What Size Wire for a 40 Amp Breaker?

Use 8 AWG for a 40 amp breaker, in either copper or aluminum. 8 AWG copper is rated 40 A at 60 °C and 50 A at 75 °C, so it works with common terminals.

The answer

8 AWG copper

8 AWG aluminum (75 °C terminals)

Wire size for a 40 amp breaker
Case Wire size Table ampacity Area
Copper · 75 °C THHN in conduit, most modern equipment 8 AWG 50 A 8.4 mm²
Copper · 60 °C NM / Romex cable, older terminals 8 AWG 40 A 8.4 mm²
Aluminum · 75 °C Needs connectors and terminals listed for aluminum 8 AWG 40 A 8.4 mm²

Calculate for your exact distance

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How the answer is worked out

The breaker is 40 A. The smallest copper wire with a 75 °C table ampacity at or above 40 A is 8 AWG, rated 50 A, with a cross-section of 8.4 mm².

For 60 °C terminals or NM cable the smallest copper size is 8 AWG, rated 40 A in the 60 °C column.

Aluminum uses its own column. The smallest aluminum size at 75 °C for this rating is 8 AWG, rated 40 A. Aluminum conductors need connectors and terminals listed for aluminum.

This example treats the load as continuous (three hours or more). 32 A × 1.25 = 40 A, so the next standard breaker is 40 A and the wire must carry at least that much.

NEC 240.4(D) also limits the breaker for small conductors: 14 AWG copper is capped at 15 A, 12 AWG copper at 20 A, 10 AWG copper at 30 A, 12 AWG aluminum at 15 A and 10 AWG aluminum at 25 A. The sizes in the table respect those caps, so a wire is never paired with a breaker larger than its limit.

Why 8 AWG

10 AWG is limited to a 30 amp breaker, so 8 AWG is the next size. It carries 40 A even in the 60 °C column, which is the one used for NM cable, so it is fine for a 40 amp breaker with either NM cable or individual wires in conduit.

EV chargers on a 40 amp breaker

EV charging counts as a continuous load, so the circuit is sized at 125% of the charger's current. A charger that draws 32 A needs a 40 A circuit (32 × 1.25 = 40). A charger that draws 40 A needs a 50 amp circuit instead; see the 50 amp guide.

Long runs

Garage-to-driveway and outbuilding runs often exceed 100 ft. Use the distance table below to check whether 8 AWG still keeps the drop under 3%, or step up to 6 AWG.

How far can you run it? (3% voltage drop)

The distance table uses the same resistance-only model as the calculator, with a 3% limit and the wire at 75 °C. Aluminum has about 1.6 times the resistance of copper for the same size, so an aluminum run of the same length drops more voltage.

Assumes the full 40 A load on a single-phase 240 V circuit, a 75 °C conductor, copper or aluminum, a 3% voltage drop limit and the one-way length in the table. Aluminum shows a dash where that size cannot carry the full current.

Maximum one-way length for 3% drop at 40 A, 240 V, 75 °C conductor
Wire size Copper Aluminum
8 AWG 115.5 ft [35.2 m] 70.1 ft [21.4 m]
6 AWG 183.6 ft [56.0 m] 111.5 ft [34.0 m]
4 AWG 292.0 ft [89.0 m] 177.3 ft [54.1 m]
3 AWG 368.2 ft [112.2 m] 223.6 ft [68.2 m]

Need a specific distance? See: What Size Wire for an EV Charger?

Before you buy wire

The table is a starting point. Work through this list before you order conductors, and keep the answer that matches your real installation.

This is general information based on NEC tables, not a permit or an installation plan. Local amendments, the equipment nameplate and an inspector decide what is acceptable. Read the disclaimer.

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