Wiring Methods and Materials
Conductors
Minimum sizes are 14 AWG copper and 12 AWG aluminum (310.106). Ampacities come from Table 310.16 based on insulation rating and terminal temperature; the 60 °C column applies unless terminals are rated higher. Adjustment for more than three current-carrying conductors in a raceway appears in Table 310.15(C)(1), and ambient temperature correction tables apply on top of that. Equipment grounding conductors are not counted as current-carrying. Neutral conductors count only when they carry harmonic current in a wye system.
Raceways and cables
Know the permitted uses and support rules: EMT supported every 10 ft and within 3 ft of a box; rigid metal conduit every 10 ft; PVC every 3 ft for 1/2 in. through 1 in.; NM cable every 4.5 ft and within 12 in. of a box; MC and AC cable every 6 ft and within 12 in. No more than 360° of bends between pull points. Conduit fill follows Chapter 9 Table 1 - one conductor 53%, two 31%, three or more 40%. Minimum cover in earth (Table 300.5): rigid metal conduit 6 in., PVC 18 in., direct-buried cable 24 in. (18 in. for residential branch circuits at 120 V/20 A or less with GFCI protection). Conductors within 1-1/4 in. of framing need nail plates or bushings (300.4).
Traps
Using NM cable outdoors or in wet locations, forgetting that ENT and PVC have temperature and sunlight limitations, miscounting current-carrying conductors for derating, and assuming derated ampacity can exceed the small-conductor limits of 240.4(D).
Worked example
Six 12 AWG THHN current-carrying conductors in one raceway: 30 A × 0.80 = 24 A after adjustment, but 240.4(D) still caps the overcurrent device at 20 A unless 240.4(B) or (E) applies.