What Size Wire for 125 Amps?
Short answer: for a 125 amp dwelling service or main feeder, use 2 AWG copper or 1/0 AWG aluminum (NEC Table 310.12). For a 125 amp subpanel feeder or a non-dwelling service, use 1 AWG copper or 2/0 AWG aluminum from the 75°C column of Table 310.16. The equipment grounding conductor for a 125 A feeder is 6 AWG copper or 4 AWG aluminum.
125 A conductor sizes
| Application | Copper | Aluminum |
|---|---|---|
| Dwelling service / main feeder (entire dwelling load, 310.12) | 2 AWG | 1/0 AWG |
| Subpanel feeder, detached building, commercial (310.16, 75°C) | 1 AWG | 2/0 AWG |
Why the service and feeder sizes differ
Table 310.12 lets a whole-dwelling service use conductors rated at 83% of the service: 125 × 0.83 = 103.75 A. At 75°C, 2 AWG copper (115 A) and 1/0 aluminum (120 A) clear that; 3 AWG copper (100 A) and 1 AWG aluminum (100 A) fall short. A subpanel feeder gets no such allowance, so it needs a conductor rated for the full 125 A: 1 AWG copper (130 A) or 2/0 aluminum (135 A). The values are in the ampacity chart.
1/0 aluminum on a 125 A subpanel breaker
1/0 aluminum is rated 120 A at 75°C, and 120 A isn't a standard breaker size. NEC 240.4(B) permits the next standard size up, 125 A, if the conductor isn't feeding a multi-outlet branch circuit for cord-and-plug loads and the calculated load is 120 A or less. Keep the load calculation with the permit documents; inspectors often ask for it.
Grounding
- GEC (Table 250.66): 2 AWG copper or 1/0 aluminum service conductors call for an 8 AWG copper GEC (6 AWG if run exposed without protection).
- EGC for a 125 A feeder (Table 250.122): 6 AWG copper or 4 AWG aluminum (the 200 A row applies to anything over 100 A up to 200 A).
- At a subpanel: neutral isolated from the enclosure, equipment grounds on a bonded ground bar. Only the service has the neutral-to-ground bond.
125 A feeder by distance (240 V)
| Conductor | 50 ft | 100 ft | 150 ft | 200 ft |
|---|---|---|---|---|
| Copper | 1 AWG (0.6%) | 1 AWG (1.3%) | 1 AWG (1.9%) | 1 AWG (2.6%) |
| Aluminum | 2/0 AWG (0.7%) | 2/0 AWG (1.3%) | 2/0 AWG (2.0%) | 2/0 AWG (2.7%) |
Smallest conductor that meets both the 75°C ampacity for a 125 A circuit and a 3% voltage drop at 100 A (80% of the rating), 240 V single-phase, one-way distance. Percent drop in brackets. If the circuit uses NM-B cable or 60°C terminations, start from the 60°C column instead.
At a 100 A load, 1 AWG copper reaches 3% drop at about 234 feet and 2/0 aluminum at about 226 feet, so voltage drop controls only on long runs to outbuildings.
Conduit size for 125 A conductors
- Service, three 2 AWG copper THHN: 3 × 0.1158 = 0.3474 in². That just misses 1" EMT (40% area 0.3456 in²), so use 1-¼".
- Subpanel feeder, three 1 AWG copper THHN plus a 6 AWG ground: 3 × 0.1562 + 0.0507 = 0.5193 in², which fits 1-¼" EMT (0.5984 in²) or 1-¼" PVC Schedule 40 (0.5812 in²).
- Aluminum XHHW-2: use the XHHW-2 areas in Chapter 9 Table 5, then check the conduit fill chart method.
Worked example: 125 A subpanel for an ADU
A 125 A feeder runs 120 feet to a backyard ADU. The load calculation for the ADU comes to 100 A. Terminations are 75°C.
- Conductors: 1 AWG copper or 2/0 aluminum, plus a 6 AWG copper (or 4 AWG aluminum) equipment ground, in four wires.
- Voltage drop, copper: 2 × 12.9 × 100 × 120 ÷ 83,690 = 3.70 V, or 1.5%.
- Voltage drop, aluminum: 2 × 21.2 × 100 × 120 ÷ 133,100 = 3.82 V, or 1.6%.
- At the ADU: a grounding electrode system and a building disconnect (a main-breaker panel does both jobs with ground rods).
Neither option needs upsizing. The main-service load calculation must include the ADU load before the feeder is added.
Where 125 A shows up
- Manufactured and mobile homes, which commonly have 100–150 A service.
- Main-lug subpanels rated 125 A fed from a 200 A service, for a kitchen remodel, addition or ADU.
- Condo and apartment units, where individual unit feeders are often 100–125 A.
Common mistakes inspectors flag
- Service sizes on a subpanel feeder, such as 2 AWG copper on a 125 A breaker feeding a garage.
- 1/0 aluminum on 125 A without a load calculation to support 240.4(B).
- A reduced EGC carried over from a 100 A job (8 AWG copper instead of 6 AWG).
Frequently asked questions
Can I use 2 AWG aluminum for 125 amps?
No. 2 AWG aluminum is rated 90 A at 75°C. For a 125 A dwelling service you need 1/0 aluminum; for a subpanel feeder, 2/0 aluminum.
Does a 125 A subpanel need a 125 A breaker?
No. The breaker protects the feeder conductors. A 125 A-rated panel can be fed by a 100 A breaker with 100 A feeder conductors, for example; the 100 amp subpanel page covers those sizes (not the smaller 100 A service sizes, which apply only to a whole-dwelling service).
What size ground wire for a 125 amp subpanel?
6 AWG copper or 4 AWG aluminum for the equipment grounding conductor (Table 250.122). If the subpanel is in a detached building, the conductor to its ground rods is sized from Table 250.66 and never needs to exceed 6 AWG copper.
What size conduit for a 125 amp service?
With three 2 AWG copper THHN conductors, 1-¼" EMT or PVC. 1" EMT is 0.0018 in² short of the 40% limit, and 1" PVC is further off.
Is 125 A service enough for a house?
For a small or mostly gas home, often yes. With electric heat, a heat pump and EV charging, a load calculation usually points to 150 A or 200 A.
Last updated: October 6, 2026. Based on NEC 2023. Check results against the code edition and local amendments your jurisdiction enforces and with the AHJ; electrical work should be done by qualified people.