Wire size
Wire size for 30 amps
A 30 amp circuit needs 10 AWG copper at the 75 degree C column of NEC Table 310.16, or 10 AWG aluminium. If the terminations are only rated 60 degrees C, which is common on equipment at 100 amps or less, you need 10 AWG copper instead.
Conductor size for 30 amps
NEC 110.14(C) limits you to the lowest-rated termination in the circuit, which on most equipment at 100 amps or less is 60 °C. Check the breaker and the device before using the 75 °C column.
| Material | 60 °C | 75 °C | 90 °C |
|---|---|---|---|
| Copper | 10 | 10 | 10 |
| Aluminium | 8 | 10 | 10 |
| One-way run | Drop | Percent | Within 3% |
|---|---|---|---|
| 50 ft | 3.7 V | 1.6% | Yes |
| 100 ft | 7.5 V | 3.1% | No, size up |
| 150 ft | 11.2 V | 4.7% | No, size up |
| 200 ft | 14.9 V | 6.2% | No, size up |
What runs on a 30 amp circuit
Electric dryers, water heaters, small welders, RV 30 A service and window air conditioners.
The thing to watch on this circuit
NEC 240.4(D) caps 10 AWG copper at 30 A regardless of insulation rating, so 10 AWG is both the minimum and the practical maximum use for this circuit.
Common questions
What size wire do I need for 30 amps?
- A 30 amp circuit needs 10 AWG copper at the 75 degree C column of NEC Table 310.16, or 10 AWG aluminium. If the terminations are only rated 60 degrees C, which is common on equipment at 100 amps or less, you need 10 AWG copper instead.
What breaker goes with a 30 amp circuit?
- A 30 amp breaker. The conductor is sized to the breaker, not the other way round, and NEC 240.4 requires the overcurrent device to protect the conductor. Note that NEC 240.4(D) caps 10 AWG copper at 30 amps regardless of its table ampacity.
How far can I run 30 amps before voltage drop matters?
- On 10 AWG copper at 240 volts, voltage drop reaches 1.6 percent at 50 feet and 6.2 percent at 200 feet. The NEC recommends staying under 3 percent on a branch circuit in the informational note to 210.19(A)(1), which is advice rather than a requirement, so beyond about 100 feet you should go up a size.
Can I use aluminium wire for a 30 amp circuit?
- Yes, at 10 AWG for the 75 degree C column, which is the same size than the copper equivalent. Aluminium is common and cost-effective on feeders and services, but the terminations must be listed for aluminium and an antioxidant compound is normally required.
What is a 30 amp circuit usually for?
- Electric dryers, water heaters, small welders, RV 30 A service and window air conditioners.
Conductor sizes are computed from NEC Table 310.16 with the 240.4(D) small-conductor cap applied. Table 310.16 assumes not more than three current-carrying conductors at 30 °C ambient; correct for ambient (310.15(B)(1)) and conductor count (310.15(C)(1)) before use, and verify against the code edition your jurisdiction has adopted.