What size wire for a 30 amp breaker?
Short answer: A 30 A breaker needs at least 10 AWG copper (10/2 or 10/3 NM-B) or 8 AWG aluminum. Past about 100 feet at 240 V, or 50 feet at 120 V, go up a size or more to keep voltage drop under 3%.
Minimum wire size for 30 A
| Wire type | Minimum size | Ampacity |
|---|---|---|
| Copper NM-B (Romex) | 10 AWG | 30 A |
| Copper UF-B (buried) | 10 AWG | 30 A |
| Copper THHN in conduit | 10 AWG | 35 A |
| Aluminum NM-B / SER | 8 AWG | 35 A |
| Aluminum XHHW in conduit | 8 AWG | 40 A |
Why 10 AWG and not 12: NEC 240.4(D) caps 12 AWG copper at a 20 A breaker and 10 AWG at 30 A, whatever the ampacity table says. For aluminum, 10 AWG is capped at 25 A, so a 30 A circuit needs 8 AWG.
When distance pushes the size up
Copper NM-B, a 24 A load (80% of the breaker), keeping voltage drop at or under 3%:
| One-way length | 240 V (water heater, dryer, AC) | 120 V (RV outlet, TT-30) |
|---|---|---|
| 50 ft | 10 AWG (1.2%) | 10 AWG (2.4%) |
| 100 ft | 10 AWG (2.4%) | 6 AWG (2.0%) |
| 150 ft | 8 AWG (2.3%) | 6 AWG (2.9%) |
| 200 ft | 6 AWG (2.0%) | 4 AWG (2.5%) |
| 250 ft | 6 AWG (2.5%) | 3 AWG (2.5%) |
The breaker stays at 30 A when you upsize for distance. One catch: NEC 250.122(B) requires the ground wire to be upsized in proportion too. With NM-B that happens automatically (8/2 and 6/2 cable come with a larger ground), but with wire in conduit you choose the ground yourself.
Common 30 A circuits
- Electric dryer: 30 A, 10/3 copper with ground (4-wire, NEMA 14-30).
- Water heater, 4,500–5,500 W: 30 A, 10/2 copper. See breaker size for a water heater.
- Generator inlet, L14-30: 30 A, 10/3 copper. See L14-30 vs 14-50.
- RV pedestal, TT-30: 30 A at 120 V, 10/2 copper; long runs to the RV pad often need 8 AWG or larger.
Safety. Check the appliance nameplate for its "maximum overcurrent protection" as well. Installing a new 240 V circuit usually needs a permit.