Box Installation: Pull-Box Sizing, Setback, and Where Boxes Are Required

Boxes protect connections and give room to pull conductors. Beyond box fill (covered earlier), you need to know how to size large pull boxes, how far a box may sit back from the wall, and where a box is required at all.

1. Sizing pull and junction boxes — 314.28

For boxes and conduit bodies with conductors 4 AWG and larger:

• Straight pulls: box length ≥ 8 × the trade size of the largest raceway. (CEC 314.28(A)(1))
• Angle or U pulls: distance from each raceway entry to the opposite wall ≥ 6 × the largest raceway (plus the sum of the other raceway diameters on the same wall). (CEC 314.28(A)(2))

Bigger raceways need bigger boxes so conductors aren't damaged during the pull.

2. Flush-box setback — 314.20

How far a flush box may sit back from the finished wall depends on the wall material:

• Noncombustible surface (drywall, plaster, tile, concrete): front edge set back no more than 1/4 in. (CEC 314.20)
• Combustible surface (wood): the box must be flush with, or project from, the finished surface. (CEC 314.20)