Wiring Methods, Materials, and Installations
What to master
Chapter 3 governs how raceways and cables are installed. Know the common methods by article number and application: NM cable (334), AC (320), MC (330), FMC (348), LFMC (350), EMT (358), IMC (342), RMC (344), PVC (352). Focus on where each is permitted and prohibited, securing and supporting intervals, the 360-degree total bend rule between pull points, physical protection requirements, and minimum cover for underground runs.
Fill and derating
Two separate calculations cause most errors. Conduit fill uses Chapter 9, Table 1: 53 percent for one conductor, 31 percent for two, 40 percent for three or more, then Table 5 for conductor areas. Box fill uses 314.16: each conductor gets a volume allowance based on size, all equipment grounds count as a single allowance based on the largest ground, each yoke or strap counts as two (based on the largest conductor attached), and internal cable clamps count as one.
Conductor derating is a third, separate step: 310.15(C)(1) applies adjustment factors for more than three current-carrying conductors in a raceway, and 310.15(B) applies ambient temperature correction.
Traps
- Using the 40 percent figure for one or two conductors.
- Counting each equipment ground separately instead of once for all grounds.
- Forgetting the device yoke allowance, or the clamp allowance when clamps are present.
- Overlooking 300.4 protection where cables pass within 1-1/4 in. of a stud edge.
- Installing NM cable in wet or damp locations.
Worked example
A box contains four 12 AWG phase/neutral conductors, one 12 AWG equipment ground, and one receptacle. At 2.25 cu. in. per 12 AWG: 4 conductors + 1 ground allowance + 2 for the yoke = 7 allowances x 2.25 = 15.75 cu. in. A 4 x 4 x 1-1/2 in. square box provides 21 cu. in., so it is acceptable.