According to NEC 300.4(F), what is the minimum AWG size for ungrounded circuit conductors that must use an insulating bushing?

In the National Electrical Code (NEC), specifically under section 300.4(F), the requirement for the use of insulating bushings is outlined to protect ungrounded conductors when they are passing through a knockout or other potentially damaging points in electrical equipment. The NEC specifies that for ungrounded conductors, the minimum size that requires an insulating bushing is 4 AWG.

This requirement is in place to ensure the safety and longevity of the conductors. Conductors smaller than this size may be less susceptible to mechanical damage as they are generally more flexible and less rigid. However, once the conductors reach the 4 AWG size, the risk of damaging the insulation increases significantly, which necessitates the protection that an insulating bushing provides. Therefore, choosing 4 AWG as the answer aligns with the code requirements for protecting larger conductors from potential physical damage during installation and operation.