Box fill on the Texas exam, where twelve items in the bank turn on how three of the six categories are counted

What the Texas exam asks here

Twelve of the 51 conduit and raceways items in our 500-item Texas bank are box fill, the hardest concentration in the area. Ten of the twelve sit in the upper two difficulty tiers, against fourteen of the twenty-seven raceway fill items, so per item this is the costlier half of the topic.

The items come in three forms. Some hand you an inventory of box contents and ask for the total allowances. Some ask a threshold question alone, such as how many grounding conductors enter before any adds more than the shared allowance. Some list something that earns nothing and ask what it adds, which is reading discipline in disguise.

Almost every miss here is a count. The multiplication at the end is trivial once the sorting is right, and no care at the lookup recovers a category read wrong at the start. The bank spends its hardest items on counting rules and leaves the volume figures to your book.

On the published PSI content outline, wiring methods and materials carries 10 of the 56 scored items on the journeyman NEC knowledge portion and 2 of the 24 on the calculations portion. Box fill shares those slots with fill, cable types and raceway rules, and using the code book and exam day covers how that weighting is published.

Two subsections, two sides of one comparison

NEC 314.16(A) works out the volume you have. NEC 314.16(B) works out the volume you need. The box is adequate where the first is at least as large as the second, and items are written to test one side or the other in language that sounds nearly identical.

An item about a plaster ring is a subsection (A) item. An item about a switch is a subsection (B) item. An item handing you both is checking whether you can run the comparison. Under (A) the volume is the total of the assembled sections plus the space provided by plaster rings, domed covers, extension rings and similar, where those carry a volume marking or are made from boxes whose dimensions appear in Table 314.16(A). A stem that looks impossible often includes a marked ring there to be added.

Barriers cut the other way, and one bank item is built on them. Where a box has securely installed barriers, the volume is apportioned and each resulting space is calculated on its own, because conductors on one side cannot use the room on the other. A barrier carrying no volume marking consumes volume of its own at figures the code prints. The step-by-step arithmetic with the tables open lives on the question set at its box fill page.

The six categories, and how each counts

NEC 314.16(B) has six categories and they are added together. What makes the section hard is that some allowances attach to each item, some attach to the whole group however many there are, and one attaches to each type of thing present.

• Conductor fill. One for each conductor originating outside the box that terminates or is spliced inside it, and one for each conductor passing through unbroken.
• Clamp fill. One allowance in total where one or more internal cable clamps are present, sized on the largest conductor in the box.
• Support fittings fill. One allowance for each type of fitting present, luminaire studs and hickeys being the two types.
• Device or equipment fill. A double allowance for each yoke or strap carrying one or more devices.
• Equipment grounding conductor fill. One allowance for up to four together, then a quarter allowance for each one after that.
• Terminal block fill. One allowance for each terminal block assembly.

The count is then turned into volume using the table printed with that subsection, which the 2026 text carries as Table 314.16(B)(4) and which three of the six categories point to by number. Read the category name and then read whether the allowance attaches to the thing or to the set of things, because that second reading is where the marks sit.

The grounding conductor count

This is the distinction that decides more box fill items than anything else, and four of the twelve in the bank turn on it. Up to four equipment grounding conductors take one allowance between them. Each one after the fourth takes a quarter allowance. Both are sized on the largest grounding conductor entering the box.

The rule a lot of people carry is that every grounding conductor in the box shares a single allowance however many there are. That was the rule for a long time and a good deal of published material still prints it, which makes this a live conflict between what a candidate has read and what the section says now. Six grounding conductors are one and a half allowances, and seven are one and three quarters.

Clamps run the other way and get miscounted in reverse. All internal cable clamps in a box take one allowance between them, so three clamps are still one. Grounding and bonding covers what those conductors are doing in the box in the first place.

Which largest conductor, for which allowance

This catches candidates who have the six categories perfectly. Four allowances are sized on four different sets, and a box wired in one conductor size makes them all agree, which is why any practice problem that mixes sizes is doing it on purpose.

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

Two counting rules sit alongside those. Support fittings are counted by type, so a luminaire stud and a hickey are two allowances while three studs together are one. One bank item puts two clamps, a stud and a hickey in the same box, and the answer is three allowances: one for the clamps together and one for each type of support fitting.

The device allowance is double for each yoke or strap, and it goes double per gang where the device or equipment is wider than a single device box. A wide device occupying two gangs is four allowances, and the stem has to tell you the width. A terminal block takes one allowance for each assembly, whatever number of terminals the assembly carries.

What earns no allowance at all

Small fittings such as locknuts, splicing connectors and bushings are named and excluded, so an inventory listing wire nuts is listing them to be ignored. Two bank items exist for that alone, and the options offer one each, one in total and the double allowance a device gets, which covers every plausible guess.

