Wiring Methods, Materials, and Installation Requirements
Chapter 3 governs installation
Wiring methods are Articles 320 through 362, general installation rules are Article 300, boxes and fill are Article 314, switches are 404, receptacles are 406, and luminaires are 410. Know which method is permitted where: NM cable (334) is for dry locations and may not be subject to physical damage; UF (340) is the wet and underground cable; EMT (358) is permitted in concrete and in wet locations; PVC (352) has expansion, temperature and support limits.
Support and protection
300.4 requires protection where conductors sit within 1 1/4 inches of a framing surface, typically a steel nail plate. 334.30 requires NM secured within 12 inches of a box and at intervals not exceeding 4.5 feet; 358.30 requires EMT secured within 3 feet of a box and every 10 feet. Minimum cover for buried raceways and cables is in Table 300.5.
Box fill, 314.16(B)
Count each conductor that enters the box and does not leave. Volume per conductor: 14 AWG is 2.00 cubic inches, 12 AWG is 2.25, 10 AWG is 2.50. Each yoke or device counts as two conductors of the largest size present. All equipment grounding conductors together count as one of the largest size. One or more internal cable clamps count as one additional conductor. Total the volume and select the box from Table 314.16(A).
Traps
Counting each ground separately instead of once in total; forgetting the device is doubled; assuming any 4-inch square box will pass; using NM outdoors or in wet locations; ignoring 110.14 bundling in a crowded box.
Worked example
A box contains two 12-2 NM cables, one duplex receptacle and internal clamps. Counts: four 12 AWG conductors = 9.00; clamps as one 12 AWG = 2.25; device as two 12 AWG = 4.50; grounds as one 12 AWG = 2.25. Total 18.00 cubic inches, so a 4 inch by 1 1/4 inch square box (18.0 cubic inches) is the minimum.