How to use this calculator
- Enter or choose conduit internal diameter.
- Enter or choose number of conductors.
- Enter or choose conductor outside diameter (with insulation).
- Enter or choose second conductor group: number.
- Enter or choose second conductor group: diameter.
- The conduit fill updates as you type. Use Copy result or Share to keep it.
Method
Fill = Σ n × π d² ÷ 4 ÷ (π D² ÷ 4) × 100 = Σ n d² ÷ D² × 100
- n, d
- number and outside diameter of each conductor group
- D
- conduit internal diameter
Worked example
- Fill = 6 × 4² ÷ 25² = 96 ÷ 625 = 15.36%.
- Conduit area = π × 25² ÷ 4 = 490.87 mm²; 40% of it is 196.35 mm².
- Each conductor is 12.57 mm², so up to 15 fit within 40%.
Six 4 mm conductors fill a 25 mm-bore conduit to 15.36%, well within a 40% limit.
What the result means
Fill limits keep conductors from being damaged when pulled and allow heat to escape. Grouped circuits may also need current derating, which this calculator does not check.
Assumptions and limits
- Conductors are round; their overall outside diameter is used.
- Fill limits depend on the number of conductors, conduit type and code; enter the one that applies.
- Jamming, bends and pull tension are not checked.
Questions
Why does the limit change with the number of conductors?
Many codes allow a higher fill for a single conductor than for several, because several round conductors pack less efficiently and are harder to pull.
Can I use the nominal conduit size?
No. Use the internal diameter for your conduit type; it differs between materials with the same trade size.
Please note: Quantities are estimates. Confirm requirements with your engineer or supplier before ordering materials.