Box Fill Practice Questions

A. The volume calculated under the box volume rules is not less than the fill calculation

One subsection computes the volume you have and the other computes the volume you need, and the box passes when the first is not less than the second. Equal is acceptable, which is why a problem that lands exactly on the box volume is a pass rather than a fail. There is no margin built into the comparison, so nothing here runs at 125 percent.

NEC 2026 314.16

C. 2.50 cubic inches

The allowance table runs 2.00 for 14 AWG, 2.25 for 12, 2.50 for 10, 3.00 for 8 and 5.00 for 6, with smaller figures above 14 for 16 and 18 AWG. The step from 10 to 8 is a half cubic inch and the step from 8 to 6 is two full cubic inches, which is worth noticing before you trust a pattern. Landing one row off is the whole of the error here.

NEC 2026 Table 314.16(B)(4)

B. 13.50 cubic inches

Only conductors that originate outside the box get counted, so the six entering conductors give six counts at 2.25 cubic inches each, or 13.50. A conductor no part of which leaves the box is not counted at all, so the three pigtails add nothing. Counting all nine gives 20.25, which is the answer waiting for anyone who counts wire rather than reading the sentence.

NEC 2026 314.16(B)(1), Table 314.16(B)(4)

D. 20.00 cubic inches

The minimum free conductor length is 6 inches, so a loop counts twice once it reaches twice that, which is 12 inches. Fourteen inches clears the threshold, so the two looped conductors give four counts and the other six give one count each, for ten counts at 2.00 cubic inches, or 20.00. Counting every conductor once gives 16.00 and counting all eight twice gives 32.00, and neither reads the length condition.

NEC 2026 314.16(B)(1), 300.16(A), Table 314.16(B)(4)

C. None at all

The clamp category is written for internal cable clamps, and it says in the same breath that no allowance is required for a connector whose clamping mechanism is outside the box. Nothing is inside taking up room, so nothing is charged. Where internal clamps are present the charge is one allowance for the whole group, not one per clamp, and it is sized on the largest conductor in the box.

NEC 2026 314.16(B)(2)

A. 4.00 cubic inches

The allowance is made for each type of fitting rather than for each fitting, and studs and hickeys are the two types. Three studs collapse to one type and the hickey is the second, so two allowances at 2.00 cubic inches gives 4.00. Counting four fittings gives 8.00, which is what happens when the word type slides past.

NEC 2026 314.16(B)(3), Table 314.16(B)(4)

D. 9.00 cubic inches

A yoke or strap carrying a device normally takes a double allowance, but equipment wider than a single 2 inch device box takes double allowances for each gang required to mount it. Two gangs at two allowances each is four allowances of 2.25 cubic inches, or 9.00. The 4.50 is the ordinary single yoke answer, and the stem tells you the width precisely because it has to.

NEC 2026 314.16(B)(4), Table 314.16(B)(4)

B. 3.75 cubic inches

Up to four grounding conductors take a single allowance between them, and each one beyond the fourth takes a quarter allowance, with every part of that sized on the largest grounding conductor entering the box. The largest here is 10 AWG at 2.50, so the first four take 2.50 and the two beyond take a quarter of 2.50 each, giving 3.75 total. Sizing on the 12 AWG because there are more of them is the near miss, and treating six conductors as one allowance is the habit left over from an older edition.

NEC 2026 314.16(B)(5), Table 314.16(B)(4)

C. One allowance for the assembly, sized on the largest conductor terminated to it

Each terminal block assembly takes a single allowance, sized on the largest conductor landed on that assembly. That is a different set to take the largest of from the one the clamps use, and mixing the two is a quiet way to lose the item. The double allowance belongs to a yoke or strap holding devices, not to a terminal block.

NEC 2026 314.16(B)(6)

D. A locknut

Small fittings such as locknuts, splicing connectors and bushings are named and excluded, so an inventory that lists wire nuts is listing them to be ignored. The other three each land in a category of their own. Reading the exclusion sentence once saves you from charging volume for hardware the code does not charge for.

NEC 2026 314.16(B)

A. The largest conductor connected to a device supported by that yoke

The device category names the largest conductor connected to a device on that yoke, which is not necessarily the largest conductor in the box. Clamps and support fittings do use the largest conductor present, and grounding fill uses the largest grounding conductor entering, so four categories name four different sets. A box wired in one size makes them all agree, which is why a practice problem that mixes sizes is mixing them deliberately.

NEC 2026 314.16(B)(4)

B. Clamps from the largest conductor in the box, grounding fill from the largest grounding conductor entering the box

Clamp fill is sized on the largest conductor present in the box, whatever it is for. Grounding conductor fill is sized on the largest grounding conductor entering the box, which in a normal installation is a smaller wire. Treating the two as one rule inflates or deflates the answer depending on which way you collapse them.

NEC 2026 314.16(B)(2), 314.16(B)(5)

D. 4-11/16 inch square, 1-1/4 inches deep, 25.5 cubic inches

Two of the four are already too small, so the question is only whether the 25.5 or the 30.3 is the answer, and 25.5 covers 24.0 with room to spare. Jumping to the deeper box of the same family is the reflex, and it works but it is not the smallest that passes. Reading down the whole table rather than staying in one box family is the habit this item is training.

