Wiring Methods and Materials
What the exam tests
Wiring-methods questions test selection, installation limits, support, fill, and physical protection for raceways and cables under NEC Chapter 3, plus conductor ampacity and adjustment.
Core rules
- NM cable is permitted in dry, protected locations but not in wet locations, exposed to physical damage, or in most hazardous locations.
- EMT needs support within 3 ft of a box and at intervals not exceeding 10 ft; PVC Schedule 80 is required where subject to physical damage.
- Raceway fill: Chapter 9 Table 1 limits one conductor to 53%, two conductors to 31%, and three or more to 40% of internal area.
- Box fill under 314.16 uses volume allowances: each 12 AWG conductor 2.25 cu in, 14 AWG 2.00 cu in; one equipment ground counts as one conductor; clamps count one; a device counts as two.
- Ampacity comes from Table 310.16 after applying 310.15 adjustment and 310.14 correction.
Worked example
A 21 cu in box contains six 12 AWG current-carrying conductors, one equipment ground, one cable clamp, and one receptacle. Count: 6 + 1 ground + 1 clamp + 2 for the device = 10 allowances. Required volume = 10 × 2.25 = 22.5 cu in. The box is 1.5 cu in short, so it fails; use a deeper box or reduce fill.
Traps
Counting the equipment ground more than once; using the 40% column for a two-conductor raceway; forgetting the 3 ft support rule at boxes; and installing NM cable in a wet location because it looks protected.