A cable connector whose clamping mechanism sits outside the box earns nothing either, since the clamp allowance exists because an internal clamp occupies room inside. The word internal is doing the work in that sentence.

On the conductor side, a conductor no part of which leaves the box is left out, so a pigtail made up inside adds nothing. A conductor originating outside and terminating or spliced inside counts once, and so does one passing through unbroken, and splices never multiply that count. The one case that counts twice is a loop or coil of unbroken conductor at least twice the minimum free conductor length, which the 2026 book sets at 300.16 after carrying it at 300.14 in the previous edition.

One narrow permission is worth marking in your own copy. Grounding conductors, or up to four fixture wires smaller than 14 AWG, or both, may be left out where they enter from a domed luminaire or similar canopy and terminate within that box. It has three conditions on it and it exists so a fixture canopy does not fail a box that is otherwise fine.

Where the wrong answers come from

Each item names the mistake behind every wrong option. Collected across the twelve box fill items, they sort into a short list.

Every one of those is a sorting error made before any table was opened. Inventory the box on paper, sort each item into its category, and write the largest conductor beside each allowance before you look anything up.

What it costs, and what to do now

An undercount selects a box that is too small, and that discovery arrives with conductors folded into it and a device fighting the cover. The remedy on site is an extension ring where one fits and a new box where one does not, which on a finished wall means patching and painting. Buying a box larger than the count required costs a few dollars and nothing else, which is why the safe direction on a box fill item is worth knowing.

A box that passes fill can still be the wrong box. Outlet and device boxes also satisfy the minimum depth rule in NEC 314.24, keyed to the largest conductor entering and to equipment projecting backward, and a box enclosing 4 AWG or larger conductors goes to NEC 314.28 for a different method entirely. Boxes and conduit bodies works through which of the three methods applies, and an item can hand you all three at once.

Before test day, tab NEC 314.16 and write the six categories in your own words on the facing page, with the four largest-conductor sets underneath. Texas adopts the 2026 National Electrical Code on 1 September 2026 under 16 TAC 73.100, and which code edition applies to you covers what moves with it. The thirty free timed questions at practice will tell you in a few minutes whether the grounding conductor count is really yours, and conduit fill is the other counting problem on the same paper.

Questions people ask

How many equipment grounding conductors share one volume allowance?

Up to four share a single allowance, sized on the largest grounding conductor entering the box. The fifth is the first one that adds anything, and it adds a quarter allowance, as does every one after it. So three grounding conductors are one allowance, six are one and a half, and seven are one and three quarters. The older rule, where every grounding conductor in the box shared one allowance however many there were, still appears in a lot of published material, and four of the twelve box fill items in our bank are written to find out which version you carry.

Do wire nuts count in box fill?

They earn nothing. Small fittings such as locknuts, splicing connectors and bushings are named and excluded from the calculation, so an inventory that lists wire nuts is listing them to be ignored. Two items in our bank exist for that alone, and their wrong options offer one allowance each, one allowance in total, and the double allowance that belongs to a device yoke. A cable connector whose clamping mechanism sits outside the box earns nothing either, since the clamp allowance exists because an internal clamp takes up room inside the box.

How many allowances does a duplex receptacle on one yoke take?

Two. Each yoke or strap carrying one or more devices takes a double volume allowance, sized on the largest conductor connected to a device on that yoke. A yoke holding two devices is still one yoke, so it is still two allowances. The exception runs the other way: a device or utilization equipment wider than a single device box takes double allowances for each gang required to mount it, so a device occupying two gangs is four allowances. The stem will always give you the width when that case is in play.

Which largest conductor sizes each allowance?

Four allowances use four different sets, and reading the wrong one is a separate trap from miscounting. Clamps and support fittings use the largest conductor present in the box. Grounding conductor fill uses the largest grounding conductor entering the box. A device or equipment allowance uses the largest conductor connected to a device on that yoke, which may be smaller than the largest conductor in the box. A terminal block uses the largest conductor landed on that assembly. A box wired in one size makes all four agree, which is why practice problems mix sizes deliberately.

How is a box with barriers calculated?

Each resulting space is calculated separately. Where a box has securely installed barriers, the volume is apportioned to the spaces they create, and the fill for each space is worked on its own, because conductors on one side cannot occupy room on the other. Pooling the whole volume gives an answer the conductors cannot use. A barrier carrying no volume marking also consumes volume of its own, at figures the code prints for metal and for nonmetallic barriers, and that part is a box volume question under NEC 314.16(A).

Does a pigtail made up inside the box count?

A conductor no part of which leaves the box is left out of the calculation, so a pigtail adds nothing. A conductor that originates outside the box and terminates or is spliced inside counts once, and a conductor passing through unbroken counts once as well, since splices never multiply that count. The single case that counts twice is a loop or coil of unbroken conductor at least twice the minimum length required for free conductors, which the 2026 book carries at 300.16 after numbering it 300.14 in the edition before.