40 amp wire size
A 40 amp circuit is normally wired with 8 AWG copper. 8 AWG copper is rated 40 A in the 60 °C column, so 8/2 or 8/3 NM-B cable reaches the full rating, and as THHN in conduit with 75 °C terminations it reads 50 A, leaving margin.
Minimum wire size for 40 A
| Conductor | 60 °C column | 75 °C column |
|---|---|---|
| Copper | 8 AWG | 8 AWG |
| Aluminum | Not listed | 8 AWG |
Usually chosen: 8 AWG copper, as 8/3 NM-B for a range or 8/2 NM-B for an EV charger or heat pump, or 8 AWG THHN in conduit. In aluminum the minimum is 8 AWG, though 6 AWG aluminum is the more common choice.
Breaker rating taken as the required ampacity; 240.4(D) limits applied. The charts list no 60 °C aluminum, and 60 °C copper only to 2 AWG (95 A), the largest NM-B. Manufacturer chart values (2017 NEC basis). Verify against the NEC edition adopted locally.
10 AWG copper falls short on two counts: its 75 °C ampacity is 35 A, and 240.4(D) limits it to a 30 A device in any case. Ranges, cooktops, wall ovens and Level 2 EV chargers are the usual 40 A loads; the details of each follow the answer box.
Where 40 amp circuits are used
NEC 210.19(A)(3) sets 40 A as the smallest branch circuit for a household range rated 8¾ kW or more. With the demand factors of Table 220.55 applied, a typical 12 kW freestanding range counts as 8 kW, about 33 A at 240 V, and a 40 A circuit is a common result; larger ranges and double ovens push the circuit to 50 A.
Separate cooktops and wall ovens are often on their own 30 A or 40 A circuits. Under 210.21(B)(3), a 40 A circuit may supply a 40 A or a 50 A receptacle, which is why a standard 50 A range receptacle can sit on a 40 A circuit.
EV charging is the fastest-growing 40 A load. NEC Article 625 treats charging as continuous, so the circuit is rated at 125% of the charger's output: a 32 A Level 2 charger needs a 40 A circuit and 8 AWG copper, and 625.40 requires the charger to have an individual branch circuit. Heat pumps, air handlers with electric strip heat and 240 V welders also commonly land at 40 A, set by their nameplates.
Voltage drop at 40 A
With 8 AWG copper carrying 40 A, the drop stays within 3% to about 115 ft at 240 V. A detached garage charger at the end of a long run, or a range at the far side of a house from the panel, can cross that line; the table shows the size that brings each distance back under 3%.
For an EV charger the full-load figures are realistic, since the car draws the set current for hours. For a range the cooking load cycles, and the full 40 A is rarely drawn for long.
| One-way length (ft) | Drop at 120 V (%) | Drop at 240 V (%) | Copper for 3% at 120 V | Copper for 3% at 240 V |
|---|---|---|---|---|
| 25 | 1.30 | 0.65 | 8 AWG | 8 AWG |
| 50 | 2.60 | 1.30 | 8 AWG | 8 AWG |
| 75 | 3.90 | 1.95 | 6 AWG | 8 AWG |
| 100 | 5.19 | 2.60 | 4 AWG | 8 AWG |
| 150 | 7.79 | 3.90 | 3 AWG | 6 AWG |
| 200 | 10.39 | 5.19 | 2 AWG | 4 AWG |
| 250 | 12.99 | 6.49 | 1 AWG | 4 AWG |
| 300 | 15.58 | 7.79 | 1/0 AWG | 3 AWG |
Full rating on a single-phase circuit, copper at 75 °C, stranded.
Breaker pairing
8 AWG copper pairs with a 40 A breaker in NM-B cable and with up to a 50 A breaker as THHN in conduit with 75 °C terminations at both ends. Two-pole breakers are used for all of these loads, since each is fed at 240 V or 120/240 V.
The next-size-up rule of 240.4(B) does not rescue 10 AWG here: although its 75 °C ampacity is 35 A, the small-conductor limit of 240.4(D) keeps it at 30 A whatever device is chosen.
NM-B cable or THHN in conduit
8/3 NM-B with ground, with a black sheath, supplies a range that needs 120 V for its controls, clock and lights. 8/2 with ground serves 240 V-only loads such as most EV chargers and heat pumps. The 60 °C limit of 334.80 is exactly 40 A at 8 AWG, so NM-B cannot go higher than 40 A at this size.
THHN/THWN-2 in conduit gets the 75 °C value of 50 A, but only when the breaker, the receptacle or the equipment lugs are all rated for 75 °C conductors. A charger mounted on a garage wall and fed through EMT is a typical case.
Copper or aluminum
The manufacturer charts list 8 AWG aluminum at 40 A in the 75 °C column, so 8 AWG aluminum with 75 °C terminations is the aluminum minimum. Many installers use 6 AWG aluminum instead, which reads 50 A and leaves margin for voltage drop.
Aluminum makes sense for long runs at this rating, such as a feeder to an EV charger in a detached garage, where its lower cost per foot outweighs the larger size. The receptacle or charger terminals must be listed for aluminum.
Common mistakes
Using 10 AWG for a 40 A range circuit
10 AWG copper may be protected at 30 A at most. A 40 A range breaker on 10 AWG cable is a code violation regardless of how small the range's demand load appears.
Sizing an EV charger at its output current
A 32 A charger needs a 40 A circuit, not a 32 A or 35 A one. The 125% continuous-load factor applies to both the breaker and the conductor.
Mixing 60 °C cable with a 75 °C ampacity
Treating 8 AWG NM-B as 50 A because THHN of the same size is rated 50 A is a frequent error. NM-B stays in the 60 °C column, which is 40 A.
Skipping the GFCI on a garage charger receptacle
When a charger plugs into a NEMA 14-50 or 6-50 receptacle in a garage, the receptacle needs GFCI protection under current NEC editions. Hard-wired chargers have their own listed protection.
Frequently asked questions
Is 8 gauge wire good for 40 amps?
Yes. 8 AWG copper is rated 40 A in the 60 °C column used for NM-B cable and 50 A in the 75 °C column, so it suits a 40 A breaker in either wiring method.
Can 10 gauge wire handle 40 amps?
No. 10 AWG copper is rated 35 A at 75 °C and is limited to a 30 A breaker by NEC 240.4(D). A 40 A circuit needs 8 AWG copper or larger.
What size wire is used for a 40 amp EV charger circuit?
A 40 A circuit serves a charger set to 32 A and is wired with 8 AWG copper. The charger's installation manual states the breaker size and allowed conductors.
Can a 50 amp range receptacle be used on a 40 amp circuit?
Yes. NEC 210.21(B)(3) permits 40 A or 50 A receptacles on a 40 A branch circuit, so the common 50 A range receptacle may be installed with a 40 A breaker.
Is 6 gauge wire required for 40 amps?
Not for copper; 8 AWG copper is enough. 6 AWG copper is used on long runs to limit voltage drop, and 6 AWG aluminum is a common aluminum choice for 40 A.