Wiring Methods, Materials, and Installation Requirements
The Chapter 3 pattern
Every Chapter 3 article follows the same order: scope, uses permitted, uses not permitted, and installation requirements. Learn that template and unfamiliar articles become readable. Common methods: NM cable (334), AC (320), MC (330), FMC (348), LFMC (350), EMT (358), IMC (342), RMC (344), PVC (352), and LFNC (356). Article 300 supplies the rules that apply to all of them - protection from physical damage (300.4), burial depths (Table 300.5), and raceway sealing.
Securing, supporting, and fill
Memorize the pairings that show up on every exam. NM cable: secured within 12 in of boxes and at intervals not exceeding 4.5 ft. EMT, IMC, and RMC: supported within 3 ft of a box and at intervals not exceeding 10 ft. PVC support spacing varies with trade size (Table 352.30). Raceway fill is 40% for three or more conductors (Chapter 9, Table 1), and conductors must be derated for ampacity when more than three current-carrying conductors share a raceway.
Boxes
Box fill is calculated with 314.16. Each 14 AWG counts 2.00 cu in, 12 AWG 2.25, and 10 AWG 2.50. Clamps, support fittings, and each device yoke add volume, and all equipment grounding conductors together count as one conductor based on the largest. Cables also have a fill limit of 314.16(B).
Worked example
A 4 x 4 x 1-1/2 in metal box has 21.0 cu in. Using 12 AWG: six current-carrying conductors (13.50) plus one EGC (2.25) plus one device yoke (4.50) = 20.25 cu in, which fits. Add a cable clamp (2.25) and the total reaches 22.50 cu in and the installation fails; step up to a 2-1/8 in deep box (30.3 cu in).