I was told as a rule of thumb, that a conventional, old style 18000 BTU a/c takes 1800 watts when running, which is about 8.2 AMPS.
It can take double that to start up, so 16.5 amps maximum.. Inverter models do not need the high current to start up.
Your 30 amp breaker should be plenty. The breaker is there to protect the wiring, not the a/c.