Journeyman Electrician Practice Exam

For a motor with a temperature rise of 46°C, what percentage of the motor full-load current rating should the overload device be selected to trip at?

1. 110 percent

2. 115 percent

3. 120 percent

4. 125 percent

The correct answer is: 115 percent

To determine the appropriate percentage of the motor full-load current rating at which the overload device should trip, it's important to consider the temperature rise and its relation to motor performance. For a motor with a temperature rise of 46°C, the National Electrical Code (NEC) generally advises setting the overload protection devices to trip at a level that balances the protection of the motor against nuisance tripping during normal operation. A temperature rise of 40°C to 49°C indicates a motor running at or near its maximum designed load, reflecting that it may be subject to higher thermal stress. Choosing 115 percent of the full-load current rating for tripping the overload device allows for a small margin above the motor's rated capacity, accommodating initial inrush currents and temporary overloads without risking overheating or damaging the motor. This setting aligns with industry standards for motor protection, ensuring that the overload device trips before the motor reaches a temperature that may cause long-term damage. Lower settings might not provide enough coverage for temporary surges, while higher settings could risk allowing too much current to flow, leading to excessive heat and potential motor failure. Therefore, selecting 115 percent effectively balances safety and operational efficiency.