Box Fill Worked Counts for Common Boxes

Why this matters

Box fill is the inspection item electricians most often fail by eye. A box that looked fine at rough-in gets a pass-through cable added, or a fan replaces a fixture, and on trim day the device will not seat, the conductors fold against a screw terminal, and a nick in the insulation becomes an arc in the wall a year later. The count takes two minutes. This card fills in four of the counts a residential and light-commercial electrician does every week, so the method is in your hand and the edge cases are already worked.

The counting rules, with the reason for each

Count per NEC 314.16(B), 2023 edition (2020 where the section moved), as adopted by your jurisdiction. Each item converts to a number of volume allowances, and each allowance is worth the Table 314.16(B) volume for the conductor size it is based on: 2.0 cu in for 14 AWG, 2.25 for 12 AWG, 2.5 for 10 AWG.

• Each conductor that enters the box and is spliced or lands on a device: one allowance. Its size sets the allowance. A 12/2 cable entering a box brings two insulated conductors and two allowances at 2.25 cu in.
• An unbroken conductor passing through: one allowance. It occupies space but has no splice. If it is looped and the loop is 12 in or longer, it counts two, because a loop that long takes the room of two conductors.
• A conductor that never leaves the box: zero. Pigtails and jumpers between devices in the same box do not count. Neither do wire connectors, which is why pigtailing adds no fill even though it adds bulk; the volume allowances already assume it.
• Up to four equipment grounding conductors together: one allowance, based on the largest one. A box with four cables, each carrying a bare ground, still adds one. Since the 2020 edition, each EGC beyond the fourth adds a quarter allowance. Where a second set of isolated-ground conductors is present under 250.146(D), that set adds its own allowance.
• Internal cable clamps: one allowance total, based on the largest conductor in the box, whether the box has one clamp or four. A cable connector installed from outside through a knockout counts zero, because it sits outside the volume.
• Each fixture stud or hickey: one allowance for each type, based on the largest conductor in the box.
• Each device yoke: two allowances, based on the largest conductor connected to that device. A receptacle on a 12 AWG circuit counts two at 2.25. A device wider than a standard 2 in yoke counts two for each gang it needs.
• Fixture wires from a domed canopy: zero, for up to four fixture wires smaller than 14 AWG plus a grounding conductor, under the exception to 314.16(B)(1). This is why a ceiling box rarely fails on the luminaire's own leads.

Box volume comes from one of two places. A steel box listed in Table 314.16(A) takes its volume from the table; a nonmetallic box, and any box not in the table, takes the volume molded or stamped inside it. Plaster rings, extension rings and domed covers add their volume only when they are marked with it or built from a box listed in the table. The table itself is in 'Electrical Box Sizing + Fill Reference'; this card uses it.

The four counts

Each count lists what is in the box, then the arithmetic. Volumes are cubic inches.

A. SINGLE-GANG SWITCH BOX, 14 AWG, NONMETALLIC
   14/2 in, 14/2 out to light, neutrals spliced
   insulated conductors      4 x 1 =  4
   grounds (all)                    1
   switch yoke                      2
   total 7 x 2.0 = 14.0   box marked 18.0  PASS
   steel 3x2x2-1/2, int clamps: 8 x 2.0 = 16.0
   against 12.5 FAIL; 3x2x3-1/2 at 18.0 PASS

B. TWO-GANG RECEPTACLE BOX, 12 AWG, NONMETALLIC
   12/2 in, 12/2 out, 12/2 passing unbroken
   insulated conductors      6 x 1 =  6
   grounds (all)                    1
   two receptacle yokes      2 x 2 =  4
   total 11 x 2.25 = 24.75  box marked 30.0 PASS

C. 4 IN SQUARE 1-1/2 DEEP + 1-DEVICE RING, 12 AWG
   three 12/2, connectors outside, one receptacle
   insulated 6, grounds 1, yoke 2 = 9 allowances
   9 x 2.25 = 20.25 against 21.0 + ring 3.5 = 24.5
   PASS. Add a fourth 12/2: 11 x 2.25 = 24.75 FAIL
   2-1/8 deep box: 30.3 + 3.5 = 33.8 PASS

D. 4 IN OCTAGON 1-1/2 DEEP, STEEL, 14 AWG
   14/2 from switch, 14/2 on to next light
   insulated 4, grounds 1, stud 1, clamps 1 = 7
   7 x 2.0 = 14.0 against 15.5  PASS
   fan swap, 14/3 from switch: 8 x 2.0 = 16.0 FAIL
   octagon 2-1/8 deep at 21.5 PASS

The ring volume in count C is illustrative. Read the volume marked on the ring you are installing; if it carries no marking and is not built from a table-listed box, it adds nothing.

