Can I divide the box volume by 2.25 to count 12 AWG wires?

That division alone omits other applicable allowances. First determine each conductor and equipment category under the adopted code.

Do all grounding conductors always count as one?

No. The rule depends on entering conductor count and the applicable code provision. Do not reuse the previous unconditional one-allowance rule.

Does every pigtail add an allowance?

No. A conductor entirely inside the box is treated differently from one entering from outside. Check grounding/bonding provisions and relevant exceptions in the adopted edition.

Box Fill Chart — Conductor Volume Allowances

Identified content errors were corrected on 2026-10-25. Sources and scope for this page: NFPA public development record — Article 314.16 (accessed 2026-10-25). The correction record (2026-10-25) reads “Final NEC 2023 text was not independently reviewed; this page no longer claims a complete NEC 2023 installation calculation”. Full sources, values and limits on this page.

Page last changed 2026-10-25 · based on Public reference accessed 2026-10-25; scope stated on page

Quick Answer

Box fill requires the sum of applicable volume allowances. Dividing box volume by the allowance for one wire size does not account for devices, internal clamps, grounds or other applicable items. Use the box’s marked volume and the locally adopted code.

Find a size

Conductor volume allowances — reference values

Conductor size^[1]	Allowance, in³^[1]
14 AWG #	2.00
12 AWG #	2.25
10 AWG #	2.50
8 AWG #	3.00
6 AWG #	5.00

These allowances are reproduced in NFPA public development records for Article 314.16. The public records are not a substitute for the final adopted code. Former generic box-dimension/capacity columns have been removed; use the actual listed box volume.

Keep the fill categories separate

Record the conductors that originate outside and terminate or splice inside, unbroken conductors passing through, devices on yokes, internal clamps, support fittings, grounding/bonding conductors, and any barriers. The applicable counting rules differ. A conductor entirely contained in the box is not simply added as another entering conductor; grounding/bonding provisions and other exceptions must be checked separately.

Why the old grounding rule was withdrawn

The previous page alternated between counting each ground separately and counting every group as a single allowance. Neither is an adequate general rule. The reviewed NFPA development text distinguishes up to four entering grounding/bonding conductors from additional conductors, for which quarter allowances appear. Confirm the exact final wording and exceptions in the NEC edition adopted for the installation.

Arithmetic example, not installation approval

For a hypothetical box, suppose the applicable code check has established four 12 AWG conductor allowances, two allowances for one device yoke, one internal-clamp allowance and one grounding allowance. Then (4 + 2 + 1 + 1) × 2.25 = 18.00 in³. This only demonstrates summation: it does not establish the correct category counts for an actual installation.

Source and edition status

NFPA Article 314.16 public development record, checked 2026-10-25. This is development material, not confirmation that every provision is unchanged in a final edition. Consult NFPA 70 and the edition adopted by the local authority before selecting an enclosure.

Frequently Asked Questions

Can I divide the box volume by 2.25 to count 12 AWG wires?: That division alone omits other applicable allowances. First determine each conductor and equipment category under the adopted code.
Do all grounding conductors always count as one?: No. The rule depends on entering conductor count and the applicable code provision. Do not reuse the previous unconditional one-allowance rule.
Does every pigtail add an allowance?: No. A conductor entirely inside the box is treated differently from one entering from outside. Check grounding/bonding provisions and relevant exceptions in the adopted edition.

Value Sources

References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.

#	Source	Type	Revision / method
[1]	NFPA public development record — Article 314.16	reference	Public reference accessed 2026-10-25 — source

Data Sources

Standard	Revision	What it covers on this page
NFPA public development record — Article 314.16	Public reference accessed 2026-10-25; scope stated on page	See source scope on page

Verification scope

2026-10-25 — Corrected 2026-10-25. Final NEC 2023 text was not independently reviewed; this page no longer claims a complete NEC 2023 installation calculation. Evidence

Derived values — the following values on this page are calculated, not taken directly from the standard:

Value	How it is derived
Illustrative total	8 × 2.25 = 18.00 in³; category counts are assumptions

Corrected 2026-10-25. Final NEC 2023 text was not independently reviewed; this page no longer claims a complete NEC 2023 installation calculation.

Accuracy and use. Use the source notes and recorded verification scope above to assess each value. Calculated values follow the stated derivations; compilation values may lack a confirmed original document or product condition. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Read the source notes, calculation methods and verification scope above before reusing a value. To cite this page:

Plain text

Engineering Reference. "Box Fill Chart." https://refmanual.com/charts/box-fill/ (accessed 2026-11-09). Data from NFPA public development record — Article 314.16, Public reference accessed 2026-10-25; scope stated on page.

BibTeX

@misc{box-fill,
  title        = {Box Fill Chart},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/charts/box-fill/}},
  note         = {Accessed: 2026-11-09},
  source       = {NFPA public development record — Article 314.16, Public reference accessed 2026-10-25; scope stated on page}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - Box Fill Chart
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/charts/box-fill/
PY  - 2026
DA  - 2026-11-09
N1  - Source: NFPA public development record — Article 314.16, Public reference accessed 2026-10-25; scope stated on page
ER  -

Each row in the tables above also has a permanent link — hover a row and use the # link to cite a single value rather than the whole page.