Boxes, Fittings, and Enclosures
Boxes exist to contain splices, support devices, and protect conductors. Two questions recur: does the box have enough volume, and is it accessible and rated for the location?
Box fill
Each box has a marked volume. The Code assigns a volume per conductor based on size, and you total: all conductors that enter and terminate in the box, one for each conductor passing through unbroken (if counted by the applicable rule), one for all equipment grounding conductors combined, one for internal cable clamps, and a set count for each yoke-mounted device or fitting. The total must not exceed the box volume. The trap is forgetting devices and clamps, or using the smallest conductor to select the per-conductor volume when the largest conductor present governs.
Enclosures and fittings
Enclosure types (NEMA or equivalent ratings) describe resistance to dust, water, and corrosion. A damp or wet location needs a listed enclosure and properly oriented hubs and gaskets. Boxes must remain accessible unless the Code specifically permits concealment, and boxes in fire-rated assemblies need the correct rating. Bonding jumpers, reducing washers, and listed fittings keep the enclosure electrically continuous.
Traps
- Ignoring the volume taken by a large device or a GFCI.
- Concealing a junction box above a finished ceiling.
- Using an ordinary fitting outdoors.
- Mixing metal and non-metallic components without proper bonding.
Rule of thumb
Count generously and compare against the stamped volume before adding a splice; if the arithmetic is close, use a larger box.