Electrical
NEC box fill sizing and calculation for electrical crews
Count the conductors, the devices, the grounds, and the clamps, total the cubic inches, and prove the box is big enough before the cover goes on.
Direct answer
Box fill is the cubic-inch volume conductors and fittings occupy inside an outlet, device, or junction box. NEC 314.16 sets a volume allowance per conductor by wire size, counts each device as two and all grounds as one, and requires the total stay at or under the box volume. The adopted code edition and AHJ govern.
Key takeaways
- NEC 314.16 requires the total required box fill volume in cubic inches stay at or under the box volume, and inspectors enforce it.
- Volume allowance per conductor from Table 314.16(B): 14 AWG is 2.0, 12 AWG is 2.25, 10 AWG is 2.5 cubic inches.
- A device on a yoke (receptacle or switch) counts as two conductor allowances at the largest conductor connected to it.
- All equipment grounds together count as one allowance at the largest ground for up to four; all internal clamps count as one; pigtails count as zero.
- Two 12/2 cables on a duplex receptacle with clamps total 18.0 cubic inches, requiring a deep single-gang box.
Box fill, and why the code caps it
Box fill is the volume the conductors, the device, the grounds, and the fittings take up inside an outlet, device, or junction box, totaled in cubic inches and held to the volume of the box. The code caps it for the same reason it caps conduit fill, and the reason bites in two places. Crowd a box and the splices cannot shed heat, so the connections run hot under load. Crowd it worse and you cannot fold the conductors back in to land the cover without nicking insulation or backing a wire nut loose against the back of the box.
A stuffed box is also a box nobody can work in later. The next person who opens it to add a circuit or chase a fault is fighting your overfill with the power off and a flashlight in their teeth. That is when a tired splice lets go.
This guide is the box side of the count, the volume inside the enclosure. The raceway side, the cross-section the conductors take up in the pipe between boxes, is a separate calculation with its own tables. See the conduit fill guide for that. The two get confused constantly, and they are not the same check.
Box fill vs conduit fill: what is the difference?
Box fill is a volume calculation. Conduit fill is an area calculation. Box fill totals the cubic inches the conductors and fittings occupy inside the enclosure and compares that to the box volume from NEC 314.16. Conduit fill totals the cross-sectional area of the conductors and compares that to the inside area of the pipe under NEC Chapter 9. One is about the room in the box. The other is about the room in the raceway feeding it.
They use different numbers and different tables, and a run that passes one can fail the other. A conduit can be legally filled and still feed a box that is too small for the splices and the device inside it. A box can have plenty of volume while the pipe leaving it is jammed past 40 percent. Run both checks. They answer different questions.
The simplest way to keep them straight: box fill is cubic inches inside the enclosure, conduit fill is percent area inside the pipe. When you are counting splices and a device, you are doing box fill. When you are counting conductors against the bore of a conduit, you are doing conduit fill. The conduit fill guide covers the raceway side in full.
What does NEC 314.16 actually require?
NEC 314.16 is the box fill section, and it works in two halves. The first half, 314.16(A), gives the volume of standard boxes and tells you to use the volume marked on the box when it is marked. The second half, 314.16(B), tells you how to total the volume the contents require. The rule passes when the required volume is at or under the box volume.
Unlike voltage drop, this is enforceable. The 3 percent and 5 percent voltage-drop figures are advisory notes. Box fill is a hard requirement with a table behind it, and an inspector who counts a box over its volume will fail it. There is no design-target softness here.
Work in cubic inches and nothing else. Box fill is volume, not the cross-sectional area you use for conduit. The subsection numbering inside 314.16, the (B)(1) through (B)(5) breakdown, has shifted between code cycles as the grounding and clamp rules were rewritten, so confirm the exact citation against the edition the jurisdiction has adopted and any local amendments before you put a section number on a submittal. The two anchors that hold across editions are 314.16(A) for the box volume and 314.16(B) for the fill.
Volume per conductor by wire size
Each conductor gets a fixed volume allowance based on its size, from Table 314.16(B). You do not measure the actual wire. The table assigns a cubic-inch value per AWG, and you multiply by how many conductors of that size you count. A 14 AWG conductor is 2.0 cubic inches, a 12 AWG is 2.25, and a 10 AWG is 2.5. Those three are the ones to carry in your head, because they cover most branch work.
