5hp 230v single phase motor popping 30amp breaker
NEC Table 430.245 states to size 5HP motor for 28A at 230V. Add 25% for start up and the circuit should be sized for 35 amps minimum. That would be #8 copper conductors with a 40A or 45A breaker. 

The only way you could verify it is to check the amperage with an ammeter at start up and see if it is over 30A. That's probably is what is tripping your breaker.