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)). 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 125 amps
| Ground | Copper | Aluminum |
|---|---|---|
| Equipment ground, 125 A feeder to a subpanel (Table 250.122) | 6 AWG | 4 AWG |
| Electrode conductor, 2 AWG Cu service (Table 250.66) | 8 AWG | 6 AWG |
| Electrode conductor, 1/0 AWG Al service (Table 250.66) | 8 AWG | 6 AWG |
The electrode conductor runs from the service to the ground rods, Ufer or water pipe; to rods alone it never has to exceed 6 AWG copper (NEC 250.66(A)), and aluminum may not terminate within 18 in of the earth (NEC 250.64(A)). The equipment ground applies when the same rating feeds a subpanel. 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 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
Parts for a 125 A service
Since the 2020 NEC (230.67), a new or replaced dwelling service must have surge protection: a Type 2 SPD at the panel is how most 125 A services meet it.
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Type 2 whole-house SPD
Siemens BoltShield FSPD140
Best for: The default whole-house pick on any brand of panel
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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