How to calculate box fill, item by item
Add up six things: conductors, cable clamps, support fittings, device yokes, equipment grounding conductors, and terminal blocks. Each gets a volume allowance from the same small table, but each counts differently — and three of them are based on a 'largest conductor' that does not mean the same thing in each case. Then compare the total to the volume marked on the box.
The volume allowance table
Everything below resolves to a number from this table. It is unchanged between the 2023 and the 2026 editions. Note that it stops at 6 AWG — anything larger falls under 314.28 instead, which is a different calculation entirely, not a bigger version of this one.
| Conductor size | Volume |
|---|---|
| 18 AWG | 1.50 |
| 16 AWG | 1.75 |
| 14 AWG | 2.00 |
| 12 AWG | 2.25 |
| 10 AWG | 2.50 |
| 8 AWG | 3.00 |
| 6 AWG | 5.00 |
1. Conductors — the count depends on where the wire goes
- Originates outside the box and is spliced or terminated inside: counts as 1.
- Passes through unbroken: counts as 1, or as 2 if you leave at least 12 inches of free length in the box. That threshold is twice the free-length minimum in 300.14.
- Both begins and ends inside the box — a pigtail or a jumper: counts as 0. This is the one people add by mistake.
2. Cable clamps — one allowance, no matter how many
One allowance total for all clamps in the box, based on the largest conductor anywhere in the box. Two clamps do not mean two allowances.
3. Support fittings — one per type
Studs, hickeys and similar fittings each get one allowance per type present, again based on the largest conductor in the box.
4. Device yokes — doubled, and measured differently
Each yoke or strap gets a double allowance. But the size it is based on is the largest conductor connected to that particular yoke — not the largest conductor in the box.
5. Equipment grounding conductors — this rule changed in 2020
| Edition | How EGCs count |
|---|---|
| 2017 | All EGCs together get a single allowance, regardless of how many there are. An isolated ground group per 250.146(D) adds one more. |
| 2020 and later | Up to four EGCs get a single allowance. Each one beyond four adds a quarter allowance. |
In a box with six grounds and 12 AWG conductors, that difference is 2.25 cubic inches under the 2017 rule versus 3.375 under the 2020 rule. On a crowded 4-inch square box that is enough to flip the answer.
6. Terminal blocks — only from the 2023 onward
314.16(B)(6) brings terminal block assemblies into the count, based on the largest conductor terminated to the assembly. The 2017 and 2020 editions stop at (B)(5) and have no equivalent requirement, so under those editions a terminal block contributes nothing.
Worked example
A 4-inch square box, 21.0 cubic inches marked. Two 12 AWG cables in (each with two current-carrying conductors plus a ground), one duplex receptacle, internal cable clamps. Working under the 2023 or 2026 edition:
| Item | Count | Basis | Volume |
|---|---|---|---|
| Conductors | 4 × 12 AWG | 2.25 each | 9.00 |
| Equipment grounds | 2, so one allowance | 12 AWG, 2.25 | 2.25 |
| Cable clamps | One allowance total | Largest in box, 12 AWG | 2.25 |
| Device yoke | 1 yoke, doubled | Largest on that yoke, 12 AWG | 4.50 |
| Total required | 18.00 |
18.00 required against 21.0 available — it fits, with 3 cubic inches to spare. Add a second device to that same box and the yoke allowance alone pushes you to 22.50, over the limit.
The mistakes that actually happen
- Counting pigtails. They start and end inside the box, so they add nothing.
- Counting one allowance per clamp instead of one for all of them.
- Forgetting that a yoke is doubled.
- Using the box's largest conductor for the yoke instead of what is landed on that yoke.
- Applying the 2020 quarter-allowance ground rule to a job inspected under the 2017.
- Using this method at all for conductors larger than 6 AWG — that is 314.28, a different calculation.
Related guides
- What actually changed in the NEC 2026A working electrician's list of the NEC 2026 changes that move numbers on the job: Table 310.16 gained rows, 250.122 letters shifted, and one renumbered article now points somewhere else entirely.
- Where the 40% conduit fill rule comes from, and when it is not 40%The 40% fill limit only applies to more than two conductors. One conductor gets 53%, two conductors get 31%, and a short nipple gets 60%. Plus the jam ratio, which the percentages will not warn you about.
Run the numbers
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