Boxes, Raceways, Panelboards, and Equipment Installation
Core concepts
Box fill (314.16) is a volume calculation. Each conductor counts once; each cable clamp counts as one conductor of the largest size; each device or strap counts as two conductors of the largest size attached; one or more equipment grounding conductors count as a single conductor of the largest EGC. Multiply each count by the cubic-inch value in Table 314.16(B) - 2.00 for 14 AWG, 2.25 for 12 AWG, 2.50 for 10 AWG - and compare to the box volume stamped on the box.
Pull boxes are sized by 314.28. For a straight pull, the box length is eight times the trade size of the largest raceway. For an angle or U pull, distance from the raceway entry to the opposite wall is six times the largest raceway plus the sum of the other raceway sizes on the same wall.
Working clearance at equipment is set by 110.26: depth based on voltage to ground (3 ft. for 0–150 V), width of the equipment or 30 in. whichever is greater, 6.5 ft. of height, and a dedicated equipment space above and in front of the panel that must remain clear. Panelboards require overcurrent protection sized to the busbar rating (408.36).
Traps
- Counting a duplex receptacle as two devices for box fill; it counts as two conductors total.
- Forgetting the clamp allowance or counting each EGC separately.
- Using the largest raceway alone for angle pulls and skipping the addition of the other raceways.
- Ignoring the dedicated space above a panelboard where foreign piping is prohibited.
Worked example
A box contains six 12 AWG conductors, two cable clamps, two receptacles, and one EGC. Fill = 6 × 2.25 (conductors) + 2.25 (clamps) + 4 × 2.25 (two devices × two conductors each) + 2.25 (EGC) = 29.25 cu. in. A 4 × 4 × 1½ box at 21 cu. in. is too small; a 4-11/16 × 1½ box at 30.3 cu. in. works.