Pick the box, count what goes into it, and read the required volume against the volume the box actually has. The box volumes are Table 314.16(A) as printed and the volume allowances are the ones printed with 314.16(B), 2026 edition.
The box fill lab draws the box and lets you drag conductors, grounds, a device yoke, clamps and support fittings into it one at a time. Each drop writes a line naming the allowance it took and the part of 314.16(B) that counted it, and the box turns red when you go over.
Nonmetallic boxes are not in the table. They carry their volume molded into them, so pick the custom row and type that figure. Add any plaster ring, extension ring or domed cover that is marked with its own volume.
| AWG | Ends here | Through | Loop |
|---|---|---|---|
| 14 | |||
| 12 | |||
| 10 | |||
| 8 | |||
| 6 |
Ends here is a conductor that comes in from outside and terminates or splices in the box. Through is one that passes through unbroken. Tick Loop when those pass-through conductors are coiled long enough to be twice the free conductor length, because a loop that long counts twice. A pigtail made up entirely inside the box counts nothing, so leave it out.
One allowance for the whole box however many clamps there are, sized on the largest conductor in the box. A connector that clamps outside the box gets nothing.
Counted by type, not by fitting. A stud and a hickey in one box is two. Two studs is one.
A yoke holding two devices is still one yoke. Each yoke is a double allowance.
Not necessarily the largest conductor in the box. Read the wire landed on the device.
The first four share one allowance. Each one after that adds a quarter allowance.
Sized on the largest one entering the box, not on the circuit conductors.
Tick it and the calculator stops. A box with conductors that size is sized under NEC 314.28, not by volume allowances.
This page holds the table values so you get an answer on a ladder. The box fill tool on the calculators page makes you type every allowance in from your own book, because looking a value up is the thing the exam is testing.
Use this one on the job. Use that one the week before the test, and read the box fill study page for what each of the six categories counts as and why.
Section 314.16 is one comparison with two halves. Table 314.16(A) prints the volume of each standard metal box, and 314.16(B) turns everything inside the box into the volume you need. The box is adequate when the printed volume is not less than the required volume.
The picker above is that table, row for row. A box that is not in it, which means any nonmetallic box and any box of 100 cubic inches or less that the table does not describe, carries its volume marked on it under 314.16(A), and that marking is the number to use.
Required volume is built from categories that do not count alike, and that is the whole difficulty. A conductor is one allowance, internal clamps are one allowance for the box, a support fitting is one allowance per type, a yoke is two, and the first four grounding conductors are one between them with a quarter each after that.
The trap the exam sets is the mix of conductor sizes. Clamps and support fittings take the largest conductor in the box, a device allowance takes the largest conductor connected to a device on that yoke, and grounding fill takes the largest grounding conductor. A box wired in one size makes all three agree, which is why an item that mixes sizes is doing it deliberately.
Order of operations, and it matters: inventory the box first, sort what you found into categories, then look up the allowances. Almost every wrong answer here is a miscount rather than bad arithmetic, and no amount of care at the multiply step gets it back.
One stopping condition. Conductors 4 AWG and larger are not sized by volume allowance at all. A box enclosing them goes to NEC 314.28, which is a different calculation, and this tool refuses rather than returning a number that looks right.
Take a 4 x 1-1/2 square box, which Table 314.16(A) gives as 21.0 cubic inches. Into it come six 12 AWG conductors that terminate in the box, two 12 AWG equipment grounding conductors, a set of internal cable clamps, and one receptacle on a yoke wired in 12 AWG.
Six conductors at 2.25 is 13.5. The two grounding conductors share one allowance, so 2.25. The clamps take one allowance on the largest conductor in the box, another 2.25. The receptacle is a double allowance at 2.25, so 4.5. The total required is 22.5 cubic inches.
22.5 against 21.0 is a fail, over by 1.5 cubic inches. The next box up in the same family is the 4 x 2-1/8 square at 30.3 cubic inches, and that one passes. Set those numbers in the calculator above and you will get the same three figures back.
The value used for your inputs is read from NEC 2026 314.16(B) and Table 314.16(A), the way you would read it in the exam room, and the working above names the row it came from. The tables themselves belong to NFPA and are not printed on this page. Bring your own copy of the 2026 code, which is what the exam requires anyway, and check the row there.
Add up the volume that everything inside the box requires, then compare that total against the volume of the box. Conductors take one allowance each, internal clamps one for the whole box, each type of support fitting one, each yoke two, and up to four equipment grounding conductors one between them. The box is adequate when its volume from Table 314.16(A), or the volume marked on it, is not less than that total.
14 AWG is 2.00 cubic inches, 12 AWG is 2.25, 10 AWG is 2.50, 8 AWG is 3.00 and 6 AWG is 5.00. Those figures come from the volume allowance table printed with NEC 314.16(B). The chart further down this page prints the same five rows, and the calculator uses no others.
They do, but not one each. Up to four equipment grounding conductors take a single allowance between them, sized on the largest of them, and each one after the fourth takes a quarter allowance. Two 12 AWG grounds are one allowance of 2.25 cubic inches, not two.
Each yoke or strap holding one or more devices takes a double allowance, sized on the largest conductor connected to a device on that yoke. A receptacle on a 12 AWG circuit is 4.5 cubic inches. A device wider than a 2 inch device box takes a double allowance for each gang required to mount it, so a two gang wide device is four allowances.
Internal cable clamps take one allowance in total, however many of them are in the box, sized on the largest conductor present. Three clamps in one box are still one allowance. A connector whose clamping mechanism sits outside the box takes nothing at all.
Then 314.16 is the wrong section to be in. A box enclosing conductors 4 AWG and larger is sized under NEC 314.28, which works from raceway trade sizes and pull distances rather than volume allowances. Tick the box for it above and this calculator stops and says so rather than returning a number.
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