What breaker size for 8 AWG wire?

8 AWG copper at 75 °C terminations supports up to a 40 A overcurrent device, based on an estimated ampacity of 44 A. Aluminium of the same size supports less: about 34 A.

How many amps can 8 AWG handle?

Estimated 37 A at 60 °C, 44 A at 75 °C and 49 A at 90 °C for copper, at 30 °C ambient with no more than three current-carrying conductors.

How does a continuous load change the breaker size?

Continuous loads are sized at 125% of the load current before rounding up to the next standard device rating, so a breaker carries no more than 80% of its rating continuously.

Can the breaker be larger than the wire ampacity?

Only in specific permitted cases such as motor and transformer circuits, where the device protects against short circuit and ground fault while overload protection is provided separately. For ordinary branch circuits, the device must not exceed the conductor ampacity after derating.

8 AWG Breaker Size — Amps & Overcurrent Rating

8 AWG copper is estimated at 44 A at 75 °C, which supports a 40 A overcurrent device. Breaker pairing, continuous-load sizing and derating shown.

Quick answer

8 AWG copper pairs with a 40 A breaker at 75 °C terminations, from an estimated ampacity of 44 A. Aluminium of the same size is about 34 A. A continuous load on that circuit should not exceed 35 A, since continuous loads are sized at 125%.

Max breaker (Cu, 75 °C)

40A

Copper 60 / 75 / 90 °C

37 / 44 / 49A

Aluminium 75 °C

34A

Max continuous load

35A

Ampacity and breaker pairing

Estimated ampacity by material and insulation temperature rating, with the largest standard device each supports.

Material	60 °C	75 °C	90 °C	Max device @ 75 °C
Copper	37 A	44 A	49 A	40 A
Aluminium	29 A	34 A	39 A	30 A

Formula estimate

Derated ampacity and the device that still fits

Ambient temperature and conductor count reduce the usable ampacity before the breaker is chosen.

Conductors in raceway	30 °C ambient	40 °C ambient	50 °C ambient	60 °C ambient
3	49 A → 40 A	45 A → 40 A	40 A → 40 A	35 A → 35 A
6	39 A → 35 A	36 A → 35 A	32 A → 30 A	28 A → 25 A
9	34 A → 30 A	31 A → 30 A	28 A → 25 A	24 A → 20 A
15	25 A → 25 A	22 A → 20 A	20 A → 20 A	17 A → 15 A
25	22 A → 20 A	20 A → 20 A	18 A → 15 A	16 A → 15 A

90 °C insulation, copper, capped in practice by the 75 °C termination rating.

Source & method: Continuous-load example: a 35 A continuous load needs 44 A of design current and a 45 A device. Size your own load.

Neighbouring sizes

Smaller10 AWG32 A copper at 75 °C Larger6 AWG59 A copper at 75 °C

Frequently asked

What breaker size for 8 AWG wire?: 8 AWG copper at 75 °C terminations supports up to a 40 A overcurrent device, based on an estimated ampacity of 44 A. Aluminium of the same size supports less: about 34 A.
How many amps can 8 AWG handle?: Estimated 37 A at 60 °C, 44 A at 75 °C and 49 A at 90 °C for copper, at 30 °C ambient with no more than three current-carrying conductors.
How does a continuous load change the breaker size?: Continuous loads are sized at 125% of the load current before rounding up to the next standard device rating, so a breaker carries no more than 80% of its rating continuously.
Can the breaker be larger than the wire ampacity?: Only in specific permitted cases such as motor and transformer circuits, where the device protects against short circuit and ground fault while overload protection is provided separately. For ordinary branch circuits, the device must not exceed the conductor ampacity after derating.