Journeyman Electrician Practice Exam

What is the minimum ampacity percentage required for branch-circuit conductors supplying multiple motors?

1. 100%

2. 110%

3. 125%

4. 150%

The correct answer is: 125%

The minimum ampacity percentage required for branch-circuit conductors supplying multiple motors is based on the NEC (National Electrical Code) guidelines. When calculating the ampacity for conductors serving multiple motors, the code specifies that the conductors must be sized to at least 125% of the largest motor's full load current plus the full load currents of the other motors in the circuit. This ensures that the conductors can handle the maximum anticipated load without overheating and to prevent voltage drop that could adversely affect motor performance. Using 125% helps to account for the inrush current that occurs when motors start, which is significantly higher than their running load. This ensures that the conductors are adequately rated to handle not just the continuous load but also the temporary spikes in current associated with motor operation. Choosing 100% or lower percentages would not provide sufficient capacity for heating or the additional starting currents that motors typically experience. A higher percentage like 150% goes beyond the necessary allowance, which could lead to oversizing, unnecessarily increasing costs and complications in installation. Thus, 125% is the precise and calculated minimum requirement for such applications, ensuring both safety and functionality.