What size wire do I need for a 40 amp circuit?

Use 8 AWG copper or 6 AWG aluminum at the 60°C column, which sets the default under 110.14(C)(1)(a) for equipment rated 100 A or less, and 334.80 and 340.80 hold NM-B and UF-B cable to that column regardless of the terminals. With 75°C terminations, 8 AWG copper or 8 AWG aluminum carries the circuit.

What size ground wire for a 40 amp circuit?

10 AWG copper or 8 AWG aluminum, taken from the 60 A row of NEC Table 250.122.

Can a higher temperature column let me use a smaller conductor on a 40 A circuit?

No. 10 AWG copper reaches 40 A in the 90°C column but NEC 240.4(D) caps its overcurrent protection at 30 A, so the 90°C column cannot carry this circuit on a smaller conductor than the 60°C column does.

What size aluminum wire for 40 amps?

6 AWG aluminum at the 60°C column carries 40 A. That equals the circuit rating, so any ambient correction, bundling adjustment, or voltage drop pushes the size up. Terminations on aluminum conductors must be identified for the material.

Is 40 A enough for EV charging?

A 32 A EV charger is the largest continuous load a 40 A circuit can supply: 625.41 sizes the branch-circuit overcurrent device at 125% of the charger's maximum load, and 210.19(A)(1) does the same for the conductor.

What Size Wire for 40 Amps? Copper and Aluminum (NEC 2023)

Minimum size by temperature column

Smallest conductor that carries 40 A. NEC Table 310.16 ampacities with the 240.4(D) small-conductor limit applied. The highlighted column is the one 110.14(C) assumes for a 40 A circuit.
Conductor	60°C	75°C	90°C
Copper	8 AWG40 A	8 AWG50 A	8 AWG55 A
Aluminum	6 AWG40 A	8 AWG40 A	8 AWG45 A

Equipment grounding conductor

A circuit protected at 40 A takes a 10 AWG copper or 8 AWG aluminum equipment grounding conductor, from the 60 A row of NEC Table 250.122. Raise the ground with the ungrounded conductors when you increase them for voltage drop or any reason other than 310.15(B) or (C) derating, because 250.122(B) requires the same proportion. The ground is never required to be larger than the circuit conductors.

Longest run at 3% voltage drop

Longest one-way run before voltage drop reaches 3%
Conductor	120 V	240 V
8 AWG copper	55 ft	115 ft
6 AWG aluminum	55 ft	110 ft

NEC 210.19(A)(1) Informational Note 4 recommends keeping branch-circuit drop at or below 3%. These lengths come from the Table 8 resistance at 75°C for a stranded conductor at unity power factor, rounded to the nearest 5 ft. Reactance is not included, so a large conductor in a steel raceway falls short of the figure. A three-phase run reaches about 15% farther than these single-phase figures.

Small-conductor limit under 240.4(D)

NEC 240.4(D) caps overcurrent protection at 15 A for 14 AWG copper, 20 A for 12 AWG copper, 30 A for 10 AWG copper, 15 A for 12 AWG aluminum, and 25 A for 10 AWG aluminum. On a 40 A circuit that cap removes a size the ampacity table would otherwise allow.

10 AWG copper carries 40 A at 90°C, but cannot be protected above 30 A.

The caps apply after ambient correction and conductor adjustment, so derating the load does not raise them.

What a 40 A circuit feeds

Electric ranges and cooktops
Level 2 EV charging on a 40 A circuit

Forty amps is a range rating and the smaller end of home charging. It takes 8 AWG copper or 6 AWG aluminum at 60°C, which is what 8/3 NM-B and 8/3 UF cable provide. The same 8 AWG copper in conduit reaches 50 A at 75°C, so this is one of the ratings where the 60°C and 75°C answers agree.

A range circuit is sized from the demand factors in Table 220.55 rather than from the nameplate, because a range rarely draws its full rating. Appliances that do run at nameplate current, such as a charger or a commercial cooking appliance, are the ones where that number sets the circuit.

This is also where aluminum becomes practical in residential work, since 6 AWG aluminum reaches the rating at 60°C at a lower cost than 8 AWG copper. Aluminum needs terminations identified for the material, and in every size above 12 AWG it takes a larger conductor than copper for the same current.

40 A wire size questions

What size wire do I need for a 40 amp circuit?: Use 8 AWG copper or 6 AWG aluminum at the 60°C column, which sets the default under 110.14(C)(1)(a) for equipment rated 100 A or less, and 334.80 and 340.80 hold NM-B and UF-B cable to that column regardless of the terminals. With 75°C terminations, 8 AWG copper or 8 AWG aluminum carries the circuit.
What size ground wire for a 40 amp circuit?: 10 AWG copper or 8 AWG aluminum, taken from the 60 A row of NEC Table 250.122.
Can a higher temperature column let me use a smaller conductor on a 40 A circuit?: No. 10 AWG copper reaches 40 A in the 90°C column but NEC 240.4(D) caps its overcurrent protection at 30 A, so the 90°C column cannot carry this circuit on a smaller conductor than the 60°C column does.
What size aluminum wire for 40 amps?: 6 AWG aluminum at the 60°C column carries 40 A. That equals the circuit rating, so any ambient correction, bundling adjustment, or voltage drop pushes the size up. Terminations on aluminum conductors must be identified for the material.
Is 40 A enough for EV charging?: A 32 A EV charger is the largest continuous load a 40 A circuit can supply: 625.41 sizes the branch-circuit overcurrent device at 125% of the charger's maximum load, and 210.19(A)(1) does the same for the conductor.