Journeyman Electrician Practice Exam

With a computed allowable ampacity of 75 amperes, what is the maximum ampere rating for the overcurrent protection device?

1. 75 amperes

2. 80 amperes

3. 90 amperes

4. 100 amperes

The correct answer is: 80 amperes

When determining the appropriate ampere rating for an overcurrent protection device, it is essential to reference the National Electrical Code (NEC) guidelines regarding the sizing of conductors and protection devices. For a computed allowable ampacity of 75 amperes, the recommended maximum rating for the overcurrent protection device is typically permitted to be greater than the allowable ampacity of the conductors, but it cannot exceed 125% of that value, which helps ensure the protection device operates effectively without nuisance tripping. In this scenario, 125% of 75 amperes equals 93.75 amperes. The next standard rating above this figure is 100 amperes. However, for effective system protection, under certain circumstances, it is acceptable to select a standard rating that is less than this calculated maximum, yet still above the conductor's ampacity. Choosing an 80 ampere overcurrent protection device is compliant with NEC standards, as it allows for a bit of overhead for temporary load increases, while still providing protection suitable for the conductors’ rated capacity. Therefore, an 80 ampere device can be considered an appropriate choice to achieve a balance between safety and efficiency.