A switch box will contain an isolated ground receptacle and a 12/2 with ground AC cable with exterior connector. The shallowest switch box you can use for this installation is 3" × 2" × ___ inches.

A switch box will contain an isolated ground receptacle and a 12/2 with ground AC cable with exterior connector. The shallowest switch box you can use for this installation is 3" × 2" × ___ inches.

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

Answer: (C)

Box-fill is the idea you must understand here. Each conductor that enters the box from outside and is either terminated or spliced inside counts as a fill unit, and all equipment grounding conductors in the box count as one unit together. A single yoke (the device strap) also counts as one fill unit. For a 12 AWG circuit, each fill unit requires 2.25 cubic inches. In this installation you have two current-carrying conductors (hot and neutral) plus two grounding conductors present: the equipment grounding conductor coming with the 12/2 plus an isolated grounding conductor that runs to the receptacle. The isolated grounding conductor is treated as a separate conductor for fill purposes, since it is not bonded to the metal box. The receptacle’s yoke also counts as one fill unit. That totals six fill units (two hots/neutrals, two grounds, plus the device strap and the isolated ground conductor). Six times 2.25 equals 13.5 cubic inches of required box volume. The box in question has a face area of 3 inches by 2 inches, which is 6 square inches; to meet 13.5 cubic inches, you need a depth of 13.5 ÷ 6 = 2.25 inches. In standard box depths, the next available depth that safely accommodates the arrangement is 2 1/2 inches. That is why the shallowest suitable box depth for this setup is 2 1/2 inches.

Box-fill is the idea you must understand here. Each conductor that enters the box from outside and is either terminated or spliced inside counts as a fill unit, and all equipment grounding conductors in the box count as one unit together. A single yoke (the device strap) also counts as one fill unit.

For a 12 AWG circuit, each fill unit requires 2.25 cubic inches. In this installation you have two current-carrying conductors (hot and neutral) plus two grounding conductors present: the equipment grounding conductor coming with the 12/2 plus an isolated grounding conductor that runs to the receptacle. The isolated grounding conductor is treated as a separate conductor for fill purposes, since it is not bonded to the metal box. The receptacle’s yoke also counts as one fill unit. That totals six fill units (two hots/neutrals, two grounds, plus the device strap and the isolated ground conductor).

Six times 2.25 equals 13.5 cubic inches of required box volume. The box in question has a face area of 3 inches by 2 inches, which is 6 square inches; to meet 13.5 cubic inches, you need a depth of 13.5 ÷ 6 = 2.25 inches. In standard box depths, the next available depth that safely accommodates the arrangement is 2 1/2 inches. That is why the shallowest suitable box depth for this setup is 2 1/2 inches.