When a box has conductors of more than one size, you total each size at its own allowance and add them up. Six 12 AWG and two 10 AWG is not eight of anything. It is six at 2.25 plus two at 2.5. The mistake of rating the whole box at the smallest or the largest size is common and it always pushes the number the wrong way.
The full table runs from 18 AWG up. The values below are the ones that show up in everyday device and junction boxes.
| Conductor size (AWG) | Volume allowance (cubic inches) |
|---|---|
| 18 | 1.50 |
| 16 | 1.75 |
| 14 | 2.00 |
| 12 | 2.25 |
| 10 | 2.50 |
| 8 | 3.00 |
| 6 | 5.00 |
How do you count conductors, devices, grounds, and clamps?
The count is where most box fill mistakes happen, because the box holds things that are not loose conductors but still take a volume allowance. Get the rules straight and the arithmetic is easy. Get them wrong and a passing box reads as failing or a failing box reads as fine.
Each conductor that enters the box from outside and terminates or is spliced inside counts as one, at its own size. A conductor that runs through the box without a splice also counts as one. A pigtail or any conductor that starts and ends inside the box, with no part leaving it, is not counted at all. That last one trips people who count the short jumpers off a wire nut.
The fittings and the device each carry their own allowance. A device on a yoke or strap, a receptacle or a switch, counts as two conductors at the volume of the largest conductor connected to that device. All the equipment grounding conductors together count as one allowance, at the size of the largest ground present, for up to four of them. All the internal cable clamps in the box together count as one allowance, at the largest conductor entering the box. Support fittings like fixture studs or hickeys each add one allowance at the largest conductor present. Recent editions added a quarter-allowance for each grounding conductor past the fourth, so confirm the grounding count rule against the adopted edition.
| Item in the box | How it counts | Sized to |
|---|---|---|
| Conductor terminating or spliced | 1 each | Its own size |
| Conductor passing through unbroken | 1 | Its own size |
| Pigtail (never leaves the box) | 0, not counted | Not applicable |
| Device on a yoke (receptacle or switch) | 2 | Largest conductor on the device |
| All equipment grounds together | 1 (up to four) | Largest ground present |
| All internal cable clamps together | 1 | Largest conductor in the box |
| Support fitting (stud or hickey) | 1 each | Largest conductor present |
Totaling the box fill
Total the box fill by adding every allowance the contents earn, then check that total against the box volume. The required fill is the sum of the conductor allowances, plus two allowances for each device, plus one for the grounds, plus one for the clamps, plus one for each support fitting. The box passes when that required volume is at or under the volume of the box.
Run it in one pass so nothing gets dropped. Conductors first, by size. Then the device count, two each at the largest conductor on it. Then the grounds, one allowance. Then the clamps, one allowance. Then any studs or hickeys. Add the cubic inches and compare. The comparison is the whole rule: required at or under available.
Vreq = Σ(Nawg × Vawg) + (2 × Vdevice) + Vgrounds + Vclamps + VfittingsVreq ≤ Vbox- N<sub>awg</sub>
- Number of counted conductors of a given wire size
- V<sub>awg</sub>
- Volume allowance for that wire size from Table 314.16(B)
- V<sub>device</sub>
- Allowance for the largest conductor connected to the device, counted twice per device
- V<sub>box</sub>
- Box volume from 314.16(A) or the volume marked on the box
Field example: a standard duplex receptacle box
A common device box: two 12/2 cables landing on a duplex receptacle, with internal cable clamps, everything 12 AWG. Two cables means two hot conductors and two neutrals entering and terminating, so four insulated conductors at 2.25 each, which is 9.0 cubic inches. The two equipment grounds together count as one allowance at 12 AWG, 2.25. The internal clamps together count as one allowance at 12 AWG, another 2.25.
The receptacle is the piece people forget. It sits on a yoke, so it counts as two conductors at the largest conductor connected to it, 12 AWG. That is 2 times 2.25, or 4.5 cubic inches, for the device alone. The pigtails between the device and the wire nuts do not count, because they never leave the box.
