Boxes, Fittings, Switches and Receptacles
What the exam targets
Box fill (314.16), pull-box sizing (314.28), device rules in Articles 404 and 406, receptacle spacing and GFCI/AFCI requirements, and luminaire support.
Concepts to own
- Box fill counting: each conductor that enters and terminates counts once; conductors passing through without splice count once. Equipment grounding conductors count as one volume based on the largest. Cable clamps get one allowance. Each yoke or strap containing a device counts as two conductor volumes based on the largest conductor connected.
- Volume per conductor: 14 AWG = 2.00 cu in, 12 AWG = 2.25, 10 AWG = 2.50. Box volumes come from Table 314.16(A) or the marked volume.
- 210.52 receptacle spacing and 210.8 GFCI locations (bathrooms, kitchens, garages, outdoors, laundry, unfinished basements, within 6 ft of a sink). 210.12 covers AFCI protection. 406.9 weather-resistant and 406.12 tamper-resistant receptacles.
- 404.2(C): a neutral conductor is generally required at switch locations in habitable rooms.
Traps
Forgetting cable clamps; counting each grounding conductor separately; using the smallest rather than the largest conductor size when a box contains mixed sizes; forgetting the device yoke's two-volume allowance; confusing the 6 ft (distance from a sink or tub) rule with the 12 ft wall-spacing rule.
Worked example
A 4 × 4 × 1.5 in box is 21.0 cu in. With 14 AWG: four circuit conductors + one EGC allowance + one clamp allowance + one device (2 volumes) = 8 allowances × 2.00 = 16.0 cu in, which fits within 21.0 cu in. Adding one more 14 AWG conductor would bring it to 18.0 cu in, still acceptable.