NEC Table 314.16
What Size Box Do I Need?
Tell it what is going in the box. It names the box to buy, what it leaves you, and whether it still fits when the next device goes in. Computed from NEC 314.16.
Add a second conductor size (a switch leg, a travellers pair)
NEC 314.16(B)(1) allows each conductor at its own size, but the yokes, the clamps, the grounds and the terminal blocks are all allowed at the LARGEST conductor in the box, so mixing sizes costs more than it looks.
All the clamps together take ONE allowance, at the largest conductor present (314.16(B)(2)).
Terminal blocks and fixture fittings
one more device
Adding one more device breaks this box. One more yoke takes the fill to 27 cubic inches against the FD double’s 24, so a second device does not go in this one. If there is any chance of one more device here, buy the 4-11/16x1-1/2 square (29.5 cu in) instead and it costs you pennies today rather than a second trip later.
Show the math: NEC 314.16(B)
| #12 conductors | 6 × 2.25 | 13.5 |
| #14 conductors | 0 × 2 | 0 |
| Device yokes | 1 × 4.5 | 4.5 |
| Internal clamps (all) | 1 × 2.25 | 2.25 |
| Equipment grounding (all) | 1 × 2.25 | 2.25 |
| Total | 22.5 |
Device yokes take two allowances each at the largest conductor connected to them (314.16(B)(4)); all internal clamps together take one (314.16(B)(2)); up to four grounds take one between them, with a quarter allowance for each one after that (314.16(B)(5), the 2020 change that stopped undercounting device boxes stuffed with grounds).
Where this number comes fromIntry Verified
- Calculated from
- NEC 314.16(B) volume allowances against the box volumes printed in NEC Table 314.16(A). Computed from the NEC 314.16(B) volume allowances for your conductors, devices, clamps and grounds, ranked against the box volumes printed in NEC Table 314.16(A). See the full receipt ↓
- Run with
- Conductors6 x #12
- Devices1
- Ground wires2
- Internal clampsyes
- Fill required22.5 cu in
- Checked
- Every figure an Intry calculator computes comes from a locked source module, not hand-typed. 21026 automated checks re-derive and source-check the numbers against the cited NEC section before each deploy. This is our own deterministic gate, not a third-party audit.
- Final say
- Your AHJ and local amendments have final say. Confirm before rough-in.
Intry Verified · NEC 2023 · Build AE2FC52 · 2026-08-21
| Box | Shape | Volume | Spare | Full | One more device |
|---|---|---|---|---|---|
| FD double | FD double | 24 | 1.5 | 94% | no |
| 4-11/16x1-1/4 square | square | 25.5 | 3 | 88% | no |
| 4-11/16x1-1/2 square | square | 29.5 | 7 | 76% | yes |
| 4x2-1/8 square | square | 30.3 | 7.8 | 74% | yes |
| 4-11/16x2-1/8 square | square | 42 | 19.5 | 54% | yes |
| 3x2x1-1/2 device | device | 7.5 | -15 | 300% | – |
| 3x2x2 device | device | 10 | -12.5 | 225% | – |
| 4x2-1/8x1-1/2 | – | 10.3 | -12.2 | 218% | – |
| 3x2x2-1/4 device | device | 10.5 | -12 | 214% | – |
| 3x2x2-1/2 device | device | 12.5 | -10 | 180% | – |
| 4x1-1/4 round/octagonal | round/octagonal | 12.5 | -10 | 180% | – |
| 4x2-1/8x1-7/8 | – | 13 | -9.5 | 173% | – |
| FS single | FS single | 13.5 | -9 | 167% | – |
| 3x2x2-3/4 device | device | 14 | -8.5 | 161% | – |
| masonry 3-3/4x2x2-1/2 | masonry | 14 | -8.5 | 161% | – |
| 4x2-1/8x2-1/8 | – | 14.5 | -8 | 155% | – |
| 4x1-1/2 round/octagonal | round/octagonal | 15.5 | -7 | 145% | – |
| 3x2x3-1/2 device | device | 18 | -4.5 | 125% | – |
| 4x1-1/4 square | square | 18 | -4.5 | 125% | – |
| FD single | FD single | 18 | -4.5 | 125% | – |
| FS double | FS double | 18 | -4.5 | 125% | – |
| 4x1-1/2 square | square | 21 | -1.5 | 107% | – |
| masonry 3-3/4x2x3-1/2 | masonry | 21 | -1.5 | 107% | – |
| 4x2-1/8 round/octagonal | round/octagonal | 21.5 | -1 | 105% | – |
Table 314.16(A) prints different shapes at identical volumes, so where two boxes tie this ranking prefers the shape that suits what is going in: a device box when there is a yoke, a round or octagonal box when there is a fixture stud. That preference only ever breaks ties, and it can never promote a box that does not fit.
