Box Fill

📍 Where this lives in the book

Rules: 314.16 · Box volumes: Table 314.16(A) · Conductor allowances: Table 314.16(B)

All three are in the same place — tab it once and you've got the whole topic.

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Why this trips people up

Box fill is one of the first calculations you'll be asked to do on an exam, and one of the few you'll actually do in your head on a job site for the next thirty years. It's also where I see apprentices lose points for reasons that have nothing to do with math.

The math is addition. That's it. You add up volume allowances and compare the total to the size of the box. Nobody fails box fill because they can't add.

They fail it because they count the wrong things.

So before we touch a single number, get this straight:

Box fill is not "how many wires are in the box." It's "how much volume does the code charge me for." Those are different, and the gap between them is where the test questions live.

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The five things that take up volume

Per NEC 314.16(B), five categories get charged volume. Learn them as a list and run the list every single time — on the test and in the field.

#	What	What it costs	NEC
1	Insulated circuit conductors	1 allowance each	314.16(B)(1)
2	Internal cable clamps	1 allowance total, based on the largest conductor in the box	314.16(B)(2)
3	Support fittings (fixture studs, hickeys)	1 allowance each, largest conductor	314.16(B)(3)
4	Devices / yokes (switches, receptacles)	2 allowances each, based on the largest conductor connected to that yoke	314.16(B)(4)
5	Equipment grounding conductors (all of them)	1 allowance total, based on the largest EGC	314.16(B)(5)

Five categories. Run them in order. You will not miss anything.

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The volume allowance table

This is NEC Table 314.16(B). You need these six numbers cold:

Conductor size	Volume allowance
#18 AWG	1.50 cu in
#16 AWG	1.75 cu in
#14 AWG	2.00 cu in
#12 AWG	2.25 cu in
#10 AWG	2.50 cu in
#8 AWG	3.00 cu in
#6 AWG	5.00 cu in

The three bolded ones are the residential world. Memorize 14 = 2.00, 12 = 2.25, 10 = 2.50. I have used those three numbers more times than I can count and the other four maybe a dozen times total.

A trick for remembering: from #14 up, each size adds a quarter — 2.00, 2.25, 2.50. Then the pattern breaks at #8. It breaks because the code says so, not because of any logic you need to understand.

Why memorize anything on an open-book exam?

Because you're not short on information. You're short on time.

Yes, this table is right there in the book. But flipping to it, finding the row, and flipping back is thirty to sixty seconds — and a box fill question may need you to do it three times for three different conductor sizes. Do that across a whole exam and it's the difference between finishing and not.

Every number you know cold is a lookup you don't make. That's the entire reason to memorize anything for an open-book test: not correctness, speed. Pick the handful of numbers you'll use over and over and burn them in. Look up the rest.

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What counts, and what doesn't

This is the part that actually decides whether you get the question right.

Counts as a conductor (1 allowance each)

• Every insulated conductor that enters the box and terminates inside it. Hot, neutral, travelers, switch legs — doesn't matter what it does, if it comes in and lands, it counts.
• Every insulated conductor that passes through the box unbroken. It came in, it left, it counts once.

Counts as TWO conductors

• A conductor that loops through the box unbroken, if the loop is 12 inches or longer. Under 12 inches, it counts once. Twelve inches or more, it counts twice.

  Here's why that number isn't arbitrary, and this is worth understanding rather than memorizing: 300.14 requires at least 6 inches of free conductor at every box for making up splices and terminations. A loop long enough to be two free conductors — 2 × 6" — is 12 inches. So the code charges you for two, because functionally that's what you've got.

  Understand the 6" rule and you never have to memorize the 12" rule.

  This is a classic exam question. They will show you a box with a wire looping through and see if you charge it once or twice. Check the stated loop length. That's the whole question.

Does NOT count at all

• Pigtails. A conductor that both starts and ends inside the box is free. This is the single most common mistake I see. You splice three grounds together and add a 6" pigtail to the device — that pigtail is not charged.
• Wire connectors, wire nuts, tape, and the splice itself. No volume.
• Locknuts, bushings, and cable connectors on the outside of the box. They're outside. They cost nothing.

Read that last one twice. Internal clamps cost you an allowance. External connectors cost you nothing. Same hardware family, opposite answer, and the test knows it.

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Devices are the other big trap

A single-gang switch or receptacle costs you two allowances, not one.

Why two? Because the code figures a device takes up about as much room as a pair of conductors. You don't need the rationale. You need the habit: see a yoke, charge two.

And it's two allowances of the largest conductor connected to that yoke — not the largest in the box. That distinction is the whole trap. If you've got #12 feeding through the box but the device is pigtailed with #14, that yoke gets charged at #14, because #14 is what's landed on the device.

Read the question carefully. It will tell you what's feeding the box and what's landing on the device, and it expects you to notice they're different.

There's also a rule for devices wider than a single gang, where the count scales with the gangs it occupies. Rare in residential. Worth knowing it exists.

⚠️ A trap worth understanding, because it caught me

Out in the field, you will functionally never charge a yoke more than #10 — 2.50 cu in, so 5.00 cu in for the yoke. That's because a standard switch or receptacle isn't listed to accept anything bigger than #10. You physically can't land #8 on it. After enough years this becomes so reliable that it starts to feel like a rule.