Add it up: 9.0 for the conductors, 2.25 for the grounds, 2.25 for the clamps, 4.5 for the device, and you land on 18.0 cubic inches. That is exactly an 18 cubic inch box, the deep single-gang device box, with nothing to spare. This is why a standard two-cable receptacle maxes out an 18 cubic inch box, and why a shallower box will not legally hold it.
| Item | Count | Allowance | Subtotal |
|---|---|---|---|
| 12 AWG insulated conductors | 4 | 2.25 | 9.00 |
| Equipment grounds (all) | 1 | 2.25 | 2.25 |
| Internal cable clamps (all) | 1 | 2.25 | 2.25 |
| Duplex receptacle (device) | 2 | 2.25 | 4.50 |
| Required fill | 18.00 cu in | ||
| Box needed | 18.0 cu in or larger |
A second example: a two-gang switch box
Switch boxes add up faster than people expect, because every device on the yoke costs two. Take a two-gang box with two single-pole switches, a 14/2 power feed in, and a 14/2 switch leg out to each fixture, with internal clamps, all 14 AWG. The feed brings a hot and a neutral. Each switch leg is two conductors. That is six insulated conductors entering and terminating, at 2.0 each, for 12.0 cubic inches.
All three cable grounds together count as one allowance at 14 AWG, 2.0. The clamps together count as one allowance, another 2.0. Now the devices: two switches on two yokes, each counting as two conductors at 14 AWG, so 4.0 per switch and 8.0 for the pair. The pigtails to the switch screws do not count.
Total: 12.0 plus 2.0 plus 2.0 plus 8.0 is 24.0 cubic inches. That box has to be at least 24 cubic inches, which rules out a lot of shallow two-gang boxes. Add a third switch or a third cable and you are past what a standard two-gang will hold, which is exactly the situation where crews cram it instead of going to a deeper box.
Where the box volume comes from
The available volume is the marked volume on the box when it is marked, or the value from Table 314.16(A) for standard boxes. Manufacturers stamp the cubic-inch capacity inside many boxes, and that marked number is the one to use, because plaster rings, extension rings, and listed accessories add their own marked volume on top. Add the marked volume of every section you assemble.
For standard boxes without a usable marking, Table 314.16(A) lists the volume by trade size. A deep single-gang device box is 18.0 cubic inches. A 4 inch square 1-1/2 inch deep box, the workhorse 1900 box, is 21.0 cubic inches, and the 2-1/8 inch deep version is 30.3. The table covers the round, octagonal, square, and device boxes the trade uses every day.
Gangable and sectional boxes are added up by their marked volumes, not guessed. A device ring or mud ring adds the cubic inches stamped on it, and that addition is often what gets a tight count over the line legally. The point is to read the number off the hardware rather than estimate it, because the inspector will read the same number.
| Box (trade size) | Volume (cubic inches) |
|---|---|
| Device box, 3 x 2 x 2-3/4 | 14.0 |
| Device box, 3 x 2 x 3-1/2 (deep) | 18.0 |
| 4 in square x 1-1/2 deep (1900) | 21.0 |
| 4 in square x 2-1/8 deep | 30.3 |
| Any box marked by the maker | Use the marked volume |
Why a device counts as two conductors
A receptacle or a switch on a yoke counts as two conductor allowances, sized to the largest conductor connected to that device. People read this as a code quirk and then forget it, which is how a count comes up short. The device body and its yoke take real room in the box, and the rule books that room as two allowances of the largest conductor landing on it.
Sized to the largest conductor is the part that gets missed. If a 10 AWG and a 12 AWG both land on the same receptacle, the device counts as two at 2.5, not two at 2.25. The largest conductor on the device sets the rate, even if most of what is on it is smaller.
A wide device matters too. Recent editions count a device or assembly that occupies more than one gang width as two allowances for each gang it spans, so a large dimmer or a wide combination device can cost more than a single duplex. Confirm the wide-device rule against the adopted edition, because it has been refined across cycles.
Conduit bodies: when a splice is allowed inside one
A conduit body, the LB, LL, LR, T, or C casting in a run of pipe, can hold a splice or a device only when it is durably and legibly marked with its volume. If the body has no marked cubic-inch volume, you cannot splice, tap, or put a device in it. That rule, in 314.16(C), is the one that catches crews who treat an LB as a junction box because it was handy.
