NEC Breaker Selection for Motor Loads
the motor overload is based on the FLA of the motor 115% and this protects the motor, in the US we normaly use class 10 overloads this measn the motor can draw 115% of the full load value on the motor nameplate for 10 seconds before it will trip out. 430.32 the conductor size is 125% these are minimum values the OCPD can be increased if needed for starting as motors generaly draw 200-800% of full load current on startup if the breaker is set two low it will trip as the time of the high current draw is only a few seconds the wiring dosent get hot but if there is a locked rotor/ or shorted phase the motor will draw enough to trip the breaker.
so the motor overloads protect the motor, circuit breakers protect the branch conductors, there are many kinds of overloads the 2 major types are electronic , and heaters, the electronic measure the current and many times are adjustable, the heaters have a small coil that is heated when it gets hot the metal in the "operator" melts and a small ratchet is released opening a switch contact that is in line with the "starter" the relay that applys line power to the motor , once the overload is cleared the heater cools and it can be reset, these are adjusted by changing the coil size that heats the "operator"