Wire size for a 40 amp breaker
| Wire | Condition | Minimum size for 40 A |
|---|---|---|
| Copper | NM-B (Romex) cable or 60°C terminals | 8 AWG |
| Copper | THHN/THWN in conduit, 75°C terminals | 8 AWG |
| Aluminum | 75°C terminals | 8 AWG |
| Aluminum | 60°C terminals | 6 AWG |
8 AWG copper is rated 40 A in the 60°C column, 50 A at 75°C and 55 A at 90°C. The size above is the smallest whose ampacity is at least 40 A. Most modern breakers and panels have 75°C terminals, but a 40 A circuit is presumed 60°C unless the equipment is marked otherwise (NEC 110.14(C)).
Voltage drop: how far can 8 AWG run at 40 A?
The 8 AWG above carries 40 A safely but loses voltage over distance. At the full 40 A and 240 V, 8 AWG copper stays within the NEC's 3% recommendation for about 115 ft one way. 6 AWG stretches that to 183 ft.
| One-way distance | 8 AWG copper | 6 AWG copper | 4 AWG copper |
|---|---|---|---|
| 25 ft | 0.7% | 0.4% | 0.3% |
| 50 ft | 1.3% | 0.8% | 0.5% |
| 100 ft | 2.6% | 1.6% | 1.0% |
| 150 ft | 3.9% | 2.5% | 1.5% |
| 200 ft | 5.2% | 3.3% | 2.1% |
Percent drop at 40 A and 240 V.
On 120 V circuits, the 40 A percentages are twice these values.
Check your own 40 A run with the voltage drop calculator, or let the wire size calculator choose the size for you.
What a 40 amp circuit is used for
- Smaller electric ranges, cooktops and wall ovens
- Level 2 EV chargers drawing up to 32 A continuous
- Tankless water heaters and welders
- Larger heat pump and air-conditioner circuits
Cable type matters
8/2 and 8/3 NM-B (Romex) is rated 40 A in the 60°C column, an exact match for a 40 A breaker. For 75°C wire in conduit, 8 AWG is good for 50 A.
Continuous loads and common mistakes
On a 40 A breaker, a continuous load running three hours or more should not exceed 80% of the rating. That is 32 A, or about 7,680 W at 240 V. A 32 A EV charger is a continuous load, so it needs a 40 A circuit. Do not fit it on a 30 A circuit even if the wire is thick.
For a 40 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, 30 amps, 50 amps, 60 amps, 100 amps.
Frequently asked questions
What size wire for a 40 amp breaker?
8 AWG copper at 75°C terminals. In aluminum use 8 AWG.
How far can 8 AWG wire run on 40 amps?
About 115 feet one way at 240 V before the voltage drop passes the NEC's 3% recommendation. Past that distance, step up to 6 AWG.
What size wire for a 40 amp breaker?
8 AWG copper. For 8 AWG aluminum, the terminals must be rated 75°C.
What size wire for a 32 amp EV charger?
8 AWG copper on a 40 A breaker, since the charger is a continuous load at 125%.