What size wire for 125 amps?
1 AWG copper — at 75 °C terminals, NEC Table 310.16. Aluminum: 2/0 AWG.
A 125 A circuit needs 1 AWG copper, rated 130 A at 75 °C. At this size the conductors are single conductors in conduit or SER cable; NM-B is not manufactured heavy enough to apply.
Safety first. This is a starting point before derating, not a load calculation. Wire sizing affects fire and shock safety — verify against the current NEC and have circuits designed or reviewed by a licensed electrician for your jurisdiction.
125 amp wire size by material and terminal rating
| Material | 60 °C (NM-B) | 75 °C (typical) | 90 °C |
|---|---|---|---|
| copper | 1/0 AWG (125 A) | 1 AWG (130 A) | 2 AWG (130 A) |
| aluminum | 3/0 AWG (130 A) | 2/0 AWG (135 A) | 1/0 AWG (135 A) |
Sizes are the smallest conductor whose ampacity and NEC 240.4(D) breaker cap both reach 125 A, before any derating. Use the 75 °C column unless every terminal, breaker and device in the circuit is rated otherwise (NEC 110.14(C)).
If the load runs continuously
A load operating for three hours or more is a continuous load, and NEC 210.19(A)(1) requires the conductor to be sized at 125% of it — 125 A becomes 156.3 A for sizing purposes. That moves this circuit to 2/0 AWG copper , one or more sizes above the 1 AWG the plain table suggests. EV chargers, electric heat and commercial lighting are the loads this catches most often.
How far can a 125 amp circuit run?
The table above only prevents overheating. On a long run the limit is voltage arriving at the far end, and it bites well before ampacity does. These are the longest one-way runs for 1 AWG copper at 125 A within the 3% drop recommended by the NEC 210.19(A) informational note — and what going up to 1/0 AWG buys you.
| System | Typical use | 1 AWG | 1/0 AWG |
|---|---|---|---|
| 120 V | standard receptacle and lighting circuits | 116 ft | 147 ft |
| 240 V | ranges, dryers, EV chargers, feeders | 232 ft | 293 ft |
| 12 V DC | automotive, marine, solar and battery runs | 12 ft | 15 ft |
Single-phase, one-way distance, copper at 20 °C. Aluminum reaches its limit at roughly 60% of these distances. The 3% target is a recommendation, not a code requirement.
What runs on a 125 amp circuit
- large subpanels
- commercial equipment
- secondary distribution panels
The catch at 125 amps
125 A copper is 1 AWG at 75 °C (130 A). Because 1 AWG is poorly stocked, 1/0 is frequently pulled instead — a substitution that is always safe, since going larger never violates ampacity.
Working the other way round? See how many amps 1 AWG carries , including its diameter, resistance and full run-length table.