What Size Wire for a 40 Amp Breaker?
Short answer: a 40 amp breaker needs 8 AWG copper, in NM-B cable (8/2 or 8/3 with ground) or as THHN in conduit. With aluminum, use 8 AWG aluminum where 75°C terminations apply, or 6 AWG aluminum in cable limited to 60°C.
40 amp wire size by wiring method
| Wiring method | Copper | Aluminum |
|---|---|---|
| NM-B or UF cable (60°C) | 8 AWG | 6 AWG |
| THHN/THWN-2 in conduit, 75°C terminals | 8 AWG | 8 AWG |
Ground wire: 10 AWG copper or 8 AWG aluminum (Table 250.122 covers 40 A in the 60 A row).
8 AWG copper is rated exactly 40 A in the 60°C column, which is why it works with NM-B. It has headroom at 75°C (50 A). Aluminum is the reverse case: 8 AWG aluminum is 35 A at 60°C and 40 A at 75°C, so the wiring method changes the aluminum answer but not the copper one.
40 amp wire size by distance (240 V)
| Conductor | 50 ft | 100 ft | 150 ft | 200 ft |
|---|---|---|---|---|
| Copper | 8 AWG (1.0%) | 8 AWG (2.1%) | 6 AWG (2.0%) | 6 AWG (2.6%) |
| Aluminum | 8 AWG (1.7%) | 6 AWG (2.2%) | 4 AWG (2.0%) | 4 AWG (2.7%) |
Smallest conductor that meets both the 75°C ampacity for a 40 A circuit and a 3% voltage drop at 32 A (80% of the rating), 240 V single-phase, one-way distance. Percent drop in brackets. If the circuit uses NM-B cable or 60°C terminations, start from the 60°C column instead.
Maximum one-way run at 3% drop, 240 V
| Conductor | At 32 A (EV charger) | At 40 A (full rating) |
|---|---|---|
| 8 AWG copper | 144 ft | 115 ft |
| 6 AWG copper | 229 ft | 183 ft |
| 6 AWG aluminum | 139 ft | 111 ft |
| 4 AWG aluminum | 221 ft | 177 ft |
Worked example: 32 A EV charger in a detached garage
A hardwired Level 2 charger set to 32 A sits 120 feet from the house panel. Conductors are THHN/THWN-2 in PVC, and all terminations are rated 75°C.
- Circuit size: EV charging is continuous, so 32 × 1.25 = 40 A breaker and conductors.
- 8 AWG copper: 2 × 12.9 × 32 × 120 ÷ 16,510 = 6.00 V, or 2.5%. Acceptable.
- 8 AWG aluminum (40 A at 75°C): 2 × 21.2 × 32 × 120 ÷ 16,510 = 9.86 V, or 4.1%. Too much.
- 6 AWG aluminum: 2 × 21.2 × 32 × 120 ÷ 26,240 = 6.20 V, or 2.6%. Acceptable.
Result: 8 AWG copper or 6 AWG aluminum, both with a 10 AWG copper equipment ground. Aluminum needs one size up for the same drop, which usually decides whether it saves money on a run like this. Try other distances in the voltage drop calculator.
Common 40 amp circuits
- Cooktops and wall ovens. Many are rated for a 40 A circuit. Household ranges rated 8¾ kW or more need at least a 40 A branch circuit (210.19(C) in the 2023 NEC; 210.19(A)(3) in 2020). A double wall oven may need 50 A; check the installation manual.
- Level 2 EV charger at 32 A, as in the example above.
- Heat pumps and AC condensers. Use the nameplate MCA to size the wire and the MOCP for the breaker; a 40 A MOCP doesn't always mean 8 AWG is required if the MCA is lower.
- Tankless electric water heaters often use several 40 A circuits; follow the manufacturer's wiring diagram.
8/2 or 8/3? And what size conduit?
A cooktop or wall oven that uses only 240 V needs two hots and a ground (8/2 with ground). A range, or any appliance with 120 V controls, needs a neutral as well (8/3 with ground).
In conduit, two 8 AWG THHN plus a 10 AWG ground total 0.0943 in², which fits ½" EMT (40% of 0.304 in²). Add a neutral and the total is 0.1309 in², which needs ¾" EMT. Areas are from Chapter 9 Table 5; see the conduit fill chart.
Common mistakes inspectors flag
- 10 AWG on a 40 A breaker for a cooktop or oven. Only Article 440 equipment with a nameplate that allows it gets that exception.
- 8 AWG aluminum on a 40 A breaker where the 60°C column applies (60°C terminations, or SE cable run in thermal insulation per 338.10(B)(4)). It's rated 35 A there.
- Stranded 8 AWG forced into terminals listed only for smaller wire. Check the device's terminal range.
- A 50 A breaker on 8 AWG NM-B for a range. The cable is limited to 40 A.
Frequently asked questions
Can I use 10 gauge wire on a 40 amp breaker?
No, for a general circuit. 10 AWG copper is limited to 30 A by 240.4(D); see 30 amp wire size. The exception is equipment like AC condensers where the nameplate MCA and MOCP permit it under Article 440.
Is 8 gauge wire good for 40 amps?
Yes. 8 AWG copper is rated 40 A at 60°C and 50 A at 75°C. See the ampacity chart for other sizes.
What size wire for a 40 amp EV charger?
A charger that draws 40 A continuous needs a 50 A circuit, so 6 AWG copper NM-B or 8 AWG copper THHN at 75°C; see 50 amp wire size. A charger set to 32 A uses a 40 A circuit with 8 AWG copper.
How far can I run 8 gauge wire on a 40 amp circuit?
About 115 feet one way at a full 40 A on 240 V, or 144 feet at a 32 A load, before exceeding 3% drop. Past that, use 6 AWG copper. The wire size calculator handles other loads.
Last updated: October 6, 2026. Based on NEC 2023. Check results against the code edition and local amendments your jurisdiction enforces and with the AHJ; electrical work should be done by qualified people.