What Size Wire for 40 Amps?

Quick answer: wire size for a 40 amp breaker

Minimum conductor allowed on a 40 A breaker, with not more than three current-carrying conductors at 30 °C (86 °F) ambient.

For copper the terminal rating does not change the answer: 8 AWG copper is 40 A in the 60 °C column, exactly enough for 40 A, so 8 AWG NM-B cable is allowed. For aluminum it does: 8 AWG aluminum is 35 A at 60 °C and 40 A at 75 °C, so it only works when the conductor and both terminations are rated 75 °C (NEC 110.14(C)). Otherwise use 6 AWG aluminum (40 A at 60 °C).

Why 10 AWG copper is not enough

10 AWG copper has a 75 °C ampacity of 35 A, but 240.4(D) still limits it to a 30 A breaker. 8 AWG copper on a 40 A breaker is also the natural maximum for NM cable: 40 A is a standard breaker size under 240.6(A), and 8 AWG NM-B is rated exactly 40 A.

Wire size for a 40 amp range, cooktop or wall oven

• NEC 210.19(A)(3) requires a branch circuit of at least 40 A for a household range rated 8¾ kW or more.
• NEC Table 220.55, Column C, counts one range of up to 12 kW at 8 kW, which is 33.3 A at 240 V. That is why a 40 A circuit with 8/3 copper is common, but the range nameplate decides: many list 50 A, which needs 6 AWG copper (see the 50 A guide).
• Use 8/3 with ground: the neutral feeds the clock, controls and oven light at 120 V. Under 210.19(A)(3) Exception No. 2 the neutral of a range circuit of 8¾ kW or more may be 70% of the circuit rating, but not smaller than 10 AWG.
• Separate cooktops and wall ovens use their own nameplate ratings; a counter-mounted cooktop and up to two wall ovens on one circuit are added together under 220.55 Note 4.

Wire size for a 32 amp EV charger

EV supply equipment is a continuous load, so NEC 625.41 sizes the breaker at 125% of the charger's setting. A charger set to 32 A needs a 40 A breaker and 8 AWG copper. Common settings:

A 32 A charger adds about 7.7 kW. If the charger plugs into a NEMA 14-50 receptacle, the receptacle needs a 50 A circuit even when the charger is set to 32 A, so the wire becomes 6 AWG copper. A hardwired charger set to 32 A can stay on 40 A.

A/C, heat pumps and other 40 amp loads

For air conditioners and heat pumps, use the nameplate minimum circuit ampacity (MCA) for the wire and the maximum overcurrent protection (MOCP) for the breaker (NEC Article 440). A unit with an MCA of 28 A and an MOCP of 40 A can use 10 AWG copper on a 40 A breaker, because 240.4(G) lets motor and A/C circuits follow the nameplate. A general-purpose circuit, a subpanel or a receptacle cannot.

Other common 40 A loads include electric tankless water heaters (often several 40 A circuits, one per element), hot tub heaters and small subpanel feeders. Storage and tankless water heaters of 120 gallons or less are continuous loads under 422.13, so check the nameplate current times 125%.

Wire size for 40 amps by distance

Smallest size that keeps the run at or under a 3% voltage drop at 240 V. The 32 A columns are for a continuous load on a 40 A breaker, such as an EV charger. Ampacity sets the floor, so the table never goes below 8 AWG.

Example: a 32 A EV charger 150 ft from the panel. 8 AWG copper reaches 147 ft at 32 A, so the run needs 6 AWG copper, which reaches 229 ft. Check that the charger and breaker terminals accept the larger wire, and upsize the ground too (see ground).

Maximum length at 40 amps by wire size

The longest one-way run, in feet, before each size reaches the drop shown. Three-phase columns use the √3 formula. These are DC resistance figures.

Copper

Aluminum

Formulas: single phase L = %·V·1000 / (2·I·R), three phase L = %·V·1000 / (√3·I·R), with R in ohms per 1000 ft. The NEC informational notes in 210.19 and 215.2 recommend 3% on a branch circuit or feeder and 5% total. Use the voltage drop calculator for your exact load.

Ground wire for 40 amps

• Equipment grounding conductor: 10 AWG copper or 8 AWG aluminum, from the 60 A row of NEC Table 250.122 (it covers breakers over 20 A up to 60 A). In 8/2 and 8/3 NM-B the bare ground is 10 AWG.
• Upsized hots: going from 8 AWG (16,510 cmil) to 6 AWG (26,240 cmil) is a ratio of 1.59, so under 250.122(B) the 10 AWG ground (10,380 cmil) must be at least 16,498 cmil, which is 8 AWG copper.

Conduit size for 40 amp wire

Smallest conduit at 40% fill (NEC Chapter 9 Table 1) using THHN/THWN-2 areas from Chapter 9 Table 5.

XHHW-2 aluminum areas differ slightly from THHN, so check the conduit fill calculator for your insulation.

How much load can a 40 amp circuit carry?

Balanced load, unity power factor. The amps to watts calculator handles power factor.

When more than three wires share a conduit

With four to six current-carrying conductors in one raceway, NEC 310.15(C)(1) cuts ampacity to 80%. Starting from the 90 °C column, 8 AWG copper gives 44 A, still enough for 40 A. At seven to nine conductors (70%) it drops to 38 A, so upsize to 6 AWG (52 A). See wire derating.

In Canada

CEC Table 2 also rates 8 AWG copper at 40 A in the 60 °C column, so a 40 A circuit is 8 AWG copper in Canada too, and CEC Rule 14-104 caps 10 AWG copper at 30 A in the same way as NEC 240.4(D). Rule 8-102 makes the 3% and 5% voltage drop limits mandatory, and distances are in metres. See CEC Table 2 for run lengths in metres by breaker, or use the Canadian wire size calculator.

For a smaller circuit, see wire size for 30 amps. For larger circuits, see 50 amps, 60 amps, 100 amps and 200 amps.

This page is a quick reference. Confirm against the code edition your jurisdiction has adopted, the equipment markings and your inspector.