Can you use a 30 amp breaker when you have need for only 20 amps?

The question is one of protection of conductors (wires). A 30 amp breaker must have a #10 copper conductor or larger for adequate protection. A smaller wire (larger number) such as #12, is not protected by a 30 amp breaker. If the equipment you are powering only requires 20 amps, using a 30 amp breaker and #10 AWG wire is acceptable provided all other elements of the circuit are listed for 30 amp circuits.