What breaker size for 400 kcmil wire?

400 kcmil copper at 75 °C terminations supports up to a 300 A overcurrent device, based on an estimated ampacity of 348 A. Aluminium of the same size supports less: about 272 A.

How many amps can 400 kcmil handle?

Estimated 295 A at 60 °C, 348 A at 75 °C and 393 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.

400 kcmil Breaker Size — Amps & Overcurrent Rating

400 kcmil copper is estimated at 348 A at 75 °C, which supports a 300 A overcurrent device. Breaker pairing, continuous-load sizing and derating shown.

Quick answer

400 kcmil copper pairs with a 300 A breaker at 75 °C terminations, from an estimated ampacity of 348 A. Aluminium of the same size is about 272 A. A continuous load on that circuit should not exceed 278 A, since continuous loads are sized at 125%.

Max breaker (Cu, 75 °C)

300A

Copper 60 / 75 / 90 °C

295 / 348 / 393A

Aluminium 75 °C

272A

Max continuous load

278A

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	295 A	348 A	393 A	300 A
Aluminium	230 A	272 A	306 A	250 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	393 A → 300 A	359 A → 300 A	321 A → 300 A	278 A → 250 A
6	314 A → 300 A	287 A → 250 A	257 A → 250 A	222 A → 200 A
9	275 A → 250 A	251 A → 250 A	224 A → 200 A	194 A → 175 A
15	197 A → 175 A	179 A → 175 A	160 A → 150 A	139 A → 125 A
25	177 A → 175 A	161 A → 150 A	144 A → 125 A	125 A → 125 A

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

Source & method: Continuous-load example: a 278 A continuous load needs 348 A of design current and a 350 A device. Size your own load.

Neighbouring sizes

Smaller350 kcmil319 A copper at 75 °C Larger500 kcmil403 A copper at 75 °C

Frequently asked

What breaker size for 400 kcmil wire?: 400 kcmil copper at 75 °C terminations supports up to a 300 A overcurrent device, based on an estimated ampacity of 348 A. Aluminium of the same size supports less: about 272 A.
How many amps can 400 kcmil handle?: Estimated 295 A at 60 °C, 348 A at 75 °C and 393 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.