Required Volume
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cu in · 314.16(B)
Selected Box
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cu in · 314.16(A)
Margin
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cu in
Status
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📊 Fill Contribution Breakdown
Each segment is one 314.16(B) subdivision, scaled against the larger of required fill and box volume
📦 Box Cross-Section
Conceptual fill level — not a physical layout
💡 Interpretation
Adjust the inputs to see how each NEC subdivision contributes to box fill.
📋 Current Assumptions
What this calculation is assuming on your behalf
🧮 Step-by-Step Calculation TraceNEC 2023
Each row traces one fill allowance category. The code path matters — the answer did not appear by magic.
| Step | Category | Count Logic | Volume Each | Subtotal | NEC Reference |
|---|
📖 Reading the Trace
Volumes from each applicable 314.16(B) subdivision are added together, then compared with the box volume marked by the manufacturer or listed in Table 314.16(A). Small fittings such as locknuts and bushings are never counted. Conductors 4 AWG and larger also invoke the sizing rules of 314.28, which this calculator does not cover.
⚖ Volume Comparison
| Required Volume — 314.16(B) | -- |
| Selected Box Volume — 314.16(A) | -- |
| Margin | -- |
📈 Category Breakdown
| Category | Volume (cu in) | % of Total |
|---|
💬 Interpretation
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📜 NEC Reference Summary
These references are paraphrased. Always consult the actual NEC edition adopted by your jurisdiction.
| Reference | Paraphrased Logic |
|---|
📏 Volume Allowance Table
Per NEC Table 314.16(B).
| AWG Size | Volume (cu in) | Volume (cm³) |
|---|---|---|
| 18 AWG | 1.50 | 24.6 |
| 16 AWG | 1.75 | 28.7 |
| 14 AWG | 2.00 | 32.8 |
| 12 AWG | 2.25 | 36.9 |
| 10 AWG | 2.50 | 41.0 |
| 8 AWG | 3.00 | 49.2 |
| 6 AWG | 5.00 | 81.9 |
📝 Edition Notes
Why This Matters
Box fill is not paperwork. An overcrowded box traps heat, damages insulation as conductors are folded back, and turns a routine device replacement into a splice-hunting exercise. NEC 314.16 sets a volume budget: every conductor, device yoke, clamp, support fitting, grounding set and terminal block draws from it.
“The NEC is trying to help, even when it feels personally inconvenient.”
✅ What to Notice
- Pigtails that originate and terminate entirely within the box are NOT counted per 314.16(B)(1).
- A looped or coiled unbroken conductor at least twice the minimum free-conductor length counts as two.
- Device yoke volume uses the largest conductor connected TO THAT YOKE, not the largest in the box.
- Internal clamps: one or more = one allowance, sized on the largest conductor in the entire box.
- Support fittings: a single allowance for each type — multiple studs still count once; a stud plus a hickey counts twice.
- Terminal block assemblies (2023/2026 only) each count as 1 allowance based on the largest conductor terminated there.
- Conductors passing through without a splice still count as 1 each.
🔄 What Changed Between Editions
- 2017: all EGCs = one allowance (largest EGC/EBJ); an additional isolated grounding set per 250.146(D) adds one more.
- 2020: up to 4 EGCs = 1 allowance; each EGC above 4 adds 0.25 allowance.
- 2023: adds 314.16(B)(6) for terminal block assemblies; the allowance table is renumbered to Table 314.16(B)(1).
- 2026: clarifies that small fittings — locknuts, splicing connectors and bushings — are not counted.
- The volume values themselves (1.50 through 5.00 cu in) have not changed across these editions.
⚠ Common Mistakes
- Counting pigtails that never leave the box.
- Counting each grounding conductor separately under NEC 2017, where they share a single allowance.
- Sizing a device yoke on the largest conductor in the box instead of the largest connected to that yoke.
- Forgetting that internal clamps add fill even though they are part of the box.
- Treating two fixture studs as two allowances — 314.16(B)(3) grants one allowance per type of fitting.
- Applying 2017 grounding rules to a 2020+ jurisdiction (or the reverse).
- Confusing conduit body fill, 314.16(C), with standard box fill under 314.16(B).
- Ignoring 314.28 when the box encloses 4 AWG or larger conductors.
“A crowded box is not a personality trait. It is a problem.”
🚫 Educational Disclaimer
- This tool is for learning and calculation support only.
- It is not a substitute for professional engineering judgment.
- Results must be verified by a qualified electrician or engineer.
- Always confirm the NEC edition adopted by your local AHJ.
- Plan review, field conditions, and AHJ interpretation supersede any calculator output.
Test Your Knowledge
Fourteen edition-aware questions on 314.16. Pick an answer, then check it — each explanation shows the arithmetic.
🎯 Box Fill Quiz