Wiring Methods and Materials
What to master
Chapter 3 tells you which wiring method is legal where, and how to support, bend, and fill it. Know NM cable (Article 334), AC and MC cable (320 and 330), EMT (358), IMC (342), rigid metal conduit (344), PVC (352), LFMC and LFNC (350 and 356), and boxes (314). Support rules are heavily tested: NM every 4.5 ft and within 12 in of a box, EMT within 3 ft of a box and every 10 ft, and PVC on tighter intervals per Table 352.30. Conduit fill comes from Chapter 9, Table 1: 53% for one conductor, 31% for two, 40% for three or more, using Table 5 dimensions. Box fill follows 314.16(B): 14 AWG = 2.0 in³, 12 AWG = 2.25 in³, 10 AWG = 2.5 in³, each device counts as two conductors, and all equipment grounds count as one.
Traps
Using NM cable in wet or damp locations; assuming MC armor is always a valid equipment ground; counting each ground wire separately in box fill; forgetting clamps and support fittings in a box-fill count; and mixing the 40% rule with the 31% two-conductor rule.
Worked example
A box holds four 12 AWG conductors, two devices, and two grounding conductors. Count 4 × 2.25 = 9.0 in³, plus 2 devices × 2 × 2.25 = 9.0 in³, plus one ground allowance of 2.25 in³, for 20.25 in³ - so select a box of at least that volume.