Wiring Methods and Materials (NEC Chapter 3)
Chapter 3 is about containment and protection
Article 300 sets rules that ride on top of every wiring method: protection from physical damage where conductors sit within 1¼ in of framing (300.4, nailing plates), burial depths (300.5), a box or fitting at every splice, angle, or termination (300.15), all conductors of a circuit grouped in the same raceway or slot (300.20), and restrictions in ducts and plenums (300.22). Then each article adds limits - EMT (358), PVC (352), FMC (348), LFMC (350), RMC/IMC - plus rules for cable assemblies, boxes, and wireways.
Fill and box-count math
Chapter 9 Table 1 caps raceway fill at 53 percent for one conductor, 31 percent for two, and 40 percent for three or more; a nipple 24 in or shorter may use 60 percent. Box fill under 314.16(B): each circuit conductor counts one volume; all equipment grounding conductors count as one; each clamp, support fitting, and grounding screw counts one; each device yoke counts as two conductors based on the largest conductor present. Volume per conductor: 14 AWG = 2.0, 12 AWG = 2.25, 10 AWG = 2.50 cu in.
Worked example: a box with two 12 AWG two-wire cables with ground, one switch, internal clamps. Conductors 4 + EGC 1 + clamps 1 + yoke 2 = 8 counts × 2.25 = 18.0 cu in, so a 4 × 4 × 1¼ box (18 cu in) just passes while a 3 × 2 × 2½ box (12.5 cu in) fails.
Traps: PVC needs expansion fittings where temperature swings and is restricted where subject to physical damage; EMT is not permitted where severe corrosion or severe damage exists.