When the body is marked, the splice count follows the same box fill logic: total the conductor allowances and the device allowance against the marked volume of the body. Conduit bodies marked for splices are larger than the standard pull casting, and the difference is the volume the maker certified for the contents.
For conduit bodies used only as pull or angle points with conductors passing straight through and no splice, a different and simpler set of cross-section and length rules applies, and the conduit and raceway guides cover the bend and pull side. The line to remember here: no marked volume means no splice and no device, full stop.
Box fill, conduit fill, and derating are three separate checks
Box fill, conduit fill, and ampacity derating are three different calculations that crews blur into one. Box fill is the cubic-inch volume inside the enclosure. Conduit fill is the percent of the pipe's cross-section the conductors occupy. Derating is the reduction in allowable ampacity when too many current-carrying conductors share a raceway and cannot shed heat. They overlap in the same run, but they are not the same number and they do not pass or fail together.
The trap is finishing one and assuming the others followed. A box can have legal fill while the conduit feeding it is over 40 percent. A conduit can be under its fill limit while the conductor bundle inside it triggers a derating that drops the ampacity below the load. Each check has its own table and its own pass condition.
Run them in sequence on any real run: size the conductor for ampacity and any derating from the bundle, check the conduit fill for the raceway, then check the box fill at every enclosure where the conductors land or splice. The conduit fill guide and the ampacity material cover the raceway and heat side. This guide covers the volume in the box.
High-density and data-center boxes
On data-center and high-density jobs the box fill count is where the home-runs and the device density collide. A box gathering many branch circuits, a bank of receptacles, or a dense rack of low-voltage and line-voltage conductors hits its volume limit long before the conduit does. The crews used to residential single-gang math underestimate these badly.
The fittings stack up. Multiple cable assemblies mean the clamp allowance is in play, multiple devices each cost two at their largest conductor, and the grounding conductors, while they pool into one allowance up to four, start adding the quarter-allowance per ground past the fourth under recent editions. A box that looks roomy fills up on the count even when it looks half empty.
The move on these is to oversize the enclosure deliberately and read the marked volumes of every ring and extension you stack, then run the count before the rough-in is set, not at trim. Pulling a too-small box after the conductors are landed is expensive, and on a dense board it is a day, not an hour.
What does the inspector check on box fill?
An inspector checks box fill by counting what is in the box and comparing it to the box volume, and they do it fast because they have the pattern memorized. They count the insulated conductors by size, add two for each device on a yoke, add one for the grounds, add one for the clamps, and add any studs or hickeys. Then they read the marked volume off the box or pull the trade size from the table.
The things they catch most are the device that was not counted as two, the grounds and clamps left out of the total, and the box with no marked volume holding a splice it should not. On a conduit body they look for the volume marking first, because an unmarked body with a splice in it is an automatic correction.
They will also look at whether the box is physically workable, not just legal on paper. A count that passes at 18.0 in an 18 cubic inch box is legal, but if the conductors are jammed so the device will not seat without forcing it, that is a workmanship flag. Legal fill and a good install are not always the same thing, and the good inspectors know the difference.
Leave room: the box you will work in later
Sizing a box to exactly its volume limit passes the calculation and creates a headache for everyone who opens it after you. Hit the limit dead on and there is no room for the change order that adds a circuit, the device swap that lands a larger conductor, or the splice somebody needs to make in two years. Box fill is a minimum, not a target to ride.
The cheap insurance is going one box size deeper than the count demands on anything that might grow, which is most boxes feeding devices and most junctions on a commercial job. The volume costs almost nothing at rough-in. The recount and re-pull when the box turns out too small costs labor and a callback.
Do not stuff the box to make a number work. If the count is over, go to a larger box or a deeper ring. Forcing conductors past the volume to seat a cover is how a splice works loose, and a loose splice in a crowded box is the failure that shows up as intermittent power or a warm cover plate long after you are gone.
