The breaker protects the wire, not the appliance

A breaker is sized to the ampacity of the wire it feeds, so the wire can never carry more than it is rated for; the load only decides whether the circuit is big enough, never how big the breaker may be.

Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.

What it is for

A 15 A breaker keeps tripping on a space heater, so somebody swaps it for a 20 A. The wire behind it is still 14 AWG, rated 15 A. Now the wire can carry 20 A all day with nothing to stop it, and the fix for a nuisance trip has become a fire.

How to think about it

Start from the wire that is there, or the wire you will pull. Its ampacity is the ceiling for the breaker. Then check the load fits under that ceiling, with 125 percent if it is continuous. If it does not fit, the answer is a bigger wire and a bigger breaker together, never a bigger breaker alone.

Worked example

Wire: 12 AWG copper is rated 20 A

The ceiling. Whatever the load, the breaker cannot exceed this.

Load: 1400 W at 120 V, I = 1400 / 120 = 11.7 A

Fits comfortably.

Breaker: 20 A

Sized to the wire. The load fitting under it is the check, not the choice.

Load grows to 22 A: not a 25 A breaker on 12 AWG

The wire is still 20 A. Pull 10 AWG, then a 30 A breaker; or split the load onto two circuits.

Your turn

A circuit is wired in 14 AWG copper. Write the biggest breaker allowed on it.

breaker =  A

The trap

Sizing the breaker to the load. The load is why the circuit exists; the wire is what the breaker is guarding. A breaker one size too big is invisible until the day it matters.

Practise sizing a breaker on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.