A box fill calculation adds up the cubic inches of everything in a box and checks that the total fits the box's volume. Each wire counts 2.00 cubic inches for 14 AWG, 2.25 for 12 AWG and 2.50 for 10 AWG. Clamps, devices and grounds add more. If the total runs over, use a bigger box or an extension ring.
| Quick facts | NEC box fill (2020 NEC) |
|---|---|
| The rule | Section 314.16 of the 2020 NEC (NFPA 70) |
| Box volume | Table 314.16(A) for standard metal boxes. Others are marked by the maker |
| What gets counted | Conductors, internal clamps, luminaire studs and hickeys, devices, and equipment grounding conductors: 314.16(B)(1) through (B)(5) |
| Pass test | Box volume equal to or larger than the fill total |
Box fill calculator
Enter what's in the box. The calculator adds up the allowances and checks the total against the box volume.
How many wires of each size (see the counting rules below):
Box fill: 18.00 in³ of 18.0 in³ Fits
Wires 9.00 + clamps 2.25 + devices 4.50 + grounds 2.25 in³
Smallest standard metal box that holds it: 18.0 in³ (4 × 1¼ in. square or 3 × 2 × 3½ in. device)
Uses NEC Table 314.16(B) (cubic inches per wire, 18 to 6 AWG) and the metal box volumes in Table 314.16(A). Clamps and studs go by the largest wire in the box. Each yoke counts double. Up to four grounds count once, and each extra adds a quarter. For plastic or other marked boxes, pick "Other" and enter the marked volume.
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How does a box fill calculation work?
You compare the box volume to the fill, the sum of the allowances for everything inside.
- Rings and domed covers add volume only if marked with it or made from a Table 314.16(A) box.
- An unmarked barrier takes up 1/2 cubic inch if metal, 1.0 if nonmetallic.
- Each side of a barrier is figured as its own box.
How to size a junction box
- Count allowances for everything in the box, using the counting rules below.
- Multiply each allowance by the cubic inches for its wire size in Table 314.16(B) of the NEC. The calculator above has every size from 18 AWG to 6 AWG built in.
- Pick a box at least that big: the Table 314.16(A) volume for a standard metal box, or the volume marked on the box, plus any marked ring or cover.
Boxes with conductors 4 AWG or larger must also meet 314.28. JCR's pull box sizing calculator does that math.
NEC box fill counting rules
- Wires spliced or terminated in the box: 1 each.
- Wires that pass through unspliced: 1 each. An unbroken loop counts 2 if it's at least twice the 6-inch free length in 300.14 (12 inches or more).
- Pigtails and jumpers that never leave the box: 0.
- Domed luminaire canopy wires ending in the box: grounds, and up to four fixture wires smaller than 14 AWG, count 0.
- Internal cable clamps: 1 total, sized by the largest conductor in the box. Connectors clamped outside the box: 0.
- Luminaire studs or hickeys: 1 for each type, sized by the largest conductor in the box.
- Devices: 2 per yoke or strap, sized by the largest conductor connected to it. A device wider than a single 2-inch device box gets 2 per gang it needs. A GFCI counts the same as any receptacle.
- Equipment grounding conductors: up to four count as 1, sized by the largest one. Each ground past four adds 1/4.
- Locknuts and bushings: 0.
Worked example 1: two 12/2 cables and a receptacle
Setup: a 3 × 2 × 3½-inch metal device box with internal clamps. Two 12/2 NM cables with ground feed a duplex receptacle.
| Item | Allowances |
|---|---|
| 12 AWG insulated conductors | 4 (two per cable) |
| Two grounds | 1 (four or fewer count once) |
| Internal clamps | 1 |
| Receptacle yoke | 2 |
| Total | 8 × 2.25 in³ = 18.00 in³ |
Result: Table 314.16(A) lists this box at 18.0 cubic inches, so it passes. The box can't be smaller than the fill, and equal isn't smaller.
Add a third 12/2 cable: that's 2 more conductors, and three grounds still count as 1. The total is 10 × 2.25 = 22.50 cubic inches, which fails. A 4 × 2⅛-inch square box (30.3 cubic inches) handles it.
Worked example 2: mixed wire sizes
Setup: a 4 × 2⅛-inch square metal box (30.3 cubic inches) with internal clamps. Two 12/2 cables feed a receptacle. A 10/2 cable passes through unspliced.
- 12 AWG conductors: 4 × 2.25 = 9.00 in³
- 10 AWG pass-through conductors: 2 × 2.50 = 5.00 in³
- Grounds: three, so 1 allowance at the largest ground (10 AWG) = 2.50 in³
- Clamps: 1 allowance at the largest conductor in the box (10 AWG) = 2.50 in³
- Receptacle: 2 allowances at the largest wire on it (12 AWG) = 4.50 in³
- Total: 23.50 in³, under 30.3, so it passes.
The lesson: clamps go by the biggest wire in the box. Devices go by the biggest wire landing on them.
Common box fill exam traps
- Using Table 314.16(A) conductor counts with clamps, a device or grounds in the box. Those counts only work when nothing else needs an allowance. Otherwise, use cubic inches.
- Stopping at box fill on 4 AWG and larger. Those boxes must also meet the pull box sizing in 314.28. See JCR's angle pull box question.
Box fill doesn't cover fixture or fan weight. JCR's ceiling box weight guide does.
Box fill questions
How many 12 AWG wires can go in a 4 × 2⅛ square box?
Table 314.16(A) lists 13, assuming no clamps, studs, devices or grounds. Otherwise, 30.3 cubic inches ÷ 2.25 gives 13 total allowances. Subtract what the clamps, device and grounds use.
Do ground wires count in box fill?
Yes, but lightly. Up to four equipment grounding conductors count as one allowance, based on the largest ground. Each ground beyond four adds a quarter allowance.
Do wire nuts count in box fill?
No. Section 314.16(B) gives allowances only for conductors, clamps, luminaire studs and hickeys, devices and grounds. Wire connectors aren't on the list, and locknuts and bushings are expressly excluded.
Do cable clamps count in box fill?
Yes, once. One or more internal clamps count as a single allowance, sized by the largest wire in the box, so four clamps still add one. Connectors clamped outside the box add nothing.
How do I find the volume of a plastic box?
Read it off the box. Nonmetallic boxes must be marked with their volume by the maker. So must metal boxes of 100 cubic inches or less that aren't in Table 314.16(A).
Make box fill a sure point on your exam
Box fill is quick points once counting is automatic. JCR's $495 self-study package drills it with video lessons, a 500+ question mock exam and the JCR workbook.
Videos and the digital practice exam open right after purchase, and the workbook ships the same business day. Plan on 6–8 weeks, about an hour a night.
Prefer a classroom? See JCR's two-day workshops in Raleigh, Asheville, Mooresville and Winston-Salem, or on Zoom.
Need the code book? Get the 2020 NEC softbound.
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Related guides: Ceiling box weight limits (314.27) · Pull box sizing for an angle pull (314.28) · The 5 most-missed NEC mock exam questions
More worked examples: conduit fill calculation and all NEC Answers.
References
- NFPA 70, National Electrical Code, 2020 edition
- 314.16, Number of conductors in boxes and conduit bodies
- 314.16(A) and Table 314.16(A), Metal boxes
- 314.16(B)(1) through (B)(5), Box fill calculations
- Table 314.16(B), Volume allowance per conductor
- 300.14, Length of free conductors
- 314.28, Pull and junction boxes (4 AWG and larger)
JCR checked every section and table value above against the 2020 NEC on October 21, 2026.