Plaster rings, extension rings, and the volume they add
The box itself is rarely the only space you get to work with. A plaster ring (also called a mud ring) raises the device mounting plane out to the finished wall surface, and it carries its own marked volume that adds to the box. So does an extension ring set behind it, and so does a raised cover on a handy box. When a fitting is marked with a cubic-inch capacity, you add that number to the box capacity and compare the total against the fill you calculated. An unmarked fitting adds nothing you can count.
This matters most on remodel and tenant work, where a shallow box was set years ago and the new device count has crept up. A deeper plaster ring or a box extender is often the cleanest way to gain the capacity you need without opening the wall and replacing the box. Pick the ring by its marked volume, not by how it looks, and keep the marking visible until the inspection is signed off.
Old-work and low-voltage rings are a separate matter. A low-voltage mounting bracket or open ring that does not enclose conductors is not a box and has no fill allowance, and you cannot run line-voltage splices into it. Confirm what you are mounting before you assume it buys you room, because the number that counts is the listed volume of an actual enclosure or fitting, not the apparent gap behind the wall.
How the equipment grounding and isolated conductors get counted
Equipment grounding conductors are the most miscounted item in the box. No matter how many enter, all of the standard equipment grounding conductors together count as a single volume allowance, based on the largest of them. Pigtailing six bare grounds to one device tail still counts as one. The reason is practical: the grounds bond together at one point, so the code treats the bundle as one conductor for the space it occupies.
There is a second allowance worth knowing. Where an isolated equipment grounding conductor is run for a clean ground, that conductor is permitted to be counted as a separate single allowance in addition to the standard grounding allowance. So a box with normal grounds and one isolated ground gets two grounding allowances total, each sized to the largest in its group, not one and not six.
Conductors that originate and terminate inside the box without leaving it, such as short jumpers and pigtails, are not counted at all. A conductor only earns a volume allowance when it enters the box from outside and is spliced or terminated, or passes through. Counting your pigtails is the fastest way to fail a fill calculation that the box would actually have passed.
What to document
Record the count, the box volume, and the result, so the box can be defended at inspection and reconstructed when someone changes the circuit later. A box fill calculation that lives only in your head is a calculation the next person has to redo from scratch, usually with the power off and the splices already made.
Capture the box and its volume source, the conductor count by size, the device count, the grounds and clamps, any support fittings, the required fill total, and the result against the box volume. On the boxes that matter, the dense ones and the ones likely to grow, write down which box size you specified and why, because the next crew will want to know whether there is room before they open it.
| Field to record | Why it matters |
|---|---|
| Box type and volume source | Marked volume or Table 314.16(A) sets the limit |
| Conductors by size | Each size carries its own allowance |
| Devices on yokes | Each counts as two at its largest conductor |
| Grounds and clamps | Each is one allowance, commonly missed |
| Support fittings | Studs and hickeys each add one allowance |
| Required fill total | The number compared to the box volume |
| Result and box specified | Proves the box was sized, not guessed |
Common mistakes
- Filling a box by feel with no count, then forcing the device to seat.
- Forgetting the device counts as two conductors at its largest connected conductor.
- Leaving the equipment grounds and the internal clamps out of the total.
- Counting pigtails that never leave the box, which the code does not count.
- Rating every conductor at one size instead of totaling each size at its own allowance.
- Mixing up box fill volume with conduit fill area and running only one of the two checks.
- Splicing in a conduit body that has no marked volume.
- Sizing a box to exactly its limit with no room for a future circuit.
Field checklist
Want this checklist to run itself on every job — with photo proof and a signed record crews can hand the customer? That's FieldOS.
Standards and references
The NEC, NFPA 70, is where box fill lives, at 314.16. The box volume comes from 314.16(A), which gives the volume of standard boxes and directs you to the volume marked on the box when it is marked. The contents are totaled under 314.16(B), which sets the per-conductor allowance in its table and the counting rules for devices, grounds, clamps, and support fittings. Conduit bodies are covered at 314.16(C), with the marked-volume requirement before a splice or device is allowed inside one.
The subsection numbering under 314.16(B) has changed across code cycles as the grounding and clamp provisions were rewritten, and the wide-device and extra-ground rules have been refined more than once. Confirm the exact subsection against the edition the jurisdiction has adopted and any local amendments before citing it on a submittal. The 314.16(A) and 314.16(B) anchors hold across editions.
