) | 2 AWG | 1/0 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 100 A load (80% of the breaker), 1 AWG copper is good for roughly:
| System voltage | Max one-way run (1 AWG Cu) |
|---|---|
| 240 V | ≈ 233 ft |
Past these distances, step up one gauge. Run your exact length and load through the voltage drop calculator.
What runs on 125 amps
- Main service
- Large subpanel feeder
Frequently asked questions
For a 125 A single-phase dwelling service or main feeder, the NEC 310.12 rule allows 2 AWG copper or 1/0 AWG aluminum. A general (non-dwelling-service) 125 A feeder sized from Table 310.16 needs 1 AWG copper or 2/0 AWG aluminum at 75°C.
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