Circuit capacity depends on the voltage and current ratings of wires and circuit breakers. When a circuit is rated for 15 amps, the wires and circuit breakers must be rated for at least 15 amps. The circuit is rated for the lowest current rating of wire or circuit breaker. For example, if the wire is rated for 15 amps, the circuit should not have a 20 amp breaker even if the breaker trips frequently.
It is very important that a 20 amp (or higher) circuit breaker NOT be used on a circuit with a wire rated for 15 amps or less.