Wiring Methods, Materials, and Installation Practices
Picking and installing a Chapter 3 method
Article 300 is the general rulebook for all wiring methods; Article 310 covers conductors, 314 covers boxes, and then one article per method - 334 NM cable, 330 MC, 338 SE, 352 PVC, 358 EMT, 342 IMC, 348 FMC, 376 wireway. Selection depends on the environment (wet, damp, dry, corrosive, exposed, buried) and the occupancy. General requirements recur across articles: protection from physical damage, protection against nails and screws, raceway sealing where temperature differs, expansion fittings, and support and securing.
Support intervals are heavily tested. NM cable: secured every 4.5 ft and within 12 in of a box. EMT: secured every 10 ft and within 3 ft of a box. PVC: within 3 ft of a box and at short intervals thereafter for small trade sizes. Bends must be made so conductors are not damaged; for small raceways a one-shot bend generally needs a radius of about 10 times the trade size, and the total bends between pull points cannot exceed 360 degrees.
Fill
Raceway fill uses Chapter 9 Table 1: 53% for one conductor, 31% for two, 40% for three or more, with conductor areas from Table 5 and raceway areas from Table 4. Box fill uses 314.16(B): count each conductor by volume (2.0 in³ for 14 AWG, 2.25 for 12 AWG, 2.50 for 10 AWG), add one allowance for all equipment grounding conductors, one for clamps, and two for each yoke or device.
Worked example: a box with 6 current-carrying 12 AWG conductors, 1 EGC, clamps, and 2 devices needs (6+1+1+4) × 2.25 = 27 in³; a 4 × 4 × 2⅛ box (30.3 in³) is adequate.