HOLT Electricians Practice Exam

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The motor short-circuit ground-fault protection device is sized up to what percentage?

200%
225%
250%
275%

The correct answer is: 250%

The correct sizing for a motor short-circuit ground-fault protection device is typically set at 250% of the motor's full-load current rating. This sizing criterion is established in the National Electrical Code (NEC), particularly in Section 430, which deals with the protection of motors. The purpose of this percentage is to allow for the inrush current that occurs when a motor starts, while still providing adequate protection against short circuits and ground faults. When motors start, they can draw significantly higher currents—often up to 600% or more of their full-load current—during the initial moments of operation. Setting the protection device at 250% helps to accommodate this inrush current while ensuring that the device will trip in the event of a real fault, thereby protecting the motor from potential damage. This balance is crucial for maintaining both operational efficiency and safety in electrical installations.