When using cable assemblies with nonmetallic sheaths, the sheath must extend at least:

The requirement for nonmetallic sheathed cables is intended to ensure that the sheath remains intact and provides protection to the conductors. The correct distance for the sheath to extend inside the box is a minimum of 1/4 inch. This ensures that the cable assembly is secured and protected from any potential abrasion or damage that might occur if the conductors were left exposed.

This specification helps to maintain the integrity of the electrical installation and ensures compliance with the National Electrical Code (NEC), which emphasizes that the cable must be properly secured to provide safe and reliable operation. Extending the sheath a minimum of 1/4 inch into the box helps prevent any sharp edges or internal components from damaging the conductors over time.