Minimum size by temperature column
| Conductor | 60°C | 75°C | 90°C |
|---|---|---|---|
| Copper | 1/0 AWG125 A | 1 AWG130 A | 2 AWG130 A |
| Aluminum | 3/0 AWG130 A | 2/0 AWG135 A | 1/0 AWG135 A |
Equipment grounding conductor
A circuit protected at 125 A takes a 6 AWG copper or 4 AWG aluminum equipment grounding conductor, from the 200 A row of NEC Table 250.122. Raise the ground with the ungrounded conductors when you increase them for voltage drop or any reason other than 310.15(B) or (C) derating, because 250.122(B) requires the same proportion. The ground is never required to be larger than the circuit conductors.
Longest run at 3% voltage drop
| Conductor | 240 V single phase | 240 V three phase |
|---|---|---|
| 1 AWG copper | 185 ft | 215 ft |
| 2/0 AWG aluminum | 180 ft | 205 ft |
NEC 210.19(A)(1) Informational Note 4 recommends keeping branch-circuit drop at or below 3%. These lengths come from the Table 8 resistance at 75°C for a stranded conductor at unity power factor, rounded to the nearest 5 ft. Reactance is not included, so a large conductor in a steel raceway falls short of the figure. A balanced three-phase run reaches about 15% farther than the single-phase figure.
Dwelling service allowance under 310.15(B)(7)
A 120/240 V single-phase dwelling service or feeder rated 100 A through 400 A may use conductors with an ampacity of 83% of the rating. For 125 A that is 103.75 A, which the 75°C column meets with 2 AWG copper or 1/0 AWG aluminum.
The allowance covers service and feeder conductors only. Branch-circuit conductors, the equipment grounding conductor, and any local amendment are unaffected, and the neutral may be reduced separately under 220.61.
What a 125 A circuit feeds
- Small residential and light commercial services
- Feeders from a larger service
One hundred twenty-five amps is a small house or accessory dwelling unit service, and it is often the rating a 60 A or 100 A service is upgraded to. 1/0 copper or 3/0 aluminum reaches it at 60°C, and 1 AWG copper or 2/0 aluminum reaches it at 75°C.
These services are normally meter mains with a 125 A bus. The grounding electrode conductor comes from Table 250.66 keyed to the largest ungrounded service conductor you install rather than to the 125 A figure, and continuous loads at this rating top out at 100 A.
Because the meter and the main breaker share one enclosure, the service conductors, the main bonding jumper, and the grounding electrode system are sized together. 250.24(A) requires the grounding electrode connection on the supply side of the service disconnect or inside its enclosure, which is why the grounding electrode conductor lands in the meter main rather than in a distribution panel.
125 A wire size questions
- What size wire do I need for a 125 amp circuit?
- 1 AWG copper or 2/0 AWG aluminum at the 75°C column, which 110.14(C)(1)(b) requires for equipment rated over 100 A. On the 60°C column the same circuit would need 1/0 AWG copper or 3/0 AWG aluminum.
- What size ground wire for a 125 amp circuit?
- 6 AWG copper or 4 AWG aluminum, taken from the 200 A row of NEC Table 250.122.
- What size aluminum wire for 125 amps?
- 2/0 AWG aluminum at the 75°C column carries 135 A. Terminations on aluminum conductors must be identified for the material.
- Can I use 2 AWG copper for 125 A?
- Table 310.16 lists 2 AWG copper at 95 A at 60°C, 115 A at 75°C, and 130 A at 90°C. It falls short at 75°C, the column this circuit uses. The 90°C column does reach 125 A, and a higher column applies only where every termination is listed for it.