Electrical Box Fill Calculator
Compare confirmed box volume with 2023 NEC conductor, grounding, clamp, fitting, device, and terminal-block allowances. New here? Read the quick guide.
Use only for 18 AWG through 6 AWG conductors in one undivided outlet, device, or junction-box space. Verify the adopted edition, marked or approved usable volume, box listing, conductor use, and AHJ requirements.
Calculation inputs
Confirm the available box volume
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Count each entering conductor that terminates or is spliced, and each unspliced pass-through, once. Count a loop or coil of unbroken conductor twice only when it is at least twice the minimum free-conductor length required by 300.14. Do not enter internal pigtails whose entire length stays in the box.
Grounding — 314.16(B)(5)
Clamps and support fittings
Device yokes — 314.16(B)(4)
Each yoke or strap receives two allowances based on its largest connected conductor. For equipment wider than one 2-inch device box, enter the number of mounting gangs.
Terminal blocks — 314.16(B)(6)
The 2023 edition requires one allowance for each terminal-block assembly, based on the largest conductor terminated to that assembly.
Small fittings such as locknuts, bushings, wire connectors, and internal conductors or equipment bonding jumpers that never leave the box do not receive separate conductor allowances in this workflow.
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Review the box-fill inputs
Confirm the usable assembled box volume before relying on the calculation.
Method and verification
How the 2023 NEC box-fill calculation works
The calculator multiplies each applicable allowance count by the conductor volume for its governing wire size, then adds the six categories in 314.16(B). The result passes only when the confirmed available volume is at least the calculated required volume. Inch inputs use the published in³ column and metric inputs use the published cm³ column directly.
Conductor allowances cover 18, 16, 14, 12, 10, 8, and 6 AWG. Device yokes receive two allowances based on the largest connected conductor. All ordinary internal cable clamps share one allowance, while luminaire studs and hickeys are treated as separate fitting types. For those clamp and support allowances, explicitly identify the largest conductor physically present, including an internal-only conductor.
Count only equipment grounding conductors that enter the box for the grounding allowance. Equipment bonding jumpers or other conductors that begin and end within the box are not added as separate conductor allowances. The 2023 terminal-block rule applies once per assembly, not once per pole.
Primary reference: NFPA 70, National Electrical Code, 2023 edition. The official NFPA development record documents the 2023 grounding clarification and new terminal-block allowance.
This calculation does not establish conductor permission, box depth, wire-bending space, grounding continuity, device listing, support, accessibility, local amendments, or complete compliance. Boxes with barriers require a separate calculation for each space. Conductors 4 AWG or larger, conduit-body sizing, and motor or generator terminal housings are outside this tool. Apply the luminaire-canopy exception only after confirming every condition in the adopted code text.
How are conductors counted for electrical box fill?
Under the supported 2023 NEC 314.16(B)(1) workflow, each conductor entering and terminating or being spliced in the box counts once, and each unspliced pass-through counts once. An unbroken loop or coil counts twice only when it is at least twice the minimum free-conductor length required by 300.14. A conductor that never leaves the box is not counted as a conductor allowance.
How many ground wires count toward box fill under the 2023 NEC?
Up to four equipment grounding conductors that enter the box use one volume allowance based on the largest entering equipment grounding conductor. Each additional entering equipment grounding conductor beyond four adds one-quarter of that allowance. An equipment bonding jumper that begins and ends within the box is not included in this count.
How does a switch or receptacle count in box fill?
Each yoke or strap holding one or more devices receives two conductor-volume allowances based on the largest conductor connected to that yoke. Equipment wider than a single 2-inch device box receives the double allowance for each mounting gang required.
Do terminal blocks count in a 2023 NEC box-fill calculation?
Yes. The 2023 NEC added 314.16(B)(6), which assigns one conductor-volume allowance to each terminal-block assembly based on the largest conductor terminated to that assembly.
Does this calculator cover boxes with barriers or conductors 4 AWG and larger?
No. Each space separated by a barrier must be calculated independently, and boxes containing 4 AWG or larger conductors are also subject to other requirements. This calculator is limited to one undivided box space with conductors from 18 AWG through 6 AWG.
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The PDF and CSV retain the available volume, each allowance, governing conductor size, required volume, remaining volume, and result.
Resolve the input issue before creating a PDF or CSV: Confirm the usable assembled box volume before relying on the calculation.
Included
Six allowance categories
Conductors, internal clamps, support fittings, device yokes, entering equipment grounding conductors, and 2023-edition terminal-block assemblies are itemized separately.
Required field checks
Scope exclusions
Separately evaluate barrier spaces, integrated clamp/termination assemblies, canopy exceptions, motor or generator terminal housings, conduit bodies, conductors 4 AWG or larger, box depth and bending space, listings, and local amendments.