Boxes, enclosures, and raceways
Scope
Boxes, enclosures, and raceways are tested through Article 314 and Chapter 9. You need to size boxes for conductors and devices, size pull boxes for conductors, and fill raceways without exceeding permitted percentages.
Core rules
- Box fill (314.16) counts current-carrying conductors, equipment grounding conductors as one based on the largest, devices as two conductors of the largest size, and fittings or clamps where present.
- Volume allowances: 14 AWG = 2.0 cu in, 12 AWG = 2.25 cu in, 10 AWG = 2.5 cu in per conductor.
- A 4 x 4 x 1.5 in box has a marked volume of 24 cu in; use the marked value, not outside dimensions.
- Pull-box sizing (314.28): straight pulls require 8 times the largest raceway trade size; angle pulls require 6 times, plus distance between entries.
- Raceway fill: 40% for three or more conductors; 31% for two; 53% for one. Use Chapter 9, Table 1 and Table 4.
- Enclosures must be suitable for the location: NEMA 1 indoors, NEMA 3R outdoors, NEMA 4/4X wet or corrosive.
Worked example
A box contains six 12 AWG current-carrying conductors, one 12 AWG equipment ground, and one duplex receptacle. Count: six conductors x 2.25 = 13.5 cu in; ground = 2.25; device = 2 x 2.25 = 4.5. Total = 20.25 cu in. A 4 x 4 x 1.5 in box with 24 cu in marked volume is adequate. A 4 x 4 x 1.25 in box at 20 cu in is not.
Traps
Forgetting the device count; counting each ground separately; using the box outside dimensions instead of marked volume; ignoring clamps; and using 40% fill for a single conductor.