Wiring Methods and Materials
Wiring Methods and Materials tests NEC Chapter 3: selecting a wiring method for the location, installing it with the required support and protection, and sizing boxes correctly. Questions often hinge on whether a method is permitted in a wet, damp, or dry location and on the securing distances.
Common methods and permitted uses
- NM-B (Article 334): dry locations only; not permitted in wet locations or as service-entrance cable.
- UF (Article 340): wet and direct-burial locations; suitable for underground feeder use.
- AC (320) and MC (330): armored and metal-clad cable; MC is common for commercial branch circuits.
- EMT (358), IMC (342), RMC (344), PVC (352), LFMC (350): raceways with different wet-location and physical-protection ratings.
Support and securing
- EMT: secured within 3 ft of a box and supported at least every 10 ft.
- NM cable: secured within 12 in of a box and supported at least every 4.5 ft; running boards or staples must not damage the sheath.
- PVC: support intervals depend on conduit size and temperature; expansion fittings are required where significant temperature change occurs.
Boxes and fill (314.16)
Table 314.16(B) volume allowances: 14 AWG = 2.0 in³, 12 AWG = 2.25 in³, 10 AWG = 2.50 in³. Each conductor counts once; all equipment grounding conductors count as one, based on the largest; one cable clamp counts as one; a device yoke counts as two of the largest conductor size.
Worked example
A 4 × 4 × 1.5 in square box has 24 in³. It contains two 12 AWG NM cables (four circuit conductors), two EGCs, one cable clamp, and one receptacle. Count: 4 × 2.25 = 9.0; EGCs = 2.25; clamp = 2.25; device = 4.5. Total = 18.0 in³, which fits in 24 in³.
Traps
- Counting each EGC separately instead of as one allowance.
- Forgetting the clamp or the device yoke.
- Using the 14 AWG allowance for 12 AWG conductors.
- Assuming a box's outside dimensions equal its cubic capacity.