Wire size for a 30 amp breaker
| Wire | Condition | Minimum size for 30 A |
|---|---|---|
| Copper | NM-B (Romex) cable or 60°C terminals | 10 AWG |
| Copper | THHN/THWN in conduit, 75°C terminals | 10 AWG |
| Aluminum | 75°C terminals | 8 AWG |
| Aluminum | 60°C terminals | 8 AWG |
10 AWG copper is rated 30 A in the 60°C column, 35 A at 75°C and 40 A at 90°C. The 75°C column lists 35 A, but NEC 240.4(D) limits 10 AWG copper to 30 A. The size above is the smallest whose ampacity is at least 30 A. Most modern breakers and panels have 75°C terminals, but a 30 A circuit is presumed 60°C unless the equipment is marked otherwise (NEC 110.14(C)).
Voltage drop: how far can 10 AWG run at 30 A?
The 10 AWG above carries 30 A safely but loses voltage over distance. At the full 30 A and 240 V, 10 AWG copper stays within the NEC's 3% recommendation for about 97 ft one way. 8 AWG stretches that to 154 ft.
| One-way distance | 10 AWG copper | 8 AWG copper | 6 AWG copper |
|---|---|---|---|
| 25 ft | 0.8% | 0.5% | 0.3% |
| 50 ft | 1.6% | 1.0% | 0.6% |
| 100 ft | 3.1% | 2.0% | 1.2% |
| 150 ft | 4.7% | 2.9% | 1.8% |
| 200 ft | 6.2% | 3.9% | 2.5% |
Percent drop at 30 A and 240 V.
On 120 V circuits, the 30 A percentages are twice these values.
Check your own 30 A run with the voltage drop calculator, or let the wire size calculator choose the size for you.
What a 30 amp circuit is used for
- Electric clothes dryers (30 A, 240 V, 10 AWG)
- Electric water heaters up to about 5,500 W
- Central and large window air conditioners
- RV pedestals rated 30 A (120 V)
Cable type matters
10/2 and 10/3 NM-B (Romex) is rated 30 A at 60°C. A dryer circuit uses 10/3 with a ground, which gives two hots, a neutral and a bare ground. Water heater circuits use 10/2.
Continuous loads and common mistakes
On a 30 A breaker, a continuous load running three hours or more should not exceed 80% of the rating. That is 24 A, or about 5,760 W at 240 V. Water heaters count as continuous loads, so a 4,500 W unit at 240 V (18.75 A) needs a circuit rated for at least 23.4 A, which a 30 A breaker meets.
For a 30 A breaker the ground wire comes from NEC Table 250.122. Use the ground wire size calculator. For other breaker sizes, see the breaker to wire size chart, or jump to 20 amps, 40 amps, 50 amps, 60 amps, 100 amps.
Frequently asked questions
What size wire for a 30 amp breaker?
10 AWG copper at 75°C terminals. In aluminum use 8 AWG.
How far can 10 AWG wire run on 30 amps?
About 97 feet one way at 240 V before the voltage drop passes the NEC's 3% recommendation. Past that distance, step up to 8 AWG.
What size wire for a 30 amp dryer?
10 AWG copper. A standard dryer outlet is a 30 A, 240 V circuit with a four-wire cable.
What size wire for a 30 amp RV outlet?
10 AWG copper for the 120 V, 30 A pedestal. For long runs, check voltage drop and consider 8 AWG.