The mounting holes in a metal box for attaching equipment grounding conductors are tapped for which screws?

• A. No. 8-32
• B. No. 10-32
• C. No. 6-32
• D. No. 12-24

Answer: (B)

The holes in a metal electrical box that are used to attach the grounding conductor are provided specifically to accept a grounding screw. These holes are tapped to take a ground screw of the standard size used for bonding metal boxes to the circuit’s equipment grounding conductor. Using this size ensures a secure mechanical fit and a solid, low-impedance electrical bond between the box and the grounding conductor, which is essential for safely directing fault currents to ground. The green grounding screw (and any bonding clamp or lug used) is installed into that tapped hole to complete the bond.