Checked against primary sources 2026-08-24

Box fill is a counting exercise, and six categories count six different ways

Almost every wrong answer here is a miscount rather than bad arithmetic, and the miscounts cluster in three of the six categories.

Two subsections, two sides of one comparison

Volume you have against volume you need. 314.16(A) computes the first, 314.16(B) computes the second, and the box is adequate when the first is not less than the second (NEC 314.16).

Keeping the two subsection letters apart is worth doing early, because items are written to test one or the other and the language they use is nearly identical. An item about a plaster ring is an (A) item. An item about a switch is a (B) item. An item that gives you both is checking whether you can run the comparison.

One exclusion sits above both. The section does not reach terminal housings supplied with motors or generators, so an item set at a motor is either testing something else or testing whether you know that.

The volume you have, under 314.16(A)

The volume of a box is the total of its assembled sections, plus the space provided by plaster rings, domed covers, extension rings and similar, where those are marked with their volume or are made from boxes whose dimensions are listed in Table 314.16(A).

That marked-or-listed condition is the whole of it. An item that looks impossible often turns out to include a marked plaster ring, and the ring is there to be added. An item that mentions a ring and says nothing about a marking is a different question.

Boxes of 100 cubic inches or less that are not described in Table 314.16(A), and nonmetallic boxes generally, are marked by the manufacturer with their volume. Where a stem gives you a marked volume, use it and leave the table shut.

Barriers cut the other way. Where a box has securely installed barriers, the volume is apportioned to each resulting space, and each space is then calculated separately for fill. A barrier that carries no volume marking consumes volume of its own, at figures the code prints for metal and for nonmetallic barriers. Look those two up rather than guessing, because a barrier is easy to forget entirely: it is not a conductor and it is not a device.

The six categories, and what each counts as

They are added together and none of them works quite like its neighbor (NEC 314.16(B)).

• Conductor fill. One count for each conductor that originates outside the box and terminates or is spliced inside it, and one for each conductor that passes through without splice or termination.
• Clamp fill. One allowance total where one or more internal cable clamps are present, sized on the largest conductor in the box. Not one per clamp. A connector whose clamping mechanism is outside the box gets nothing.
• Support fittings fill. One allowance for each TYPE of fitting present, luminaire studs and hickeys being the two types, sized on the largest conductor in the box.
• Device or equipment fill. A DOUBLE allowance for each yoke or strap carrying one or more devices, sized on the largest conductor connected to a device on that yoke.
• Equipment grounding conductor fill. One allowance for up to four of them together, then a quarter allowance for each additional one, all sized on the largest grounding conductor entering the box.
• Terminal block fill. One allowance for each terminal block assembly, sized on the largest conductor terminated to that assembly.

Some count per item, some count once for the whole group however many there are, and one counts once per type. That split is where the marks are, and it is why reading the category name is not enough. You have to read whether the allowance attaches to the thing or to the set of things.

Support fittings are counted by type, not by fitting

This one is stated in a single word and the word is easy to read past. The allowance is made for each type of fitting.

A luminaire stud and a hickey in the same box are two types, so two allowances. Two studs in the same box are one type, so one allowance. A box with three studs and one hickey is still two allowances.

Count fittings first and then collapse them into types. Doing it the other way around, counting types you expect and then looking for fittings, is how a hickey gets missed.

The device allowance is double, and it can be double per gang

A yoke or strap holding one or more devices takes a double volume allowance. One switch is two allowances. A yoke holding two devices is still one yoke, so still two allowances.

Devices or utilization equipment wider than a single 2 inch device box take double allowances for each gang required to mount them. A wide device occupying two gangs is four allowances, and the stem will tell you the width because it has to.

The size used is the largest conductor connected to a device supported by that yoke, which is not necessarily the largest conductor in the box. That difference is a separate trap and it has its own section below.

The grounding conductor count, where old habits produce a small box

The rule most people carry is that every equipment grounding conductor in the box, however many, adds up to a single allowance. That was the rule for a long time and a good deal of published material still prints it.

What the section says now: up to four grounding conductors take one allowance between them, and each one after the fourth takes a quarter allowance. Both are sized on the largest grounding conductor entering the box (NEC 314.16(B)).

