Wire size by breaker rating
Read across from the breaker. The first column is NM cable (Romex), which NEC 334.80 limits to the 60°C ampacity. The second is copper building wire such as THHN in conduit landing on terminals listed for 75°C, which is most modern breakers and panels. The third is aluminum at 75°C.
| Breaker | Copper, 60°C (NM cable) | Copper, 75°C | Aluminum, 75°C |
|---|---|---|---|
| 15 A | 14 AWG | 14 AWG | 12 AWG |
| 20 A | 12 AWG | 12 AWG | 10 AWG |
| 25 A | 10 AWG | 10 AWG | 10 AWG |
| 30 A | 10 AWG | 10 AWG | 8 AWG |
| 40 A | 8 AWG | 8 AWG | 8 AWG |
| 50 A | 6 AWG | 8 AWG | 6 AWG |
| 60 A | 4 AWG | 6 AWG | 4 AWG |
| 70 A | 4 AWG | 4 AWG | 3 AWG |
| 80 A | 3 AWG | 4 AWG | 2 AWG |
| 90 A | 2 AWG | 3 AWG | 2 AWG |
| 100 A | 1 AWG | 3 AWG | 1 AWG |
The small sizes are set by the breaker limits in NEC 240.4(D), not by the table: 14 AWG copper on 15 A, 12 AWG copper on 20 A, 10 AWG copper on 30 A, and 12 AWG aluminum on 15 A, 10 AWG aluminum on 25 A. That is why 30 A aluminum jumps to 8 AWG.
The table sizes the wire to the full breaker rating. NEC 240.4(B) also lets a conductor whose ampacity falls between two standard breaker sizes go on the next size up, up to 800 A, as long as the circuit is not a multioutlet branch circuit for plug-in loads and the load itself does not exceed the conductor's ampacity. That is how 6 AWG NM cable (55 A) can be protected by a 60 A breaker when the load it serves is no more than 55 A. Continuous loads still need wire rated for 125% of the load. The wire size calculator applies that rule when you enter the load.
Common circuits and the wire they take
Appliance circuits are sized from the nameplate. Dryers and ranges list the circuit they need; the figures below are typical, and the label on the appliance governs. Water heaters and EV chargers are continuous loads, so the circuit is sized at 125% of their current.
| Circuit | Load | Breaker | NM cable | Copper, 75°C |
|---|---|---|---|---|
| Electric dryer | Typically a 30 A circuit (nameplate) | 30 A | 10 AWG | 10 AWG |
| Electric range, smaller | Typically a 40 A circuit (nameplate) | 40 A | 8 AWG | 8 AWG |
| Electric range, larger | Typically a 50 A circuit (nameplate) | 50 A | 6 AWG | 8 AWG |
| 4,500 W water heater | 18.75 A × 125% = 23.44 A | 25 A | 10 AWG | 10 AWG |
| 32 A EV charger | 32 A × 125% = 40 A | 40 A | 8 AWG | 8 AWG |
| 40 A EV charger | 40 A × 125% = 50 A | 50 A | 6 AWG | 8 AWG |
| 48 A EV charger | 48 A × 125% = 60 A | 60 A | 4 AWG | 6 AWG |
Ranges and dryers usually need a 4-wire circuit (two hots, neutral and ground) in new work; check the appliance instructions for the cable type. EV chargers often let you set the current to match the circuit you have; set it to no more than the breaker rating divided by 1.25.
Why NM cable and THHN give different answers
Table 310.16 has three columns, 60°C, 75°C and 90°C, and a conductor can only be used at the rating of the weakest part of the circuit. NEC 110.14(C) sends circuits of 100 A or less to the 60°C column unless the breaker and the equipment terminals are listed for 75°C, which most current equipment is. NM cable is held to the 60°C column by 334.80 no matter what the terminals are rated. So 8 AWG copper is a 50 A conductor as THHN on 75°C terminals but a 40 A conductor as NM cable.
When to go up a size
Long runs. Voltage drop is not a code limit for most circuits, but the NEC's informational notes suggest keeping a branch circuit under 3%. A 50 A load at 240 V shows how quickly distance adds up:
| Run | Minimum size | Its drop | Size for 3% | Its drop |
|---|---|---|---|---|
| 100 ft | 8 AWG | 3.26% | 6 AWG | 2.05% |
| 150 ft | 8 AWG | 4.88% | 4 AWG | 1.93% |
| 200 ft | 8 AWG | 6.51% | 4 AWG | 2.58% |
Heat and bundling. Table 310.16 assumes a 30°C (86°F) ambient and no more than three current-carrying conductors together. A hot attic, a conduit on a sunny roof or several cables bundled through the same bored holes all lower the ampacity, and the corrections in NEC 310.15 can push the wire up a size or more.
Main services. For the service conductors or main feeder of a house, NEC 310.12 lets you size to 83% of the service rating, which is why a 100 A service often runs on 2 AWG aluminum. That allowance does not apply to subpanels or branch circuits.
Electrical work usually needs a permit, and in many places a licensed electrician. Use these sizes to plan the job or check a quote, then follow the code edition your town has adopted.
Sources
- NFPA 70, National Electrical Code, 2023: Table 310.16 (ampacity), 240.4(B) and (D) (overcurrent protection of conductors), 110.14(C) (termination temperature), 334.80 (NM cable), 210.19(A)(1) and 210.20(A) (continuous loads), 422.13 (water heaters), 625.41 (EV equipment), 310.12 (dwelling services)
- Ampacity values as tabulated in our wire ampacity chart