Calculation breakdown
| Compartment | Fill category | Quantity or equivalents | Allowance basis | Required volume | Reference |
|---|
Warnings and review notes
Shareable plain-text summary
Formula used
Total required volume combines every applicable fill category. Available volume includes only listed or marked usable space. Each barrier-separated space must pass its own separate calculation.
| Total required volume | Conductor fill + clamp fill + support fitting fill + device fill + grounding fill + terminal-block fill |
|---|---|
| Remaining volume | Available volume − required volume |
| Fill percentage | Required volume ÷ available volume × 100 |
How to use this calculator
- Select the NEC edition adopted for your project.
- Enter manufacturer-marked volume or choose a planning preset.
- Add every entering, terminating, spliced, passing, or looped conductor.
- Enter grounding conductors, clamps, fittings, devices, and terminal blocks.
- Assign every item to the correct barrier compartment.
- Calculate, review warnings, and verify the result with your AHJ.
Conductor volume allowance reference
| AWG | Volume per allowance | Metric volume |
|---|---|---|
| 18 | 1.50 in³ | 24.6 cm³ |
| 16 | 1.75 in³ | 28.7 cm³ |
| 14 | 2.00 in³ | 32.8 cm³ |
| 12 | 2.25 in³ | 36.9 cm³ |
| 10 | 2.50 in³ | 41.0 cm³ |
| 8 | 3.00 in³ | 49.2 cm³ |
| 6 | 5.00 in³ | 81.9 cm³ |
Worked example data
This example uses a 30.3 cubic-inch reference box. It includes four 12 AWG conductors and one receptacle. Grounds and an internal clamp receive separate allowances here.
| Item | Input | Equivalent count | Volume |
|---|---|---|---|
| Insulated conductors | 4 × 12 AWG | 4 | 9.00 in³ |
| Grounding conductors | 3 × 12 AWG | 1 | 2.25 in³ |
| Internal clamp | 12 AWG basis | 1 | 2.25 in³ |
| Receptacle yoke | 1 gang, 12 AWG | 2 | 4.50 in³ |
| Total | 8 | 18.00 in³ |
Common box-fill mistakes
Installers often forget internal clamps, device yokes, and long loops. Barrier-separated spaces cannot share unused volume across compartments. Always verify box markings before approving final field work.
- Counting internal-only pigtails like entering conductors.
- Ignoring equipment wider than a single gang.
- Using geometric volume instead of the listed box volume.
- Applying older grounding rules to newer adopted editions.
- Forgetting that 4 AWG and larger require separate review.
Frequently asked questions
Does every conductor inside the box count?
No. Conductors that never leave the box are generally excluded. Entering, terminating, spliced, passing, and qualifying looped conductors require review.
How are device yokes counted?
Each yoke normally receives two conductor volume allowances. Use the largest conductor connected to that supported device. Wider equipment receives allowances for every mounting gang.
Do external cable clamps add box fill?
External clamps normally consume no internal box volume. Internal factory or field clamps require the applicable allowance. Confirm the actual fitting arrangement before final approval.
Why does the grounding result change by edition?
The 2020 and later workflow adds quarter allowances beyond four grounds. Earlier rules used one combined allowance in typical cases. Local adoption determines which method applies.
Can extension rings increase available volume?
Listed extension rings can contribute their marked usable volume. Add only volume permitted for the installed assembly. Manufacturer instructions remain essential for every component.
Can unused compartment volume offset another compartment?
No. A barrier creates separately calculated spaces within the box. Every compartment must independently satisfy its own fill requirement.
Does this calculator handle conductors larger than 6 AWG?
No automatic allowance is provided above 6 AWG. The tool produces a manual-review warning instead. Apply the correct enclosure and conductor-bending requirements.
Are terminal blocks always counted automatically?
The helper activates terminal-block allowances for 2023 and 2026. The 2017 and 2020 modes offer an optional conservative allowance. Verify adopted text and local amendments.
Can geometric dimensions prove compliance?
No. Dimension-based volume is only a planning estimate. Use listed, marked, or code-recognized usable box volume.
Is a passing conductor counted once?
Yes, an unspliced conductor passing through is normally counted once. A qualifying long loop can require two allowances. Select the matching row type carefully.
Does a passing result replace inspection?
No. This helper supports planning and documentation only. The adopted NEC, manufacturer instructions, and AHJ control approval.