What is a 30 amp circuit breaker used for?

A 30 amp circuit breaker is needed unless the load is a motor circuit, then it has to be sized to 250 percent of the motors full load amps.

For 30A circuit is needed breaker 30*1.25=37.5 --> 40A.

If load is lamp or heater, then use breaker of group A or B.

If load is motor, then use breaker of groupC or D (very hard start) or special safe breaker for motor - with variable amp setpoint.

See related link also.

By code you are only allowed 80% of the rating of a breaker. So 30x80%=24amps.

24 amps is the MAX allowed on a 30 amp breaker. You would need a 40amp breaker for a 30 amp circuit. 40x80%=32. So you would want a double pole 40amp breaker.