When stripping NM cable in a metal box, at least____ of sheath should be extended into the box.

• A. 1/8 inch
• B. 3/8 inch
• C. 1/2 inch
• D. 1/4 inch

Answer: (D)

The key idea here is how the NM cable is secured and protected as it passes into a metal box. The outer jacket must extend into the box far enough to be captured by the cable clamp, so the clamp can grip the sheath rather than the copper insulation. Extending the sheath at least a quarter of an inch into the box provides a proper grip for the clamp and keeps the conductors insulated from the box edge where they enter the box. If you strip less than this, the clamp may not hold securely or could damage the conductor insulation; stripping more isn’t necessary and can crowd the box or complicate connections. Therefore, a quarter inch is the appropriate minimum.