The box maker's marked volume governs the available capacity, and the listing of the box and its accessories controls what can be assembled and counted. Where the project specification or the AHJ is stricter than the minimum, that controls. Cite the section that governs the point, and let the adopted edition settle the exact number.
Units, terms, and conversions
Box fill is worked in cubic inches in the NEC, and the same idea reads in cubic centimeters on metric drawings and equipment, where 1 cubic inch is about 16.4 cubic centimeters. Keep the volume work in the unit the box is marked in and convert only when a drawing forces it.
Conductor size is American Wire Gauge, where the number runs opposite to the size: a smaller AWG number is a larger conductor, so 10 AWG is bigger than 14 AWG and carries the larger allowance. The volume allowance per conductor is the cubic inches the table assigns each size, separate from the conductor's actual physical volume.
- Box fill
- The cubic-inch volume the conductors, device, grounds, clamps, and fittings occupy inside a box
- Volume allowance
- The cubic inches Table 314.16(B) assigns to each conductor size for the count
- Yoke / strap
- The mounting frame of a device, the reason a receptacle or switch counts as two
- EGC
- Equipment grounding conductor, all of them counted as one allowance up to four
- Conduit body
- A fitting like an LB or T in a raceway run, which needs a marked volume to hold a splice
- AWG
- American Wire Gauge; the smaller the number, the larger the conductor and the allowance
FAQ
What is box fill?
Box fill is the total cubic-inch volume the conductors, the device, the grounds, the clamps, and any fittings occupy inside an outlet, device, or junction box. NEC 314.16 sets a volume allowance per item and requires the total stay at or under the box volume so splices stay cool and workable.
How do you calculate box fill?
Count the insulated conductors by size at their Table 314.16(B) allowance, add two allowances for each device, one for all grounds, one for all clamps, and one per support fitting. Total the cubic inches and confirm it is at or under the box volume from the marking or Table 314.16(A).
Does a device count in box fill?
Yes. A receptacle or switch on a yoke counts as two conductor allowances, sized to the largest conductor connected to that device. A duplex receptacle landing 12 AWG counts as two at 2.25 cubic inches, or 4.5 total for the device, before you count any of the actual conductors.
Do grounds count in box fill?
Yes, but all the equipment grounding conductors together count as a single allowance at the largest ground present, for up to four of them. Recent editions add a quarter-allowance for each ground past the fourth, so confirm the grounding rule against the adopted code edition before you finalize a dense count.
How much volume does a 12 AWG conductor need in box fill?
A 12 AWG conductor needs 2.25 cubic inches under Table 314.16(B). For comparison, 14 AWG is 2.0 and 10 AWG is 2.5. When a box mixes sizes, total each size at its own allowance rather than rating the whole box at one number, or the result comes out wrong.
Do pigtails count in box fill?
No. A conductor that begins and ends inside the box, with no part leaving it, is not counted. The short jumpers off a wire nut to a device are pigtails and add nothing to the box fill total. Only conductors that enter, leave, or are spliced through the box earn an allowance.
What size box do I need for two cables and a receptacle?
Two 12/2 cables on a duplex receptacle with internal clamps need 18.0 cubic inches: four conductors at 2.25, one ground allowance, one clamp allowance, and the device counted twice. That is a deep single-gang device box. A shallower box will not legally hold the standard two-cable receptacle.
Box fill vs conduit fill: what is the difference?
Box fill is a cubic-inch volume calculation inside the enclosure under NEC 314.16. Conduit fill is a percent-of-area calculation inside the pipe under Chapter 9. A box can pass while its feeding conduit is overfilled, and the reverse, so run both checks on any real raceway run.
Can I splice inside a conduit body?
Only if the conduit body is durably and legibly marked with its volume. Without a marked cubic-inch volume, an LB, T, or C body cannot hold a splice, a tap, or a device. When it is marked, the splice count follows the same box fill logic against the marked volume.
Is box fill a code requirement or just a recommendation?
Box fill is an enforceable requirement under NEC 314.16, not advisory like voltage drop. An inspector can fail a box that exceeds its volume, with the table behind the call. The adopted code edition and the AHJ govern the exact rule, and a stricter project specification controls when it applies.
People also ask
Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.