Wiring Methods, Materials, and Raceways
Wiring methods, materials, and raceways
Chapter 3 is the heart of this domain. Article 300 gives general requirements for wiring methods, including protection from physical damage, conductor identification, and support. Articles 310 through 362 cover conductors, cable assemblies, and raceways such as NM cable, AC/MC cable, EMT, PVC, rigid metal conduit, and flexible connections. You must match the wiring method to the environment: wet, damp, dry, corrosive, exposed, concealed, or subject to physical damage. Article 314 covers outlet, device, and junction boxes, including box fill; Chapter 9 provides conduit and tubing fill tables and Annex C provides preset combinations.
Traps and a worked example
Common traps include using NM cable in a wet location, ignoring support and securing distances, forgetting that raceways need expansion fittings where significant temperature change occurs, and miscounting box fill. For box fill, count each current-carrying conductor and neutral, count equipment grounding conductors as one allowance based on the largest EGC, count clamps and support fittings if present, and count each device as two conductors based on the largest conductor connected. Conduit fill also trips candidates: 1 conductor 53%, 2 conductors 31%, and 3 or more 40% of the raceway area.
Worked example: a box contains four 12 AWG current-carrying conductors, one 12 AWG equipment grounding conductor, and one device. Each 12 AWG conductor requires 2.25 in³. Four conductors = 9.00 in³; one EGC allowance = 2.25 in³; one device = two conductors = 4.50 in³. Total = 15.75 in³, so select a box with at least that volume. Rule of thumb: box fill is a volume calculation, not a wire-count guess.