RACEWAY · CALCULATOR

Box fill calculator

Free box fill calculator: get required volume from verified conductor equivalents, volume per equivalent, available box volume and non-ground conductor count.

Written by Chameera Sampath · Not yet reviewed by a licensed electrician. See the methodology.

US (NEC) only · NFPA 70 / NEC — locally adopted editionDefaults are US values. Verify the locally adopted NEC edition and project data.Official reference ↗

Project inputs

Measurements

Counting methodVerified conductor equivalents

Volume per equivalent

in³

Available box volume

in³

Updates as inputs change. Unit changes preserve the same physical values.

THE MATH BEHIND THE ANSWER

Method & assumptions

Vrequired = conductor equivalents × volume allowance

NEC example applies the 2020-and-later grounding count and assumes one conductor size. Mixed sizes, fixture supports and other fittings need verified additional allowances. NEC counting is not a UK/CEC rule.

Results do not check every aspect of an electrical installation. The calculators follow US (NEC) practice only; a regional preset changes defaults such as voltage and units, and no result certifies NEC compliance.

Sources and validation →

Worked example

Counting method: User-verified conductor equivalents (all regions) · Verified conductor equivalents: 8 · Volume per equivalent: 2.25 in³ · Available box volume: 18 in³

Required volume: 18 in³ · Remaining volume: -0 in³ · Counted equivalents: 8

This example uses the stated inputs; results for the entered values appear above.

Explore the raceway branch →See the related electrical concepts →

Estimating and invoicing tool

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