How do you find the current rating of a circuit breaker?

A circuit breaker, like a switch, turns power on and off by moving one conductor to touch or not touch another conductor. The gap when not touching has to be large enough that electrons cannot pass through the air to the other conductor, producing a spark. The larger the voltage, the farther it can pass through the air, and the larger the gap must be.

So the rated voltage of a circuit breaker or a fuse is the largest voltage it can safely control.