Wiring Methods and Materials
Chapter 3 is the "which wiring method where" domain, and Articles 300 and 310 supply the rules that apply to all of them: protection from damage, support, burial depth, ampacity, and derating. Expect questions that give a location and ask which method is permitted, or give a raceway and a conductor set and ask whether it fits.
Method selection and installation
- Table 300.5 sets minimum cover: 24 in for direct-buried cable under a street or driveway, 18 in for PVC conduit, 6 in for rigid metal conduit under a building, and 12 in for a GFCI-protected residential branch circuit at 120 V, 20 A or less.
- EMT needs support within 3 ft of a box and every 10 ft (358.30). NM cable needs support every 4.5 ft and within 12 in of a box (334.30).
- 300.4 requires a steel plate or a 1¼ in setback where a raceway or cable sits closer than 1¼ in to the framing face.
- 334.80 limits NM cable ampacity to the 60°C column where it is surrounded by thermal insulation.
- Ampacity comes from 310.16 after applying the 110.14(C) termination limits and the 310.15(C)(1) adjustment for bundled current-carrying conductors.
Worked example: fill
Six 10 AWG THHN conductors in EMT. Table 5 gives 0.0211 in² each, so the total is 0.1266 in². More than two conductors means 40% fill (Chapter 9 Table 1). 1/2 in EMT holds 0.304 in² and 40% of that is 0.122 in² - too small. 3/4 in EMT at 40% is 0.213 in², so 3/4 in is the answer. The reasoning shortcut: compare total conductor area to the 40% figure, never to the raceway's full area.
Traps
- Using the 60% nipple allowance for a run longer than 24 in.
- Forgetting derating when four or more current-carrying conductors share a raceway.
- Assuming EMT is acceptable in direct burial or a corrosive area without supplementary protection.