Oklahoma Electrical Practice Exam

According to the NEC, how many running overload relays are necessary for a 3-phase motor without other approved protection?

1. 1

2. 2

3. 3

4. 4

The correct answer is: 3

For a three-phase motor, the NEC (National Electrical Code) requires that a minimum of three running overload relays be provided when there is no other approved protection in place. This requirement is rooted in the need for effective motor protection to prevent overheating, which can occur if any of the phases are carrying overload current. Utilizing three overload relays allows for monitoring each phase of the motor operation independently. This independent monitoring is crucial because an overload condition could affect just one of the three phases while the others may be operating correctly. By having three relays, each relay can trip individually if it detects an overload condition specific to its phase, thus enhancing the safety and reliability of the motor's operation. The requirement for three running overload relays ensures that adequate protection is provided against conditions that could lead to motor failure, thereby extending the life of the motor and improving overall system safety.