MasterTool

Conduit Fill Calculator

Calculate circular conductor area as a proportion of conduit area.

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What this tool does

Calculate circular conductor area as a proportion of conduit area. Use actual insulated conductor diameter and conduit internal diameter. Special cable types and local code may change the permitted fill.

How to use the Conduit Fill Calculator

  1. Enter or select conduit internal diameter in in.
  2. Enter or select insulated conductor diameter in in.
  3. Enter or select conductor count.
  4. Read the calculated result; change any measurement to compare alternatives.

Formula

fill%=count×wireDiameter²/conduitDiameter²×100; limit=53% for one, 31% for two, 40% for three or more
diameter
Conduit internal diameter (in, before metric conversion)
wire
Insulated conductor diameter (in, before metric conversion)
count
Conductor count

Use actual insulated conductor diameter and conduit internal diameter. Special cable types and local code may change the permitted fill.

Worked example

For conduit fill calculator, the following measurements illustrate the exact method: Conduit internal diameter: 1; Insulated conductor diameter: 0.25; Conductor count: 4.

Inputs

  • Conduit internal diameter1 in
  • Insulated conductor diameter0.25 in
  • Conductor count4

Result

  • Conduit fill (%)25
  • Common fill limit (%)40
  • Geometric fill checkWithin entered geometric limit

Results explained

Conduit fill (%)
Conduit fill (%) from the formula above. Use actual insulated conductor diameter and conduit internal diameter. Special cable types and local code may change the permitted fill.
Common fill limit (%)
Common fill limit (%) from the formula above. Use actual insulated conductor diameter and conduit internal diameter. Special cable types and local code may change the permitted fill.
Geometric fill check
Geometric fill check from the formula above. Use actual insulated conductor diameter and conduit internal diameter. Special cable types and local code may change the permitted fill.

Frequently asked questions

fill%=count×wireDiameter²/conduitDiameter²×100; limit=53% for one, 31% for two, 40% for three or more

Use actual insulated conductor diameter and conduit internal diameter. Special cable types and local code may change the permitted fill.

No. Electrical and HVAC results are estimates. Installation must comply with local codes and be checked by a qualified licensed professional.

Nameplate values, conductor temperature, power factor, efficiency, weather and actual installation conditions may differ from your entered assumptions.

Confirm units, manufacturer data and the formula's assumptions, then verify sizing with the equipment instructions and applicable local standards.