NEC 2026 314.16(A), Table 314.16(A)

C. 18.00 cubic inches, and it passes

Four conductors give 9.00, three grounding conductors give one allowance at 2.25, the clamps give one allowance at 2.25, and the yoke gives a double allowance at 4.50, which totals 18.00 cubic inches. The table lists that box at 18.0 cubic inches, and since the volume is not less than the fill, it passes. Charging three separate allowances for the grounding conductors is what produces the 22.50.

NEC 2026 314.16(A), 314.16(B), Table 314.16(A), Table 314.16(B)(4)

A. Where it is marked with its volume, or is made from a box whose dimensions are listed in the metal box table

The marked or listed condition is the whole of it, and it reaches plaster rings, domed covers, extension rings and similar items. A problem that looks impossible often turns out to include a marked ring that is there to be added. A stem that mentions a ring and says nothing about a marking is asking a different question.

NEC 2026 314.16(A)

B. Half a cubic inch if it is metal, and one cubic inch if it is nonmetallic

An unmarked barrier takes up volume of its own, and the section prints the two figures: half a cubic inch for metal and a full cubic inch for nonmetallic. A barrier is easy to forget entirely because it is neither a conductor nor a device, so nothing in the six categories reminds you it is there. Where the barrier is marked with its volume, that marking governs instead.

NEC 2026 314.16(A)

C. Each space is calculated separately

Where barriers are installed, the volume is apportioned to each resulting space and each space is then run as its own fill problem. Two halves passing on a combined basis can easily have one half failing on its own, which is the whole reason the rule is written this way. The barrier itself still consumes volume unless it is marked with its own.

NEC 2026 314.16(A)

A. It may be marked with its volume, and that marking is what gets used

Boxes described in the table that have more volume than the table gives are permitted to be marked with their actual volume, and a marked volume is a real number rather than an estimate. The general marking rule applies to boxes of 100 cubic inches or less that are not described in the table, and to nonmetallic boxes. Where a stem hands you a marked volume, use it and leave the table shut.

NEC 2026 314.16(A)(2)

D. A terminal housing supplied with a motor or generator

One short sentence near the front of the section takes motor and generator terminal housings out of scope and points at the motor article for their volume instead. An item set at a motor is either testing something else entirely or testing whether you know that exclusion is there. The other three all sit inside the article, though a conduit body is sized by its own subsection rather than by box fill unless something is spliced in it.

NEC 2026 314.16

B. Equipment grounding conductors, or not more than four fixture wires smaller than 14 AWG, or both

The exception is narrow and it carries three conditions at once: the conductors enter from a domed luminaire or similar canopy, they terminate within that box, and they are either grounding conductors or fixture wires smaller than 14 AWG limited to four, or both. It exists so a fixture canopy does not fail a box that is otherwise fine. Widening it to any small conductor is the reading that gets people into trouble.

NEC 2026 314.16(B)(1)

A. A depth not less than the depth of the equipment plus a quarter inch

Depth is a separate test from volume, and the large equipment case sets the depth at the depth of the equipment plus a quarter inch. A shallow box can pass on cubic inches and still fail here, which is why a stem that mentions a device projecting into the box is pointing at the depth section rather than at the fill rules. Extension boxes, plaster rings and raised covers are included when that depth is measured.

NEC 2026 314.24

D. Six

Six categories are added: conductor fill, clamp fill, support fittings fill, device or equipment fill, equipment grounding conductor fill and terminal block fill. Some count per item, one counts once for the whole group and one counts once per type, so knowing the count is only the start. Anything in the box that does not land in one of the six is either excluded or is not a fill question.

NEC 2026 314.16(B)

B. 4.00 cubic inches

The allowance attaches to the yoke or strap rather than to each device on it, so a yoke carrying one or more devices is still one double allowance. Two allowances of 2.00 cubic inches gives 4.00. Charging a double allowance per device gives 8.00, which is the answer this item is built to catch.

NEC 2026 314.16(B)(4), Table 314.16(B)(4)

C. 31.50 cubic inches, and it fails

Eight conductors give 18.00, the four grounding conductors give one allowance at 2.25, both clamps together give one allowance at 2.25, and two yokes give two double allowances at 4.50 each, for 31.50 cubic inches. The table lists that box at 30.3 cubic inches, so it comes up 1.20 short and fails. Charging two allowances for the two clamps is what produces the 33.75.

NEC 2026 314.16(A), 314.16(B), Table 314.16(A), Table 314.16(B)(4)

D. 5.00 cubic inches

The 6 AWG row reads 5.00 cubic inches, which is a two cubic inch jump from the 3.00 at 8 AWG rather than the half inch steps further up the table. Six AWG is also the last row on the table, because a box enclosing 4 AWG or larger is sized by the pull and junction box rules instead. Guessing a smooth progression from the smaller sizes gives the 3.50 or 4.00 shape of answer.

NEC 2026 Table 314.16(B)(4), 314.16