Wire size
8 AWG
Copper
What size wire for a 40 amp 240 volt circuit?
Short answer
8 AWG copper
A 40 A circuit at 240 V takes 8 AWG copper wire on a 40 A breaker, as long as the run stays under 143 ft.
Breaker
40 A
Longest run
143 ft
Longest run at 3% drop
Why 8 AWG
- Ampacity: 8 AWG copper is rated 40 A in the 60 degC column of NEC Table 310.16, which covers the 40 A this circuit needs.
How far you can run it
Past a certain length the wire still carries the current safely, but too much voltage is lost on the way. These are the lengths where you should move up a size.
| Wire size | Longest run at 3% drop |
|---|---|
| 8 AWG | 143 ft |
| 6 AWG | 227 ft |
| 4 AWG | 362 ft |
| 3 AWG | 456 ft |
| 2 AWG | 575 ft |
One-way distance, copper, 240 V.
Before you buy
- Check what your breaker terminals are rated for. Most residential breakers are 60 degC or 75 degC, and that is the column you must use, not the 90 degC column on the wire jacket.
- If the load runs three hours or more at a stretch, size for 125% of it. That is most heating, EV charging and shop lighting.
- Local amendments beat the national Code. Your inspector has the last word.