How do you count cable clamps in a box?

NEC 314.16(B)(2): if one or more internal cable clamps are present, ALL clamps together count as ONE conductor of the largest AWG entering the box. External cable connectors that mount through a knockout are not counted.

Do equipment grounding conductors get counted?

Yes, but with a twist. NEC 314.16(B)(5): all equipment-grounding conductors entering a box, regardless of how many, together count as ONE conductor of the largest EGC AWG. A second isolated EGC counts as one additional conductor.

What conductors do NOT count toward box fill?

NEC 314.16(B)(1) Exception: conductors that originate and terminate inside the same box (such as a pigtail wire-nut splice) and conductors smaller than #18 AWG fixture wire used for luminaire taps are not counted. Conductors that pass through the box without splice are still counted once.

FIG. 01 — BOX FILL CALCULATOR

Box Fill Calculator

Q: How are device yokes counted?

NEC 314.16(B)(4): each yoke (the mounting strap of a switch, receptacle or similar device) counts as TWO conductors of the largest AWG connected to that device. A duplex receptacle and a single switch on a two-gang yoke each count as 2× their largest AWG.

Cubic-inch box-fill computation per NEC 314.16(B). Counts conductors, internal clamps, device yokes, equipment-grounding conductors, and support fittings — all in the largest-AWG terms NEC requires.

FIG. 02 — INPUTS

Box volume (in³)

Conductors entering the box

#18#16#14#12#10#8#6

Box-fill extrasInternal cable clamps (B-2 — count as 1× largest AWG)Equipment-grounding conductors present (B-5 — all EGCs = 1× largest)Yokes(B-4 — each yoke = 2× largest AWG)Support fittings(B-3 — luminaire stud, hickey = 1× largest)

FIG. 03 — RESULT

Required volume

22.50 in³

Box volume

21.00 in³

OVER — need 1.50 in³ more (use a larger box)

Per-conductor allowance (NEC Table 314.16(B))

#18 = 1.5 in³
#16 = 1.75 in³
#14 = 2 in³
#12 = 2.25 in³
#10 = 2.5 in³
#8 = 3 in³
#6 = 5 in³

FIG. 04

HOW NEC 314.16(B) COUNTS CONDUCTORS

(B)(1) — Conductor fill

Each conductor that originates outside the box and terminates or passes through the box counts as one. Conductors that originate and terminate inside (pigtails) do not count.

(B)(2) — Cable clamps

All internal cable clamps together count as a single conductor of the largest AWG entering the box. External connectors mounting through a knockout do not count.

(B)(3) — Support fittings

Each luminaire stud, fixture hickey, or similar support fitting counts as a single conductor of the largest AWG entering the box.

(B)(4) — Device yokes (straps)

Each device yoke counts as TWO conductors of the largest AWG connected to that device. A duplex receptacle on one yoke = 2×.

(B)(5) — Equipment grounding conductors

All EGCs together count as a single conductor of the largest EGC AWG. One additional isolated EGC counts as one more.

Multiply by the right AWG allowance

Convert each conductor count into cubic inches using NEC Table 314.16(B): #14 = 2.0 in³, #12 = 2.25 in³, #10 = 2.5 in³, #8 = 3.0 in³, #6 = 5.0 in³. Sum everything. Pick a box that meets or exceeds the result.

FIG. 05

FREQUENTLY ASKED QUESTIONS

Box fill (NEC 314.16) measures cubic-inch volume inside a junction box or device box, summing conductor allowances, clamps, yokes, EGCs and support fittings. Conduit fill (NEC Chapter 9) measures cross-sectional area in a raceway, capping conductor area at 53%/31%/40% by count. They use different units (in³ vs in²) and entirely different tables.

↗ Box Fill Chart (Reference)↗ Conduit Fill Calculator