Wire Size for 125 Amp Circuit
Find the copper or aluminum wire for a 125 A subpanel, service or feeder, and check the voltage drop for a long run.
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Assumes NEC 2023 Table 310.16: copper or aluminum, up to 3 current-carrying conductors in a raceway or cable, 30 °C ambient, 75 °C column above 100 A with 14 to 1 AWG held to 60 °C (110.14(C)); the dwelling service size follows NEC 310.12 (83 %, 75 °C); 3 % drop target; ground wire not included. Runs in your browser. Verify with your local code and a licensed electrician before you install.
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How to use the 125 Amp Wire Size Calculator
- Keep 125 in “Breaker rating (A)”, or type the rating of your breaker or main.
- Enter the nameplate or calculated current in “Load current (A)”. The starting load is 100 A. Leave it empty to size for the full 125 A.
- Tick “Continuous load (3 hours or more)” for a charger or heater. The design current becomes 125 % of the load and must stay at or under 125 A.
- Pick the “Supply type” and enter “Voltage (V)”: 240 for a residential subpanel or service, or the line-to-line voltage for three phase, for example 208.
- Type the “One-way length” in ft or m, measured along the route from the main to the subpanel.
- Choose Copper or Aluminum under “Conductor” and the “Insulation temperature rating” printed on the cable, (the terminal setting still caps the answer at the 60 or 75 °C column).
- Leave “Allowed voltage drop (%)” at 3 unless your project sets another limit.
- Open the advanced settings only when they apply: “Terminal temperature rating” (set “75 °C (listed equipment)” only when the breaker and the lugs are listed for it), “Ambient temperature” and the number of current-carrying conductors.
- Read “Recommended wire size”, “Deciding limit”, the “Dwelling service or feeder” rows and “Longest run of the recommended size”, then the steps and warnings.
The example buttons load a 100 A run at 75 ft, a 125 A run at 30 ft, a 100 A continuous load at 40 ft, an aluminum 150 ft run and a copper run with 60 °C terminals at 40 ft.
What size wire do I need for a 125 amp breaker?
A 125 A breaker needs a conductor whose allowed ampacity reaches 125 A. For circuits above 100 A, Auto terminals use the 75 °C column, so the answer is 1/0 AWG copper (150 A) or 2/0 aluminum (135 A). 1 AWG copper has 130 A in the 75 °C column, but Auto terminals hold it to the 60 °C column (110 A), which is too small for 125 A. It fits only when the equipment is listed for 75 °C: select “75 °C (listed equipment)” to get 1 AWG copper. Aluminum stays at 2/0 either way.
| Settings (125 A breaker, 100 A load, 240 V, 75 ft) | Copper | Aluminum |
|---|---|---|
| Terminals Auto | 1/0 AWG | 2/0 AWG |
| Terminals listed for 75 °C | 1 AWG | 2/0 AWG |
| Dwelling service or feeder, NEC 310.12 | 2 AWG | 1/0 AWG |
What size wire for a 125 amp service?
For the service or feeder that carries the whole load of a one-family dwelling, NEC 310.12 lets the conductors be 83 % of the rating: 83 % of 125 A is 103.75 A. In the 75 °C column that is 2 AWG copper or 1/0 aluminum for the default 100 A load. If the calculated load is 125 A, the rows show 1 AWG copper (130 A) or 2/0 aluminum (135 A) instead. With the load left empty they show no 83 % size.
The dwelling size applies only to the service or feeder that carries the whole dwelling load, nominal 120/240 V single phase (not 208 V), conductors rated 75 °C or more, up to 3 current-carrying conductors and a 30 °C ambient. Outside those conditions the tool says so and gives no smaller size. It does not apply to a garage or shop subpanel, which uses the breaker-rated size above. Check that your utility and inspector accept it.
How far can you run 125 amp wire?
At a full 125 A load on 240 V, 1/0 AWG copper reaches about 236 ft one way before the drop passes 3 %. At a 100 A load it reaches about 295 ft. Beyond that the calculator moves up a size.
| Copper unless noted, 240 V | One-way length | Result |
|---|---|---|
| 100 A load | 75 ft | 1/0 AWG, 0.76 % drop |
| 100 A load | 100 ft | 1/0 AWG, 1.02 % drop |
| 125 A load (full breaker) | 75 ft | 1/0 AWG, 0.95 % drop |
| 125 A load | 200 ft | 1/0 AWG, 2.54 % drop |
| 125 A load | 300 ft | 3/0 AWG, 2.40 % drop |
| 125 A load | 400 ft | 4/0 AWG, 2.53 % drop |
| 100 A load, aluminum | 150 ft | 2/0 AWG, 1.99 % drop |
A 100 A continuous load has a design current of 125 %, or 125 A, so it needs 1/0 AWG copper on this breaker. The dwelling service rows then show 1 AWG copper or 2/0 aluminum.
The ground wire is separate. For a 125 A breaker the equipment grounding conductor is 6 AWG copper or 4 AWG aluminum (NEC Table 250.122, 100 to 200 A), found with the equipment grounding conductor calculator.
Worked examples
| Input | Result |
|---|---|
| 125 A breaker, 100 A load, 240 V, 75 ft, copper 75 °C | 1/0 AWG (0.762 %) |
| 125 A breaker, 125 A load, 240 V, 30 ft, copper 75 °C | 1/0 AWG (0.381 %) |
| 125 A breaker, 100 A load, 240 V, 40 ft, copper 75 °C | 1/0 AWG (0.406 %) |
| 125 A breaker, 100 A load, 240 V, 150 ft, aluminum 75 °C | 2/0 AWG (1.99 %) |
| 125 A breaker, 100 A load, 240 V, 40 ft, copper 60 °C | 1/0 AWG (0.406 %) |
Calculated by the tool above; open it with the same values to see each step.
Questions, briefly answered
What size wire for a 125 amp breaker or subpanel?
1/0 AWG copper or 2/0 aluminum with Auto terminals. A 125 A subpanel fed from a 125 A breaker uses the same size. With a 100 A load over 75 ft, 1/0 copper gives a 0.76 % drop.
What size wire for a 125 amp service?
Under NEC 310.12 a one-family dwelling service can use 83 % of 125 A (103.75 A): 2 AWG copper or 1/0 aluminum for a 100 A load, or 1 AWG copper or 2/0 aluminum for a 125 A calculated load. Use this only for the service or feeder that carries the whole dwelling load.
Can I use 1 AWG copper on a 125 amp breaker?
Only when the breaker and the equipment lugs are listed for 75 °C. 1 AWG copper is 130 A in the 75 °C column but only 110 A in the 60 °C column that Auto terminals use. Select “75 °C (listed equipment)” to see it.
What size aluminum wire for 125 amps?
2/0 aluminum with Auto terminals, which is rated 135 A in the 75 °C column. For a dwelling service under NEC 310.12, 1/0 aluminum is enough for a 100 A load. Aluminum needs lugs marked AL/CU.
How far can 1/0 copper run at 125 amps?
About 236 ft one way at a full 125 A load on 240 V with a 3 % target, or about 295 ft at a 100 A load. At 200 ft and 125 A the drop is 2.54 %; at 300 ft the answer moves to 3/0 AWG (2.40 %).
What about a 100 amp continuous load on a 125 amp breaker?
The design current is 125 %, or 125 A, so the answer is 1/0 AWG copper. The dwelling service rows show 1 AWG copper or 2/0 aluminum for that design load.
What size ground wire for a 125 amp breaker?
6 AWG copper or 4 AWG aluminum under NEC Table 250.122 (100 to 200 A). It is not included in the wire size above.