So six grounding conductors are not six allowances and they are not one either. They are one and a half. Counting six invents volume that is not required. Counting one understates the fill, which is the dangerous direction, because it selects a box that is too small and the error ends up in the wall rather than on the answer sheet.

Clamps genuinely do work the old way and get miscounted the same way in reverse. Three internal clamps are one allowance, not three.

Which largest conductor, for which allowance

This catches people who have the categories perfectly.

Clamps and support fittings are sized on the largest conductor present in the box. Grounding conductor fill is sized on the largest grounding conductor entering the box. A device or equipment allowance is sized on the largest conductor connected to a device on that yoke. A terminal block is sized on the largest conductor landed on that assembly.

Four different sentences, four different sets to take the largest of. A box wired in one size makes them all agree, which is why every practice problem that mixes sizes is doing it deliberately.

Read which set each allowance names before you write a size down. It takes a few seconds and it is the difference between understanding the section and having memorized a shape.

What counts nothing at all

Small fittings get no allowance. Locknuts, splicing connectors and bushings are named and excluded, so an inventory that lists wire nuts is listing them to be ignored (NEC 314.16(B)).

A conductor no part of which leaves the box is not counted. A pigtail made up inside the box never originates outside it, so it adds nothing.

The opposite case is easy to state backwards, so state it in full. A loop or coil of unbroken conductor not less than twice the minimum length required for free conductors is counted TWICE. A short loop counts once like any pass-through, a long one counts twice, and nothing about a loop ever counts less than once.

There is also an exception worth marking in your own code book. Equipment grounding conductors, or not more than four fixture wires smaller than 14 AWG, or both, are permitted to be left out of the calculation where they enter a box from a domed luminaire or similar canopy and terminate within that box. That is a narrow permission with three conditions on it, and it exists so that a fixture canopy does not fail a box that is otherwise fine.

The method

1. Inventory everything in the box and sort it into the six categories. Anything that does not land in one of them is either excluded or is not a fill question.
2. For each allowance, identify which largest conductor the code names for that category.
3. Total the allowances, remembering that clamps are one regardless of quantity, support fittings are one per type, devices are double per yoke, and grounding conductors are one for the first four and a quarter thereafter.
4. Convert the allowance count to volume using the table printed with 314.16(B).
5. Compute the box volume under 314.16(A), adding any marked ring or cover and deducting for any unmarked barrier.
6. Compare. If required exceeds provided, go up a size.

Do the sorting on paper before opening a table. Almost every error happens in step one, and no amount of care in steps four and five recovers it.

Two neighbors finish the job on a real installation. A box enclosing 4 AWG or larger conductors also has to satisfy 314.28, and an outlet or device box also has to satisfy the minimum depth rule in 314.24. Both are separate calculations from this one and an item can hand you all three at once.

What this page cites

• NEC 314.16 Number of conductors in outlet, device and junction boxes, and conduit bodies. Carries the comparison between (A) and (B), the exclusion of motor and generator terminal housings, and the pointers to 314.24 and 314.28. source
• NEC 314.16(A) Box volume calculations. Assembled sections plus marked or listed rings, covers and extensions; the marking requirement for boxes of 100 cubic inches or less not described in Table 314.16(A) and for nonmetallic boxes; and the apportioning of volume where barriers are installed, including the volume an unmarked barrier consumes.
• NEC 314.16(B) Box fill calculations. Six categories, (B)(1) through (B)(6), added together, with the small fittings exclusion and the separate calculation of each barriered space. The per-conductor volume allowance table is printed with this subsection. Category list confirmed against NFPA Code-Making Panel 8 first draft report for the A2025 cycle, First Revision 7529. source
• NEC Table 314.16(A) Metal boxes and their volumes. Read it in your own code book; nothing from it is printed here.
• NEC 300.16 Minimum length of free conductors, which 314.16(B)(1) cross-references for the loop that counts twice. This is the rule that sat at 300.14 in the previous edition.
• NEC 314.24 Minimum depth of outlet and device boxes, driven by the largest conductor entering. A separate test from fill.
• NEC 314.28 Pull and junction boxes and conduit bodies containing 4 AWG or larger conductors. A separate sizing method again, and the one an item reaches for when the conductors get big.