HOLT Electricians Practice Exam

What percentage of conductor ampacity must motor control circuit conductors have for short-circuit and ground-fault protection in Table 430-72(B)?

1. 125 percent

2. 200 percent

3. 250 percent

4. 300 percent

The correct answer is: 300 percent

Motor control circuit conductors must have a minimum ampacity of 300 percent for short-circuit and ground-fault protection according to Table 430.72(B) in the National Electrical Code (NEC). This requirement ensures that the conductors can safely handle short-circuit currents which are significantly higher than the normal operating currents of the motor. By specifying a percentage as high as 300 percent, the code aims to prevent damage to the conductors themselves and reduce the risk of fires, equipment failure, or personal injury which can result from electrical faults. In practical terms, this means that if a motor has a specific full-load current rating, the ampacity of the conductors must be calculated based on three times that rating, thereby providing a robust safety margin for unexpected fault conditions. The high percentage reflects the critical nature of the applications involved and the inherent risks associated with electrical faults in motor control circuits.