Boxes, Raceways, and Conductor Fill
Article 314 governs boxes: volume for conductors, devices, and clamps; pull-box dimensions; and conductor length. Chapter 9 supplies the fill tables that pair with it.
Box fill under 314.16
- Table 314.16(B) volume per conductor: 14 AWG = 2.00 in³, 12 = 2.25, 10 = 2.50, 8 = 3.00, 6 = 5.00.
- Count every ungrounded and grounded conductor once. All equipment grounding conductors together count as one allowance based on the largest.
- One allowance for one or more cable clamps, one for a support fitting, and two for a device or equipment based on the largest conductor connected.
- Compare the total to the cubic inches stamped on the box (Table 314.16(A)), not to its outside dimensions.
Worked example
A metal box with two NM cables (two conductors plus ground each), a single receptacle, and two cable clamps. Conductors: 4 × 2.25 = 9.00. EGCs: one allowance = 2.25. Clamps: one allowance = 2.25. Device: two allowances = 4.50. Total 18.00 in³. A 3 × 2 × 3½ in device box is stamped 18.0 in³, so it is the minimum; a 4 × 4 × 1½ in box (21.0 in³) gives margin.
Pull boxes and conductor fill
- 314.28: for conductors 4 AWG and larger, a straight pull needs 8× the largest raceway trade diameter, an angle pull 6×, and the distance between raceway entries must meet the same minimums.
- Chapter 9 Table 1: 53% for one conductor, 31% for two, 40% for more than two.
- 300.14: at least 6 in of free conductor, extending at least 3 in outside the box opening.
Traps
- Counting each EGC separately instead of as one allowance.
- Forgetting the clamp allowance in a metal box.
- Using 314.16 for 4 AWG and larger conductors, which go to 314.28.