240v AC Circuit Breaker for 24v AC
Whatever amp rating is on your breaker will hold true up to its voltage limit of 240 VAC. It is sensing only AC current, not the line voltage, and it does not care what the AC source is at long as it does not exceed the 240 VAC limit.

24 VAC is a bit low but enough so the breaker contacts behave correctly. They may not conduct current if the source voltage is just a few volts AC. If you have a datasheet on the breaker it should state minimum and maximum VAC limits.