Wiring Methods and Materials
Core concepts
Chapter 3 of the NEC is organized one article per wiring method: 320 AC cable, 330 MC cable, 334 NM cable, 338 SE cable, 342 rigid metal conduit, 344 IMC, 348 FMC, 350 LFMC, 352 PVC, 358 EMT, 362 ENT. For each you must know three things: where it is permitted, where it is prohibited, and how it must be secured and supported.
Support and securing rules follow a pattern worth memorizing. Cable assemblies such as NM must be secured within 12 in. of a box and supported at intervals not exceeding 4.5 ft. Rigid raceways such as EMT, IMC, and RMC must be secured within 3 ft. of a box and supported at intervals not exceeding 10 ft. Fastening must be listed for the environment: standard EMT fittings cannot be used with PVC, and PVC in exposed outdoor runs needs expansion fittings to absorb thermal movement.
Traps
- NM cable in wet or damp locations, in masonry voids, or embedded in concrete - all prohibited.
- Assuming metal raceways are acceptable in corrosive or classified areas without examining the article for that location.
- Ignoring conductor identification requirements (grounded conductor white/gray, equipment grounding conductor green or bare).
- Exceeding Chapter 9 conduit fill because the pull "looked easy."
- Using a fitting listed only for a different raceway type.
Rule of thumb
Ask yourself two questions for any method: "Is it allowed here?" and "Is it strapped right?" If either answer is uncertain, the question likely turns on that point.