What breaker size for 300 kcmil wire?

300 kcmil copper at 75 °C terminations supports up to a 250 A overcurrent device, based on an estimated ampacity of 289 A. Aluminium of the same size supports less: about 225 A.

How many amps can 300 kcmil handle?

Estimated 245 A at 60 °C, 289 A at 75 °C and 326 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.

300 kcmil Breaker Size & Ampacity

300 kcmil copper is estimated at 289 A at 75 °C, supporting a 250 A overcurrent device. Breaker pairing and derating shown.

Quick answer

300 kcmil copper pairs with a 250 A breaker at 75 °C terminations, from an estimated ampacity of 289 A. Aluminium of the same size is about 225 A. A continuous load on that circuit should not exceed 231 A, since continuous loads are sized at 125%.

Max breaker (Cu, 75 °C)

250A

Copper 60 / 75 / 90 °C

245 / 289 / 326A

Aluminium 75 °C

225A

Max continuous load

231A

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	245 A	289 A	326 A	250 A
Aluminium	191 A	225 A	254 A	225 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	326 A → 250 A	298 A → 250 A	266 A → 250 A	230 A → 225 A
6	261 A → 250 A	238 A → 225 A	213 A → 200 A	184 A → 175 A
9	228 A → 225 A	208 A → 200 A	186 A → 175 A	161 A → 150 A
15	163 A → 150 A	149 A → 125 A	133 A → 125 A	115 A → 110 A
25	147 A → 125 A	134 A → 125 A	120 A → 110 A	104 A → 100 A

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

Source & method: Continuous-load example: a 231 A continuous load needs 289 A of design current and a 300 A device. Size your own load.

Neighbouring sizes

Smaller250 kcmil257 A copper at 75 °C Larger350 kcmil319 A copper at 75 °C

Frequently asked

What breaker size for 300 kcmil wire?: 300 kcmil copper at 75 °C terminations supports up to a 250 A overcurrent device, based on an estimated ampacity of 289 A. Aluminium of the same size supports less: about 225 A.
How many amps can 300 kcmil handle?: Estimated 245 A at 60 °C, 289 A at 75 °C and 326 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.