40 amp wire size
40 amp wire size: 8 AWG copper or 8 AWG aluminum, with 75 °C terminations.
40 amp wire size by termination rating
| Termination rating | Copper | Aluminum |
|---|---|---|
| 60 °C, NM-B | 8 AWG | 6 AWG |
| 75 °C, THHN | 8 AWG | 8 AWG |
| Ground wire | 10 AWG | 8 AWG |
40 amp wire size by distance at 240 V
Sizes that keep the voltage drop within 3% on a 240 V circuit loaded to the full 40 A, 75 °C terminations. The copper ground wire grows with the hot wires on long runs.
| One-way distance | Copper | Drop | Copper ground | Aluminum | Drop |
|---|---|---|---|---|---|
| 25 ft | 8 AWG | 0.7% | 10 AWG | 8 AWG | 1.1% |
| 50 ft | 8 AWG | 1.3% | 10 AWG | 8 AWG | 2.1% |
| 100 ft | 8 AWG | 2.6% | 10 AWG | 6 AWG | 2.7% |
| 150 ft | 6 AWG | 2.5% | 8 AWG | 4 AWG | 2.5% |
| 200 ft | 4 AWG | 2.1% | 4 AWG | 3 AWG | 2.7% |
| 300 ft | 3 AWG | 2.5% | 4 AWG | 1 AWG | 2.5% |
- Copper
- Aluminum
Check your own 40 amp run
What are you sizing for?
A
ft
From the panel to the load, not there and back
Voltage
Conductor
Termination rating
Result
8 AWG copper
Ampacity sets the size; the voltage drop is within 3%.
- Breaker
- 40 A
- Voltage drop
- 3.1 V (1.3%)
- Volts at the load
- 236.9 V
- Ground wire
- 10 AWG copper
How it is calculated
- Breaker: 40 A rounded up to a standard size = 40 A.
- Smallest copper conductor rated for 40 A at 75 °C: 8 AWG.
- Drop = 2 × K × 40 A × 50 ft ÷ 16,510 cmil = 3.13 V, target 3% of 240 V.
What a 40 amp circuit is used for
- Electric cooktop
- Level 2 EV charger drawing 32 A