These are the METAL box volumes. NEC Table 314.16(A) is titled Metal Boxes, and a nonmetallic box of the same nominal dimensions is very often a different volume. Every nonmetallic box, and any box not on this table, is required to be durably marked with its own cubic-inch capacity, and that marking governs over anything here. Read the box.
314.16 is a volume rule and nothing else. It does not check the six inches of free conductor 300.14 requires at each opening, whether the yoke and the wirenuts physically clear the box, or how the cable clamps eat the corners. A box that passes here at 95 percent is legal and still a brutal make-up, which is what the “one more device” column is for.
box_fill tool live, handing back the verdict with its cited source instead of a guess. Then keep the reading as one job in the Intry app.Intry Verified. The volumes and box sizes THE CALCULATOR ON THIS PAGE RETURNS are rendered from the same module our box fill calculator computes from, cited to NEC 314.16(B)(1) and 314.16(A) (NFPA 70-2023). Allowances quoted in the FAQ and the surrounding copy are read from the same printed table rather than computed. Confirm the edition your jurisdiction has adopted; a listed box marked with its own volume governs instead. Those figures are computed by a locked engine straight from the source it cites, not hand-typed, and that computation re-runs inside a deterministic test on every deploy. Source-checked. Locked. This is our own deterministic gate, not a third-party audit. What Intry Verified means
The Five Things That Count
Box fill is the sum of five categories, each from NEC 314.16(B), and the total must stay at or below the box volume from 314.16(A):
- Conductors (B)(1): each conductor that runs through or terminates counts as one volume at its own size. A conductor that only passes through counts once. A loop or coil of unbroken conductor at least twice the 300.14 minimum free-conductor length (12 in. or longer) counts as two conductor volumes.
- Clamps (B)(2): all internal cable clamps together count as one volume at the largest conductor size.
- Support fittings (B)(3): each fixture stud or hickey counts as one volume at the largest conductor.
- Devices (B)(4): each yoke or strap (receptacle, switch) counts as two volumes at the largest conductor connected to it.
- Grounds (B)(5): all equipment grounding conductors together count as one volume at the largest ground, covering up to four EGCs; since the 2020 NEC, each additional EGC beyond four adds another 1/4 of that largest-ground volume. A second isolated-ground system adds one more.
Worked Example
Device box with two 12-2 cables, one receptacle, internal clamps
#12 conductors: 4 x 2.25 = 9
Device yokes: 1 x 4.5 = 4.5
Internal clamps (all): 1 x 2.25 = 2.25
Equipment grounding (all): 1 x 2.25 = 2.25
Total = 18 cu in
Result: the box must be at least 18 cubic inches. The smallest box NEC Table 314.16(A) prints at or above that is the 3x2x3-1/2 device at 18 cu in, leaving 0 to spare. Add one more device and the same make-up needs 22.5 cubic inches, which takes you to the FD double (24 cu in).
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Frequently Asked Questions
What is the box fill volume for #12 wire?
Each #12 conductor counts as 2.25 cubic inches for box fill per NEC Table 314.16(B)(1). A #14 is 2.00, #10 is 2.50, #8 is 3.00, and #6 is 5.00 cubic inches.
What is the box fill volume for #14 wire?
Each #14 conductor counts as 2.00 cubic inches per NEC Table 314.16(B)(1). That is the most common branch-circuit conductor, so a box with six #14 conductors uses 12.0 cubic inches of conductor fill before devices and clamps.
How much box fill does a receptacle or switch take?
Each device yoke (receptacle or switch) counts as two conductor volumes based on the largest conductor connected to it, per NEC 314.16(B)(4). For a device on #12 that is 2 x 2.25 = 4.50 cubic inches.
How do you count grounds in box fill?
All equipment grounding conductors in the box together count as one single conductor volume, based on the largest ground, per NEC 314.16(B)(5). That single volume covers up to four EGCs; since the 2020 NEC, each additional EGC beyond four adds another 1/4 of that largest-ground volume. Four #12 grounds still count as just 2.25 cubic inches, not four.
Do cable clamps count toward box fill?
Yes. One or more internal cable clamps together count as one conductor volume based on the largest conductor in the box, per NEC 314.16(B)(2). Cable connectors on the outside of the box do not count.
How many #12 conductors fit in a 4-square box?
A 4 x 1-1/2 inch square box is 21.0 cubic inches (NEC Table 314.16(A)). At 2.25 cubic inches each, it holds up to 9 #12 conductors (21.0 / 2.25 = 9.3) if there are no devices, clamps, or grounds. Each of those subtracts from the count.
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Know the volumes. Let the calculator do the count.
Box fill is easy to get wrong by one device or ground. The box fill calculator applies all five 314.16(B) rules automatically and tells you the minimum box size in cubic inches.
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