Electrical Box Fill Calculator

Inputs

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Usable volume marked on the box, extension ring or box assembly.
in³
Use extra groups when the box contains different AWG sizes.
Volume allowance for insulated conductors in the primary group.
Count insulated conductors that enter the box and are spliced, terminated or pass through as counted by the box-fill rule.
conductors
Equipment grounding conductors in the box are treated as one volume allowance using the largest grounding conductor present.
Enter zero if no equipment grounding conductors are present. Any positive count contributes one grounding-conductor allowance in this screen.
grounds
Largest conductor connected to the device yokes counted in this box.
Each device yoke counts as two conductor volume allowances of the largest conductor connected to that yoke.
yokes
Internal clamps count as one conductor allowance of the largest conductor present in this simplified screen. External clamps do not.
Support fittings are screened as one largest-conductor allowance each.
fittings

Results

Default result
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Required box volume(V_req)
15.75in³
Pass

Insulated groups: 4 x #12 AWG.

Also computed

Volume margin(V_margin)Pass2.25in³

Marked box volume exceeds required fill volume.

Box fill(F)Pass87.5%

Additional primary conductors(N_add)1conductors

Same-size reference using the primary conductor allowance.

Insulated conductor volume(V_cond)9in³

4 counted insulated conductors.

Grounding conductor volume(V_g)2.25in³

All equipment grounding conductors are counted as one grounding allowance in this screen.

Device yoke volume(V_yoke)4.5in³

Each device yoke is counted as two conductor allowances.

Method notes 3 notes
  • Formula: required box volume = insulated conductor allowances + equipment grounding allowance + device yoke allowances + internal clamp/support allowances.
  • This calculator uses NEC-style #18 through #6 AWG volume allowances. Conductors larger than #6 AWG, pull boxes, conduit bodies, raceway fill, ampacity adjustment and local amendments need separate checks.
  • Pigtails that originate and terminate in the same box are usually not counted; conductors entering the box and spliced, terminated or passing through generally are. Verify the adopted code text and box listing.

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