What breaker size for 18 AWG wire?

18 AWG is below the smallest standard branch-circuit device rating and is normally used for control, signal or fixture wiring.

How many amps can 18 AWG handle?

Estimated 8 A at 60 °C, 10 A at 75 °C and 11 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.

18 AWG Breaker Size — Amps & Overcurrent Rating

18 AWG copper is estimated at 10 A at 75 °C, which supports a — A overcurrent device. Breaker pairing, continuous-load sizing and derating shown.

Quick answer

18 AWG copper pairs with a — A breaker at 75 °C terminations, from an estimated ampacity of 10 A. Aluminium of the same size is about 8 A. A continuous load on that circuit should not exceed 8 A, since continuous loads are sized at 125%.

Max breaker (Cu, 75 °C)

—A

Copper 60 / 75 / 90 °C

8 / 10 / 11A

Aluminium 75 °C

Max continuous load

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	8 A	10 A	11 A	—
Aluminium	6 A	8 A	9 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	11 A → —	10 A → —	9 A → —	8 A → —
6	9 A → —	8 A → —	7 A → —	6 A → —
9	8 A → —	7 A → —	6 A → —	5 A → —
15	6 A → —	5 A → —	4 A → —	4 A → —
25	5 A → —	5 A → —	4 A → —	3 A → —

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

Source & method: Continuous-load example: a 8 A continuous load needs 10 A of design current and a 15 A device. Size your own load.

Neighbouring sizes

Larger16 AWG13 A copper at 75 °C

Frequently asked

What breaker size for 18 AWG wire?: 18 AWG is below the smallest standard branch-circuit device rating and is normally used for control, signal or fixture wiring.
How many amps can 18 AWG handle?: Estimated 8 A at 60 °C, 10 A at 75 °C and 11 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.