RACEWAY · CALCULATOR

Conduit fill calculator

Free conduit fill calculator: get fill and minimum internal diameter from conductor count, outside conductor diameter and internal conduit diameter.

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

Conductor count

Outside conductor diameter

Internal conduit diameter

Verified fill limit

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

THE MATH BEHIND THE ANSWER

Method & assumptions

Fill = N × (d / D)² × 100

Use outside insulated diameters. Verify jurisdictional limits; no automatic UK/Canadian application of NEC percentages.

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

Conductor count: 4 · Outside conductor diameter: 0.212 in · Internal conduit diameter: 0.824 in · Verified fill limit: 40 %

Fill: 26.478 % · Minimum internal diameter: 0.67 in

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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