Six metal device boxes ganged together to form a 6-gang box. One GFCI receptacle, four single-pole switches, and one motion switch will be installed in the box assembly. The box assembly contains four ungrounded conductors, five grounded conductors, five switch legs, and six grounding conductors. Five cable clamps are contained in the assembly. All the conductors are 12 AWG. Each of the ganged boxes is required to be 3" × 2" × ___ inches.

Six metal device boxes ganged together to form a 6-gang box. One GFCI receptacle, four single-pole switches, and one motion switch will be installed in the box assembly. The box assembly contains four ungrounded conductors, five grounded conductors, five switch legs, and six grounding conductors. Five cable clamps are contained in the assembly. All the conductors are 12 AWG. Each of the ganged boxes is required to be 3" × 2" × ___ inches.

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

Answer: (C)

Box fill is the key concept here. NEC box-fill rules say you must provide enough cubic inches inside a box to accommodate all conductors that originate outside and terminate or splice inside, plus all equipment grounding conductors treated as a single contribution, internal clamps, and the yokes (straps) of devices mounted in the box. For this 6-gang assembly, you count: - Conductors entering from outside and terminating/splicing inside: four ungrounded conductors, five grounded conductors, and five switch legs create a substantial number of conductor equivalents. - All equipment grounding conductors together count as one conductor. - Internal clamps: there are five clamps, each adds to the fill. - Device yokes: with six devices mounted in the box, the yokes occupy space in the box and contribute to fill. When you total these up for #12 conductors (2.25 cu in per conductor) and compare to the available volume of the six-gang box (each of the six boxes is 3" by 2" in cross-section, so total internal volume is 6 × 3 × 2 × depth, i.e., 36 × depth cubic inches), the minimum depth needed to satisfy NEC box-fill becomes 2 1/2 inches. At that depth, the box provides 90 cubic inches (36 × 2.5), which comfortably accommodates the required conductor equivalents, clamps, and device yokes for this six-device, six-gang installation. So, the box must be 2 1/2 inches deep to meet the NEC box-fill requirements for this arrangement.

Box fill is the key concept here. NEC box-fill rules say you must provide enough cubic inches inside a box to accommodate all conductors that originate outside and terminate or splice inside, plus all equipment grounding conductors treated as a single contribution, internal clamps, and the yokes (straps) of devices mounted in the box.

For this 6-gang assembly, you count:

• Conductors entering from outside and terminating/splicing inside: four ungrounded conductors, five grounded conductors, and five switch legs create a substantial number of conductor equivalents.

• All equipment grounding conductors together count as one conductor.

• Internal clamps: there are five clamps, each adds to the fill.

• Device yokes: with six devices mounted in the box, the yokes occupy space in the box and contribute to fill.

When you total these up for #12 conductors (2.25 cu in per conductor) and compare to the available volume of the six-gang box (each of the six boxes is 3" by 2" in cross-section, so total internal volume is 6 × 3 × 2 × depth, i.e., 36 × depth cubic inches), the minimum depth needed to satisfy NEC box-fill becomes 2 1/2 inches. At that depth, the box provides 90 cubic inches (36 × 2.5), which comfortably accommodates the required conductor equivalents, clamps, and device yokes for this six-device, six-gang installation.

So, the box must be 2 1/2 inches deep to meet the NEC box-fill requirements for this arrangement.