TryBuildCalc5+ EGC Box Fill Calculator (NEC 314.16(B)(5), 2020+ Quarter-Allowance Rule)
Check box fill with 5+ ground wires.
🕒 Last updated: September 2, 2026
Inputs
ℹ️Standard NEC Table 314.16(A) metal box sizes. Nonmetallic boxes and any box not in the table must be marked by the manufacturer with their actual capacity — use Custom for those.
Conductors
Clamps & Fittings
Device/Equipment Straps
A strap/yoke holding a switch, receptacle, or other device — a duplex receptacle or a 2-gang switch on one strap still counts as ONE strap, not two. Each allowance is based on the largest conductor actually connected to that specific strap, which can be smaller than the box's overall largest conductor.
ℹ️Not the box's overall largest conductor — only the largest conductor actually connected to the device(s) on this specific strap.
Equipment Grounding Conductors
ℹ️Total equipment grounding/bonding conductors entering the box. Up to 4 share one volume allowance; each one beyond 4 adds only a 1/4 allowance (NEC 314.16(B)(5), 2020+).
This box fits your fill:
23.625 of 42 cu in (56.3%)Spare capacity: 18.375 cu in
Volume Allowance Breakdown
Box: 4-11/16" x 2-1/8" Square (42 cu in)
Conductor fill: 13.5 cu in (6× 12 AWG)
Internal clamp fill: 2.25 cu in (1 allowance, largest conductor: 12 AWG)
Device/equipment strap fill: 4.5 cu in (1× strap @ 12 AWG, each strap × double allowance based on the conductor connected to it)
Equipment grounding conductor fill: 3.375 cu in (6× 12 AWG, 1.5 allowance — beyond 4, each extra EGC adds only 1/4 allowance)
Total required volume: 23.625 cu in
Assumptions Used
NEC Table 314.16(A) (standard box volumes), NEC Table 314.16(B) and 314.16(B)(1)-(5) (box fill calculation rules, 2020+ EGC quarter-allowance rule). Clamp and support-fitting fill use the largest conductor present anywhere in the box (314.16(B)(2)/(3)); device/equipment strap fill instead uses the largest conductor actually connected to each specific strap (314.16(B)(4)), entered separately per strap group above. This is a reference estimate — confirm against the code edition adopted in your jurisdiction and a licensed electrician before final installation.
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Electrical Box Fill Calculator.
Box fill with more than 4 equipment grounding conductors
A box that consolidates several circuits — common in junction boxes or multi-circuit device boxes — can easily end up with more than 4 equipment grounding conductors. The 2020+ NEC changed how these are counted, and applying the pre-2020 'always just one allowance' rule here understates the required volume.
This page is pre-set to a larger square box with 6 equipment grounding conductors — edit the inputs above to match your actual count.
- The first 4 EGCs share one volume allowance based on the largest EGC size — the 5th and every one after adds only a 1/4 allowance each, not a full additional allowance.
- This rule change reflects that boxes with high conductor density genuinely need more physical room for ground splicing and bonding, even though each individual added EGC is proportionally smaller in its volume impact.
- Confirm which NEC edition your jurisdiction has adopted — pre-2020 editions still use the simpler 'always one allowance' rule regardless of EGC count.
Box Fill Formula: How Is It Determined?
Four separate volume allowances, summed and compared against the box's actual capacity.
Step 1 — Conductor Fill (NEC 314.16(B)(1))
For each conductor size present: Volume += Count × Table 314.16(B) allowance for that size
Each conductor that originates outside the box and terminates or is spliced within it counts once, at its own size's allowance — a conductor that both originates and terminates inside the box (never leaving it) is not counted.
Step 2 — Clamp & Support Fitting Fill (NEC 314.16(B)(2)-(3))
Internal clamp(s) present: Volume += 1 × allowance for the largest conductor anywhere in the box
Fixture stud present: Volume += 1 × allowance for the largest conductor anywhere in the box
Hickey present: Volume += 1 × allowance for the largest conductor anywhere in the box
These three allowances are based on the single largest conductor present anywhere in the box (including the equipment grounding conductor, if it happens to be the largest) — a fixture stud and a hickey each get their own allowance if both are present, but multiple studs alone still only count once.
Step 3 — Device/Equipment Strap Fill (NEC 314.16(B)(4))
For each strap group: Volume += Straps × 2 × allowance for the largest conductor CONNECTED TO THAT STRAP
This is a genuinely different basis than Step 2 — 314.16(B)(4) sizes each strap's allowance from the largest conductor actually connected to the device(s) on that specific yoke, not the box's overall largest conductor. A box with a large pass-through conductor that never connects to any device should size its device-strap fill from the smaller, actually-connected conductor instead.
Step 4 — Equipment Grounding Conductor Fill (NEC 314.16(B)(5), 2020+)
Up to 4 EGCs: Volume += 1 × allowance for the largest EGC
Each EGC beyond 4: Volume += 0.25 × allowance for the largest EGC
Total Required Volume = Step 1 + Step 2 + Step 3 + Step 4. The box fits if this total does not exceed its actual (or marked) cubic-inch capacity.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Box type | 4-11/16" x 2-1/8" Square | Sets the available cubic-inch capacity |
| Conductors | 6× 12 AWG | Each sized at its own Table 314.16(B) allowance |
Step 1 — Conductor Fill (NEC 314.16(B)(1))
| Calculation | Result |
|---|---|
| 6 × 12 AWG conductor(s) × 2.25 cu in | 13.5 cu in |
| Conductor fill subtotal | 13.5 cu in |
Step 2 — Clamp & Support Fitting Fill (NEC 314.16(B)(2)-(3))
| Calculation | Result |
|---|---|
| Internal clamp(s): 1 × largest conductor in box (12 AWG, 2.25 cu in) | 2.25 cu in |
| Clamp + fitting fill subtotal | 2.25 cu in |
Step 3 — Device/Equipment Strap Fill (NEC 314.16(B)(4))
| Calculation | Result |
|---|---|
| 1 strap × 2 × largest conductor CONNECTED TO THAT STRAP (12 AWG, 2.25 cu in) | 4.5 cu in |
| Device/equipment strap fill subtotal | 4.5 cu in |
Step 4 — Equipment Grounding Conductor Fill (NEC 314.16(B)(5), 2020+)
| Calculation | Result |
|---|---|
| 6 × 12 AWG EGC — 4 share 1 allowance + 2 beyond 4 × 0.25 each = 1.5 allowance × 2.25 cu in | 3.375 cu in |
| Equipment grounding conductor fill subtotal | 3.375 cu in |
Total Required Volume
| Step | Volume (cu in) |
|---|---|
| Step 1 — Conductor fill | 13.5 |
| Step 2 — Clamp + fitting fill | 2.25 |
| Step 3 — Device/equipment strap fill | 4.5 |
| Step 4 — Equipment grounding conductor fill | 3.375 |
| Total required volume vs. box capacity | 23.625 of 42 cu in (56.3%) |
Therefore, this box requires 23.625 cu in against its 42 cu in capacity (56.3% fill) — it fits, with room to spare.
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Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.