Conductors, Boxes, Raceways, and Cable Systems
Ampacity
Use Table 310.16 ampacities, but the termination temperature rating limits you: 60°C for most small conductors, 75°C for most equipment, 90°C only when terminations are rated for it. Then apply correction for ambient temperature and adjustment for more than three current-carrying conductors in a raceway or cable. The neutral counts as a current-carrying conductor on some multiwire branch circuits on wye systems.
Boxes
Box fill is computed in cubic inches per Article 314.16: each 14 AWG counts 2.0 in³, 12 AWG 2.25 in³, 10 AWG 2.5 in³. Count conductors, add one for clamps, two for a device, and one for all grounding conductors. A box marked with its volume makes the math straightforward. Boxes must remain accessible.
Raceway rules
Bends between pull points cannot exceed 360°. Pull boxes use 314.28 dimensions: 8 times the trade size for straight pulls, 6 times for angle pulls. Conductors 1/0 and larger may be paralleled if they are the same length, same conductor material, same insulation, and terminate in the same manner.
Worked example
A box contains six 12 AWG circuit conductors, one 12 AWG ground, two cable clamps, and one duplex receptacle. Count: 6 conductors + 1 ground + 1 clamp + 2 for the device = 10 × 2.25 = 22.5 in³. A 4 × 4 × 2-1/8 box is 30.3 in³, so it is acceptable.