Wiring Methods and Materials
The Wiring Methods and Materials domain tests Chapter 3 selection and installation: cable and raceway types, permitted uses, support and securing, conductor ampacity, adjustment and correction, conduit fill, and box fill. The exam pairs a code rule with a physical installation detail.
Method selection
- NM cable (Article 334) is for dry locations and is common in dwellings; it is not permitted in wet, corrosive, or hazardous locations or where subject to physical damage. MC cable (330) and AC cable (320) have specific permitted uses; MC includes a grounding path, while AC uses a bonding strip.
- EMT (358), IMC (342), RMC (344), PVC (352), LFMC (350), and FMC (348) each have permitted uses and fittings. Wet locations require listed wet-location fittings and conductors; PVC needs expansion fittings where temperature changes.
- Securing and supporting: NM every 4.5 ft and within 12 in of boxes; EMT and RMC every 10 ft and within 3 ft of boxes; PVC per Table 352.30. Splices and junctions must be accessible and enclosed in boxes.
Ampacity and fill
- Table 310.16 gives conductor ampacity at 60°C, 75°C, and 90°C; use the termination rating per 110.14(C). NM uses the 60°C column per 334.80. Apply 310.15(B)(1) ambient correction and 310.15(C) bundling adjustment.
- Chapter 9 Table 1 conduit fill: 53% for one conductor, 31% for two, and 40% for three or more. Annex C gives exact conductor combinations.
- Box fill 314.16(B): 14 AWG = 2.00 in³, 12 AWG = 2.25 in³, 10 AWG = 2.50 in³. Count each current-carrying and neutral conductor, one largest equipment ground, clamps, and device yokes.
Worked example
A box contains four 12/2 NM cables, one duplex receptacle, two 12 AWG grounds, and internal clamps. Count 8 circuit conductors × 2.25 = 18.0 in³; one ground × 2.25 = 2.25 in³; clamps = 2.25 in³; device yoke = 4.50 in³. Total = 27.0 in³. A 4 × 4 × 1.5 box has 24 in³, so it fails. A 4 11/16 × 4 11/16 × 2 1/8 box has 42 in³ and is acceptable.
Traps
- Using 90°C ampacity for terminations rated only 75°C.
- Forgetting the 60°C limit for NM cable.
- Counting equipment grounds as full circuit conductors.
- Using 40% conduit fill for a two-conductor raceway.
- Mixing cable types without checking permitted uses.