Which box and plaster ring combination is used in the two-gang installation described in the first scenario to meet the NEC volume requirement?

Which box and plaster ring combination is used in the two-gang installation described in the first scenario to meet the NEC volume requirement?

• A. 4 × 2 1/8 square box with a 4.1 in3 two-gang plaster ring
• B. 4 × 21/8 square box with a 4.1 in3 two-gang plaster ring
• C. 4 × 1 1/2 square with a 4.1 in3 two-gang plaster ring
• D. 4 × 11/2 square with a 4.1 in3 two-gang plaster ring

Answer: (A)

Box fill is the key idea here. NEC requires you to add up the volume needed for every conductor that enters the box, plus any clamps and the volume taken by the devices themselves (the yokes). A plaster ring contributes additional internal volume to the box, and when you’re dealing with a two-gang installation, you must have enough total cubic inches to cover both the conductors and the two devices mounted on the yokes. The two-gang plaster ring in this scenario provides 4.1 cubic inches of volume. The chosen base box must supply enough base volume so that, when you add the ring’s 4.1 in³, the total meets or exceeds the box-fill required for the two devices and their conductors. The 4 × 2 1/8 square box has sufficient interior capacity (more base volume than the smaller options) so that, with the 4.1 in³ ring, the overall box fill meets NEC requirements for the described two-device installation. The other box sizes would not provide enough total volume even after adding the ring. So, using the 4 × 2 1/8 square box together with a 4.1 in³ two-gang plaster ring is the correct choice because it yields the needed total cubic inches for fill.

Box fill is the key idea here. NEC requires you to add up the volume needed for every conductor that enters the box, plus any clamps and the volume taken by the devices themselves (the yokes). A plaster ring contributes additional internal volume to the box, and when you’re dealing with a two-gang installation, you must have enough total cubic inches to cover both the conductors and the two devices mounted on the yokes.

The two-gang plaster ring in this scenario provides 4.1 cubic inches of volume. The chosen base box must supply enough base volume so that, when you add the ring’s 4.1 in³, the total meets or exceeds the box-fill required for the two devices and their conductors. The 4 × 2 1/8 square box has sufficient interior capacity (more base volume than the smaller options) so that, with the 4.1 in³ ring, the overall box fill meets NEC requirements for the described two-device installation. The other box sizes would not provide enough total volume even after adding the ring.

So, using the 4 × 2 1/8 square box together with a 4.1 in³ two-gang plaster ring is the correct choice because it yields the needed total cubic inches for fill.