A motor has two forms of protection. One for the over load and one for the short circuit-ground fault. The overload could be an integral part of the motor or starter. If not you must use the service factor or temperature rise rating to determine the overload separately. The short circuit-ground fault is based on the full load current multplied by a factor depending on what type of overcurrent protection device that you use. The full load current is based on the phases and horsepower using the the tables in artilce 430 of the national electrical code.