According to the NEC, when must the frame of a stationary motor be grounded?

The correct answer relates to the specific safety requirements outlined in the National Electrical Code (NEC) regarding the grounding of stationary motors. When a motor is supplied by metal-enclosed wiring, grounding the motor frame is necessary to ensure safety and prevent electrical shock hazards. Metal-enclosed wiring can conduct electricity, and if a fault occurs, that electricity could energize the motor housing, posing a risk to anyone who comes into contact with it. Grounding the motor frame provides a safe path for any fault current to travel back to the ground, thereby protecting both the equipment and individuals from electrical hazards.

In contrast, other options present scenarios that do not necessitate grounding under NEC guidelines. For example, motors that use non-conductive materials or are encased in non-metallic conduits (as mentioned in the other choices) do not create the same risk, as they typically do not allow for the conduction of fault currents that could be harmful. Understanding these distinctions is crucial for ensuring the safe operation of electrical equipment and compliance with regulatory standards.