Boxes: Fill Calculations
(314.16), Supports, Depth, Luminaire and Fan Boxes, and Pull-Box Sizing (314.28)
NFPA 70 National Electrical Code, 2023 edition · Reviewed 2026-09-21
"Boxes and Enclosures" is a named topic in the Wiring Methods and Materials part of the Georgia PSI outline, and box fill is one of the two calculations the Residential exam is built around. Written from the 2023 text of Article 314 (with 300.14/300.15).
1. Where a box is required and how much wire to leave — 300.14, 300.15
A box or conduit body is required at every outlet point, switch point, splice point, junction point, termination point, wiring-method transition point and pull point (300.15). Exceptions worth knowing: wiring methods with accessible interiors (wireways, gutters, surface raceways); a device with an integral enclosure listed for NM cable; direct-buried splices per 300.5(E); conduit bodies and identified fittings with no splices; luminaires used as raceways (410.64).
Free conductor (300.14): at least 6 in. measured from where the conductor emerges from its cable sheath or raceway inside the box; where the box opening is smaller than 8 in. in any dimension, each conductor must also reach 3 in. outside the opening. Conductors passing through unspliced are exempt.
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