Drag a conductor, a ground, a device yoke, a clamp or a support fitting into the drawn box and the fill rises by the allowance NEC 314.16(B) gives that item, with the rule that counted it named on every line.
The volumes sit in the script and only the box you pick is printed. A nonmetallic box is marked with its own volume, so it is not on this list.
Conductors carry their own size. This sets the size of the next grounding conductor and of the conductor landed on the next device. Items already in the box keep the size they went in with.
Nothing in the box yet. Drag an item out of the tray, or use one of the buttons.
Box fill goes wrong in one of two places: forgetting a line, or counting a line once when the code counts it twice. Watching the bar move as each item lands is a faster way to learn which is which than reading the section again.
Set up a box you have actually wired, then take one item out at a time and see what it was costing you. When you want the pass through, long loop and wide device cases, go to the box fill calculator.
The rule starts at 314.16. A box has to give free space to everything inside it, and the volume you get from 314.16(A) is not allowed to be less than the fill you calculate under 314.16(B). Boxes enclosing conductors 4 AWG and larger also have to meet 314.28, which is a different calculation and not what this lab does.
314.16(B) tells you to add the applicable parts together, and it also says small fittings such as locknuts, splicing connectors and bushings get no allowance at all. Each item in the tray belongs to one of those parts, and the line the lab writes names it.
Conductor fill, 314.16(B)(1). A conductor that comes in from outside and ends or is spliced in the box counts once, and one that passes through without a splice or a termination counts once as well. A loop of unbroken conductor left at least twice the free conductor length counts twice, and a conductor that never leaves the box does not count. There is an exception for grounding conductors, or up to four fixture wires smaller than 14 AWG, or both, where they come in from a domed luminaire or a similar canopy and end in that box.
Clamp fill, 314.16(B)(2). Internal cable clamps take a single allowance for the whole box, sized on the largest conductor in it. Drop a second clamp in the lab and the total does not move, which is the point. A cable connector whose clamping mechanism sits outside the box takes nothing.
Support fitting fill, 314.16(B)(3). A luminaire stud and a hickey are two types of fitting, and each type present takes one allowance sized on the largest conductor in the box. Two studs are still one type, so the second one adds nothing to the total.
Device or equipment fill, 314.16(B)(4). Each yoke or strap carrying one or more devices takes a double allowance, sized on the largest conductor connected to a device on that yoke. A device wider than a single 2 inch device box takes that double allowance for each gang its mounting requires, which is a case the calculator handles and the tray does not.
Grounding conductor fill, 314.16(B)(5). Up to four equipment grounding conductors entering the box share one allowance, sized on the largest of them, and each one after the fourth adds a quarter of an allowance on the same basis. Eight 12 AWG grounds therefore come to 4.50 cubic inches rather than 18.
The allowance per conductor comes from the table printed with 314.16(B), which the 2026 edition numbers Table 314.16(B)(4). The box volumes come from Table 314.16(A). Both sit in the script behind this page rather than on it, so the page can print your box and your drops and still send you to your own book for the rest.
Two more lines belong to the same calculation and are not in the tray. 314.16(A) lets the volume of plaster rings, domed covers and extension rings count when they are marked with their volume, and it splits a box with a barrier into spaces that are calculated separately. Terminal blocks have their own line at 314.16(B)(6).
Leave the wire size on 12 AWG. Add six 12 AWG conductors, one cable clamp, one device on a yoke and two grounding conductors.
The answer. The fill needs 22.5 cubic inches and the box holds 21.0, so it is over by 1.5. The six conductors are 13.5, the clamps take one more allowance at 2.25, the receptacle on its yoke is doubled to 4.5, and the two grounds share a single allowance at 2.25. The next size in the same family that holds it is the 4 x 2-1/8 square at 30.3 cubic inches.
Start with a conductor so the clamp has a size to work from, then add two clamps.
The answer. The first clamp takes one allowance sized on the largest conductor in the box. The second takes nothing, because 314.16(B)(2) gives the box a single clamp allowance however many clamps are in it.
Put a conductor in first, then work down the second row of the tray.
The answer. The stud takes an allowance and the hickey takes another, because 314.16(B)(3) counts one for each type of fitting. The second stud adds nothing, because it is the same type as the first.
Pick the 3 x 2 x 2 device box and set the wire size to 14 AWG before you add the ground and the device.
The answer. It lands on 10.0 cubic inches against the 10.0 the box holds, so it passes with nothing to spare. Two conductors are 4.0, the yoke is doubled to 4.0 and the single ground takes 2.0. Add one more conductor and the same box is over.
The box volumes and the per conductor allowances are the published values from NEC 2026 Table 314.16(A) and the allowance table printed with 314.16(B), and they were copied into this page from the box fill calculator along with the calculation itself, so the two pages cannot drift apart. Neither table is printed here. Bring your own copy of the 2026 code, which is what the exam requires anyway, and read the rows there.
It is the calculation in NEC 314.16(B) that adds up the volume everything inside a box is credited with, then checks that total against the volume of the box from 314.16(A). Conductors, devices on yokes, internal cable clamps, support fittings and equipment grounding conductors each have a line in it. If the total comes out larger than the box, the box is too small for what you are putting in it.
It depends on the size, and the figure comes from the allowance table printed with 314.16(B), which the 2026 edition numbers Table 314.16(B)(4). A 12 AWG conductor is credited with 2.25 cubic inches, which is what one drop adds in this lab. The other sizes are in your own book, and that is where to read them.
Twice. 314.16(B)(4) gives each yoke or strap holding one or more devices a double allowance, sized on the largest conductor connected to a device on that yoke. A device wider than a single 2 inch device box gets that double allowance for every gang its mounting requires.
Under 314.16(B)(5) the first four that enter the box share a single allowance, sized on the largest of them. Each one after the fourth adds a quarter of an allowance on the same basis. One ground and four grounds therefore cost the same, and the fifth is where the total starts moving again.
Yes, and they are counted differently from each other. 314.16(B)(2) gives the box one allowance when internal cable clamps are present, however many of them there are. 314.16(B)(3) gives one allowance for each type of support fitting, so a stud and a hickey are two, and both are sized on the largest conductor in the box.
It does when the ring is marked with its volume, or when it is made from a box whose dimensions are listed in Table 314.16(A). 314.16(A) counts the assembled sections of the enclosure plus the space from plaster rings, domed covers and extension rings on that basis. This lab does not model rings, so add the marked volume yourself.
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