Motor and branch-circuit overload protection is selected to trip based on a percentage of which of the following?

The correct answer is based on the motor nameplate full-load current. This value represents the maximum current that the motor is designed to carry continuously under full load conditions without overheating. Overload protection devices are set to trip when the current reaches a certain percentage of this full-load current to prevent the motor from drawing excessive current, which can lead to thermal damage and failure.

By using the full-load current indicated on the motor's nameplate, code and safety standards ensure that the protection mechanism is accurately calibrated to the motor's specifications. This approach helps achieve reliable operation and enhances safety by preventing overload conditions that could result in equipment failure or fire hazards.

The other options do not directly pertain to the selection of overload protection based on standard practices laid out in the National Electrical Code (NEC). Amperes based on motor horsepower, while related, do not provide the same specificity as the motor nameplate's full-load current. Average circuit load and total system capacity refer to broader shifts in load and overall system design rather than the specific parameters necessary for setting protection for an individual motor.