It isn't one. There is no cap in 314.16(B)(4). The code says largest conductor connected to the yoke, full stop — and if an exam hands you a heavy-duty device fed with #8 or #6, you charge two allowances of that conductor. Not #10.

I'm telling you this because I believed the cap was in the code, went and read 314.16(B)(4) expecting to find it, and it wasn't there.

This is the single most useful habit in this whole book: the field teaches you patterns, the exam tests the text, and the gap between them is where experienced people lose points. Thirty years of a rule being true in practice does not put it in the code. When it matters, open the book.

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Grounds: all of them, one charge

Every equipment grounding conductor in the box — however many there are — gets you a single allowance, sized to the largest EGC present.

Six grounds of #12 in a box? One allowance of 2.25 cu in. Not six.

This is a gift, and apprentices routinely throw it away by charging each ground separately.

One wrinkle, and it's an edition-dependent one: starting with the 2020 NEC, each EGC beyond the first four adds a quarter allowance.

So five #12 grounds isn't 2.25 cu in — it's 2.25 + (1 × 0.25 × 2.25) = 2.81 cu in.

If you're testing in a jurisdiction still on the 2017 code, that quarter-allowance rule doesn't exist yet and all grounds are a single allowance regardless of count. Know which edition your exam is written to.

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Box volumes

For metal boxes, you look up the volume in NEC Table 314.16(A). Here are the ones you'll actually see in a house:

Box	Volume
4" round/octagon × 1¼"	12.5 cu in
4" round/octagon × 1½"	15.5 cu in
4" round/octagon × 2⅛"	21.5 cu in
4" square × 1¼"	18.0 cu in
4" square × 1½"	21.0 cu in
4" square × 2⅛"	30.3 cu in
4-11/16" square × 1½"	29.5 cu in
4-11/16" square × 2⅛"	42.0 cu in
3" × 2" × 2¼" device	10.5 cu in
3" × 2" × 2½" device	12.5 cu in
3" × 2" × 2¾" device	14.0 cu in
3" × 2" × 3½" device	18.0 cu in

For nonmetallic boxes — the blue and orange plastic you're putting in all day — you don't look anything up. The volume is molded into the box. Look inside. It's stamped right there, usually on the back or a side wall.

That's a field habit worth building now: before you start stuffing a plastic box, glance at the number molded in it. Takes one second and it's the actual legal answer.

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Worked example: the one you'll actually see

Here's a box I've wired a thousand times. Kitchen, 14-2 in and 14-2 out, one receptacle, metal device box with internal clamps.

What's in it:

• 14-2 coming in: 1 hot, 1 neutral, 1 ground
• 14-2 going out: 1 hot, 1 neutral, 1 ground
• One duplex receptacle
• Internal cable clamps
• Grounds spliced with a pigtail to the device

Run the five categories:

1. Insulated conductors. Two hots and two neutrals = 4 conductors at #14.

4 × 2.00 = 8.00 cu in

(The grounds are not counted here — they get their own category. And the ground pigtail to the device is free, because it starts and ends in the box.)

2. Internal clamps. One allowance, largest conductor is #14.

1 × 2.00 = 2.00 cu in

3. Support fittings. None.

0.00 cu in

4. Device. One yoke, two allowances, #14 on the yoke.

2 × 2.00 = 4.00 cu in

5. Grounds. Two #14 EGCs — one single allowance.

1 × 2.00 = 2.00 cu in

Total required: 8.00 + 2.00 + 0.00 + 4.00 + 2.00 = 16.00 cu in

Now look at the box table. A 3" × 2" × 3½" device box is 18.0 cu in — that works. A 3" × 2" × 2¾" box is 14.0 cu in — that does not.

And that right there is why the deep boxes are worth carrying on the truck. A standard two-cable, one-device kitchen box needs 16 cubic inches and half the device boxes at the supply house don't have it.

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What this looks like on the job

A few things the code book won't tell you:

Overfilled boxes are an inspection item, and inspectors know the common ones. Two cables and a device in a shallow metal box is the classic fail. If you're remodeling and reusing existing boxes, this is the one to check first.

Old work boxes are small. Those plastic remodel boxes with the wings are often 14–18 cubic inches. They fill up fast, especially if somebody before you already ran three cables into it.

When you're over, your options are: a deeper box, an extension ring on a metal box, or reducing what lands in the box (moving a splice elsewhere). An extension ring adds its own marked volume and is often the cheapest fix on a remodel.

If you're within a fraction of a cubic inch, go bigger. I have never once regretted a box that was too big. I have absolutely regretted the other one, usually while trying to fold #12 into a box in a ceiling at the end of a long day.

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Check yourself

Work these without looking back up the page.

1. A box has three 12-2 cables entering and all three terminate inside. There's one single-pole switch, no clamps, all grounds spliced together. What's the minimum box volume?

2. Same box, but one of the cables passes straight through without being spliced. Does your answer change? Why or why not?

3. You have a plastic old-work box marked 18.0 cu in. You need to land two 14-2 cables and a duplex receptacle. Does it fit? What's your margin?

4. Why does an internal cable clamp cost you volume when the cable connector on a metal box's knockout doesn't?

Answers with full work-throughs are just below. Work the problems first.

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Cross-reference: the free Box Fill Calculator at askthejourneyman.com/calculators/box-fill runs this exact math. Use it to check your work on these problems — and to check a box on your phone when you're standing in front of one.