What size wire for 40 amps?
8 AWG copper — at 75 °C terminals, NEC Table 310.16. Aluminum: 8 AWG.
A 40 A circuit needs 8 AWG copper, rated 50 A at 75 °C. NM-B cable at the 60 °C column gives the same answer.
Safety first. This is a starting point before derating, not a load calculation. Wire sizing affects fire and shock safety — verify against the current NEC and have circuits designed or reviewed by a licensed electrician for your jurisdiction.
40 amp wire size by material and terminal rating
| Material | 60 °C (NM-B) | 75 °C (typical) | 90 °C |
|---|---|---|---|
| copper | 8 AWG (40 A) | 8 AWG (50 A) | 8 AWG (55 A) |
| aluminum | 6 AWG (40 A) | 8 AWG (40 A) | 8 AWG (45 A) |
Sizes are the smallest conductor whose ampacity and NEC 240.4(D) breaker cap both reach 40 A, before any derating. Use the 75 °C column unless every terminal, breaker and device in the circuit is rated otherwise (NEC 110.14(C)).
If the load runs continuously
A load operating for three hours or more is a continuous load, and NEC 210.19(A)(1) requires the conductor to be sized at 125% of it — 40 A becomes 50 A for sizing purposes. That moves this circuit to 8 AWG copper — the same size, so there is nothing to change here. EV chargers, electric heat and commercial lighting are the loads this catches most often.
How far can a 40 amp circuit run?
The table above only prevents overheating. On a long run the limit is voltage arriving at the far end, and it bites well before ampacity does. These are the longest one-way runs for 8 AWG copper at 40 A within the 3% drop recommended by the NEC 210.19(A) informational note — and what going up to 6 AWG buys you.
| System | Typical use | 8 AWG | 6 AWG |
|---|---|---|---|
| 120 V | standard receptacle and lighting circuits | 72 ft | 114 ft |
| 240 V | ranges, dryers, EV chargers, feeders | 143 ft | 228 ft |
| 12 V DC | automotive, marine, solar and battery runs | 7 ft | 11 ft |
Single-phase, one-way distance, copper at 20 °C. Aluminum reaches its limit at roughly 60% of these distances. The 3% target is a recommendation, not a code requirement.
What runs on a 40 amp circuit
- electric ranges and cooktops
- EV chargers
- heat-pump air handlers
The catch at 40 amps
40 A needs 8 AWG copper, but if the run is NM-B (Romex) it is limited to the 60 °C column by 334.80 — where 8 AWG is exactly 40 A, with zero margin.
Working the other way round? See how many amps 8 AWG carries , including its diameter, resistance and full run-length table.