Motor and branch-circuit overload protection is designed to trip based on a percentage of what quantity?

• A. Amperes based on motor horsepower in the motor ampacity tables located in Article 430
• B. Motor nameplate full-load current
• C. Motor starting current
• D. Maximum continuous current rating

Answer: (B)

Overload protection for motors is fundamentally about protecting the winding from overheating under load. The device is designed to trip when current exceeds a threshold that reflects how much heating the motor can safely tolerate at its rated load. That threshold is based on the motor’s own full-load current as listed on the nameplate (the FLA). Using the nameplate full-load current provides a motor-specific reference so the protection responds appropriately to each motor’s design, regardless of horsepower or other factors.

Starting current is intentionally treated differently because it’s a short inrush during startup, not a sustained overload. The protection uses a time-delayed response to allow that transient surge without nuisance tripping. Ampacity tables and horsepower alone don’t define the trip threshold for overload protection, since they relate more to conductor sizing and relative motor power rather than the actual current the motor is designed to carry at full load. The maximum continuous current rating also isn’t the trigger for overload trip; it’s a rating context, not the protective setting.

So the trip is based on a percentage of the motor’s nameplate full-load current to align protection with the motor’s rated heating and performance.