Electrical Box Fill Calculator
Calculate electrical box fill capacity according to NEC requirements to ensure code compliance and safe installation.
Box Fill Parameters
Calculate electrical box fill capacity according to NEC Article 314 requirements
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Box Fill Analysis
NEC Article 314 compliance check and volume breakdown
Configure box and conductors to see fill analysis
NEC Article 314 compliance check
NEC Box Fill Allowance Chart and Calculation Method
Reference material for the calculator above. It lists the per-conductor volume allowances the tool applies and explains how conductors, equipment grounds, device yokes and internal clamps are counted.
Code edition. The allowances below are taken from the 2023 NEC (NFPA 70), Table 314.16(B). Confirm which edition, and which local amendments, the authority having jurisdiction (AHJ) for your project has adopted before relying on any figure on this page.
Volume allowance per conductor (2023 NEC / NFPA 70 Table 314.16(B))
| Conductor size | Free space per conductor | Metric |
|---|---|---|
| 18 AWG | 1.50 in³ | 24.6 cm³ |
| 16 AWG | 1.75 in³ | 28.7 cm³ |
| 14 AWG | 2.00 in³ | 32.8 cm³ |
| 12 AWG | 2.25 in³ | 36.9 cm³ |
| 10 AWG | 2.50 in³ | 41.0 cm³ |
| 8 AWG | 3.00 in³ | 49.2 cm³ |
| 6 AWG | 5.00 in³ | 81.9 cm³ |
The table ends at 6 AWG. Boxes that contain 4 AWG or larger conductors are sized under NEC 314.28 instead, which this calculator does not cover.
How the allowances are counted
- Insulated conductors. Each insulated conductor that enters the box and is spliced or terminated inside it counts as one allowance for its size. A conductor that passes through the box unbroken also counts once. A conductor that never leaves the box, such as a pigtail, is not counted. The calculator applies one allowance per conductor in each wire row.
- Equipment grounding conductors. All equipment grounding conductors and bonding jumpers that enter the box are counted together as a single allowance, based on the largest one present. Editions from 2020 onward add a quarter allowance for each such conductor beyond four. The calculator adds one allowance at the entered conductor size whenever one or more grounds are entered, regardless of how many.
- Device yokes. Each yoke or strap that carries one or more devices, such as a switch or receptacle, counts as two allowances, based on the largest conductor connected to that device. A device wider than a single 2 in. device box counts as two allowances for each gang it occupies. The calculator applies two allowances for every device entered.
- Internal clamps. Where the box has one or more internal cable clamps, factory or field supplied, a single allowance is made for all of them together, based on the largest conductor present. Connectors whose clamping mechanism is outside the box are not counted, and neither are small fittings such as locknuts and bushings. Enter 1 under Cable Clamps for a box with internal clamps and 0 for external connectors.
- Luminaire studs and hickeys. Where one or more studs or hickeys are present, the code makes a single allowance for each type, based on the largest conductor. The calculator applies one allowance per stud and per hickey entered.
- Wire connectors. The NEC does not assign a volume allowance to wire nuts or other splicing connectors. This calculator counts one allowance for each wire nut entered as a conservative margin; enter 0 to follow the code method exactly.
Worked example
Two 12/2 nonmetallic-sheathed cables, each with two insulated 12 AWG conductors and a bare equipment grounding conductor, enter a 4 in. square by 1½ in. deep metal box fitted with internal clamps. The box holds one duplex receptacle. No wire nuts are counted, following the code method.
| Item | Allowances | Volume |
|---|---|---|
| Insulated 12 AWG circuit conductors (two cables, two conductors each) | 4 × 2.25 | 9.00 in³ |
| Equipment grounding conductors (two enter the box, counted once) | 1 × 2.25 | 2.25 in³ |
| Internal cable clamps (counted once) | 1 × 2.25 | 2.25 in³ |
| Duplex receptacle yoke (double allowance) | 2 × 2.25 | 4.50 in³ |
| Total volume required | 18.00 in³ | |
| Box volume, 4 in. square × 1½ in. deep (Table 314.16(A)) | 21.0 in³ | |
| Result | 18.00 ≤ 21.0 | Compliant, 85.7 % fill, 3.0 in³ spare |
To reproduce this in the calculator, choose the Square 4" x 1½" box (21 cu.in.), set one wire row to 12 AWG with a count of 4, and enter Devices 1, Wire Nuts 0, Cable Clamps 1, Equipment Grounding 2, Fixture Studs 0 and Hickeys 0. The result panel reports 18 cu.in. required against 21 cu.in. available, 85.7% fill and NEC Compliant, with a note that fill is above 80%. The Wire Volume line of 11.25 cu.in. includes the single ground allowance.
Limits of this calculator
- Local amendments and the AHJ. States and municipalities adopt NEC editions on different schedules and may amend Article 314. The ruling of the authority having jurisdiction governs over anything shown here.
- Conductor sizes and types. Only conductors from 18 AWG through 6 AWG have allowances in Table 314.16(B). Boxes containing 4 AWG and larger conductors, conduit bodies, and the exception for up to four fixture wires from a luminaire canopy are outside the scope of this tool.
- Mixed conductor sizes. The code bases device, clamp, stud, hickey and ground allowances on the largest conductor present. When a box holds conductors of different sizes, check the result against the table by hand.
- Box volumes. The preset volumes come from Table 314.16(A) for standard metal boxes. Nonmetallic boxes, extension rings, plaster rings, domed covers and any box marked with its own volume must be entered through the Custom Box option using the marked volume.
- Not a substitute for the code. This page and the calculator are design aids. They do not replace the adopted code text, the manufacturer markings on the box, or the judgment of a licensed electrician or engineer.