Reading count A: the steel box that fails on its clamps

The nonmetallic box in count A passes with 4.0 cu in to spare (18.0 minus 14.0). The tempting substitution is a 3x2x2-1/2 steel device box from the van, rated 12.5 cu in in Table 314.16(A). Its internal clamps add one allowance, taking the count from seven to eight, and eight at 2.0 is 16.0, which is 3.5 cu in over. The 3x2x3-1/2 steel box at 18.0 passes the same eight allowances with 2.0 to spare.

Two things in that count are worth noticing. The neutral is in the box because the switch box carries one now: NEC 404.2(C), as adopted, requires a grounded conductor at most switch locations, which is part of why switch boxes fill faster than they did. See related 'How to Get a Neutral to a Switch Box for a Smart Switch'. And a smart switch's body is deeper than a toggle; a box that passes on count may still not physically take the device. The count is the code floor, not a promise the device fits.

Reading count B: the pass-through cable

The pass-through 12/2 adds two allowances, 4.5 cu in, for conductors that do nothing in this box. It is legitimate, and it is the item an inspector looks for first when a two-gang box looks crowded. The count passes at 24.75 against 30.0, a margin of 5.25 cu in.

If the installer had looped the pass-through conductors 12 in or longer inside the box to leave slack, each would count two: the insulated conductors would go from six to eight allowances, the total from 11 to 13, and 13 at 2.25 is 29.25, still under 30.0 but with 0.75 cu in to spare. A third receptacle cable in the same box would then fail. Cut slack loops down to what you need before you count.

Reading count C: the ring that makes the box pass

A 4 in square box 1-1/2 in deep is 21.0 cu in in Table 314.16(A). Nine allowances at 2.25 is 20.25, which would pass in the bare box. The ring's marked volume adds headroom; in this count the ring brings the total to 24.5.

Now the fourth cable, the one added on a change order. It brings two insulated conductors, taking the count to 11 allowances and 24.75 cu in, which is 0.25 over the 24.5 available. The fix is the 2-1/8 in deep box at 30.3 cu in, 33.8 with the same ring. Swapping a box in a finished wall is a different job, which is why this count belongs at rough-in, when the extra cable is on the drawing.

Reading count D: the fan that fails a box the light passed

A light fixture on a 4 in octagon box 1-1/2 in deep is a clean pass at 14.0 against 15.5, with the domed canopy's fixture wires costing nothing. The customer then buys a ceiling fan with a light kit and wants separate wall control. The 14/2 from the switch becomes a 14/3, adding one insulated conductor, and the count goes to 16.0, which is 0.5 over.

The fill failure is not the most important thing wrong with that swap. A fan needs a box listed for fan support under 314.27(C), as adopted, and a shallow octagon nailed to a joist usually is not one. The fill count is a second reason to change the box, not the only one. See related 'Ceiling Box and Fan-Rated Support Verification'.

Mixed conductor sizes in one box

When 12 AWG and 14 AWG share a box, each conductor counts at its own size: a 12/2 brings two allowances at 2.25 and a 14/2 brings two at 2.0. Clamps and fixture supports take the largest conductor present in the box, the grounds take the largest EGC, and the device yoke takes the largest conductor connected to that device. A switch on a 14 AWG switch leg in a box that also carries a 12 AWG feed counts its yoke at 2.0, not 2.25, if only 14 AWG lands on it.

Before you open an existing box to count it

Counting a box that is already in service means opening it. Turn off the breaker, lock it out, and verify absence of voltage at the box with a CAT III or better meter proven on a known live source before and after, per NFPA 70E Article 120 (120.5 in the 2021 edition, renumbered in 2024) and 29 CFR 1910.333(b)(2). A switch box often carries a second circuit or a traveler fed from elsewhere; test every conductor in the box, not only the one you meant to turn off. When the box is closed again and the breaker is back on, confirm each device works and each receptacle reads correctly before you leave: see related 'How to Check Receptacle Wiring with a Meter'.

What the count cannot tell you

A box can pass on volume and still be wrong. The count does not check that each cable is secured to the box or within the distance 314.17 allows, that each conductor leaves at least 6 in of free length, 3 in of it outside a box opening smaller than 8 in, per 300.14, or that the device is rated for the conductors on it. Those are trim-out checks: see related 'The Electrical Trim-Out SOP'. And a box that passes with no margin will often fail in practice when a deeper device or a bulky splice goes in, so leave a few cubic inches where the plan allows.

References

• NFPA 70, National Electrical Code, 2023 edition as adopted by your jurisdiction: 300.14, 314.16(A), 314.16(B), 314.17, 314.27(C), 404.2(C)
• NFPA 70E, Standard for Electrical Safety in the Workplace, current edition, Article 120; 29 CFR 1910.333(b)(2)
• See related: Electrical Box Sizing + Fill Reference
• See related: Ceiling Box and Fan-Rated Support Verification; The Electrical Trim-Out SOP