Boxes, Raceways, Conductors, and Cable Fill
Boxes, Raceways, Conductors, and Cable Fill
This domain tests Articles 300, 310, 314, and Chapter 9: box fill, conduit fill, conductor ampacity, derating, pull boxes, and support.
Core rules
- Box fill is calculated by counting conductors, clamps, devices, and grounds using Table 314.16(B) volumes; grounds count once.
- Conduit fill: 40% for three or more conductors; 60% for nipples; 53% for two conductors.
- Pull-box sizing for straight pulls: 8 × trade size; angle pulls use 6 × trade size plus distance rules.
- Conductor ampacity comes from Table 310.16; derating applies for ambient temperature and more than three current-carrying conductors in a raceway.
- Raceways must be supported and secured per their article; ENT, PVC, and EMT differ.
Worked example
A box contains 8 current-carrying 12 AWG conductors, 1 clamp, 1 device, and 1 ground. Using 2.25 in³ per 12 AWG: 8 × 2.25 = 18.0; clamp = 2.25; device = 4.5 (two wire equivalents); ground = 2.25. Total = 27.0 in³. A 4 × 4 × 2⅛ in box has 30.3 in³, so it is adequate. If the box were only 21 in³, it would fail.
Traps
- Counting each ground conductor separately.
- Using 40% fill for a nipple.
- Forgetting derating when adding conductors to a raceway.
- Sizing pull boxes by wire count instead of trade size.