What size wire for a 40 amp breaker?
Short answer: A 40 A breaker needs at least 8 AWG copper or 8 AWG aluminum as individual wires in conduit with 75 °C terminations, or 8 AWG copper in NM-B (Romex) cable, which is held to the 60 °C column. Long runs need a bigger size to keep voltage drop under 3%.
Minimum wire size for 40 A by wire type
| Wire type | Minimum size | Ampacity (NEC 310.16) |
|---|---|---|
| Copper NM-B (Romex) | 8 AWG | 40 A |
| Copper THHN/THWN-2 in conduit, 75 °C | 8 AWG | 50 A |
| Aluminum SER / NM-B (60 °C) | 6 AWG | 40 A |
| Aluminum XHHW-2 in conduit, 75 °C | 8 AWG | 40 A |
The column matters as much as the breaker. NEC 334.80 limits NM-B cable to its 60 °C ampacity, so the same 40 A breaker needs a larger gauge in Romex than in conduit. Individual THHN or XHHW-2 wires can use the 75 °C column, but only when both ends terminate on lugs rated 75 °C, which is true of nearly all residential breakers and panels sold today but should be checked on the label. NEC 240.4(B) also lets you protect a wire with the next standard breaker above its ampacity (up to 800 A) when the load itself fits the wire, which is how 40 A breakers end up on some sizes listed one step smaller in other charts.
When distance pushes the size up
Smallest size that is both legal for the 40 A breaker and keeps voltage drop at or under 3% at 240 V, with a load of 32 A (80% of the breaker):
| One-way length | Copper, 75 °C | Aluminum, 75 °C |
|---|---|---|
| 50 ft | 8 AWG (1.0%) | 8 AWG (1.7%) |
| 100 ft | 8 AWG (2.0%) | 6 AWG (2.2%) |
| 150 ft | 6 AWG (2.0%) | 4 AWG (2.0%) |
| 200 ft | 6 AWG (2.6%) | 4 AWG (2.7%) |
| 300 ft | 4 AWG (2.5%) | 2 AWG (2.6%) |
Upsizing for distance does not change the breaker. It does change the ground: NEC 250.122(B) requires the equipment grounding conductor to be increased in proportion when the hot conductors are upsized for voltage drop.
Ground wire for a 40 A circuit
NEC Table 250.122 sizes the equipment ground from the breaker: 10 AWG copper or 8 AWG aluminum for 40 A. Cable assemblies (NM-B, SER, UF) include a ground already sized for their gauge. A 40 A circuit carries up to 9,600 W at 240 V, or 7,680 W for a continuous load.
Common 40 A circuits
- Electric range or cooktop rated 8–9.6 kW: 40 A, 8/3 copper with ground.
- 32 A Level 2 EV charger: 40 A circuit (32 A × 1.25), 8 AWG copper THHN or 6 AWG NM-B. See the EV charger calculator.
- Large tankless point-of-use heater or a 3–3.5-ton heat pump: follow the nameplate MOCP.
- Small subpanel for a shed or garage.
Feeding a subpanel? It needs four wires (two hots, a neutral and a ground), with the neutral and ground kept separate in the subpanel, and a detached building needs its own grounding electrode (NEC 250.32).
Safety. Installing a 40 A circuit or feeder needs a permit almost everywhere in the US. Equipment nameplates (minimum circuit ampacity and maximum overcurrent protection) override any general table, and local amendments to the NEC can be stricter.