Conduit Fill Calculator
Check conductor fill percentage against the allowable limit for a conduit.
About Conduit Fill Calculator
Fill limits exist so conductors can be pulled without damage and so heat can escape. The limit tightens for two conductors because that arrangement leaves the least usable space. Overfilled conduit is a common inspection failure, and it also forces ampacity derating that can undo the point of the larger cable.
Formula
Conduit Fill
Fill = total conductor area ÷ conduit internal area; the limit is 40 per cent above two conductors
- 40 per cent
- The allowable fill for three or more conductors
- 53 per cent
- The allowable fill for a single conductor
Worked example
With the default values below, Conduit Fill Calculator returns Conduit Fill of 15.247%. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Conduit internal diameterminimum 5 mm
- 21 mm
- Number of conductorsminimum 1
- 4
- Conductor outer diameterminimum 0.5 mm
- 4.1 mm
Results
- Conduit Fill
- 15.247%
- Allowable Fill
- 40.0%
- Within Limit
- Yes
- Maximum Conductors Permitted
- 10
- Conduit Internal Area
- 346.36 mm2
Step by step
- 1Values used
- 2Conduit Fill
- 3Conduit Fill= 15.247
- 4Allowable Fill= 40.0
- 5Within Limit= Yes
- 6Maximum Conductors Permitted= 10
- 7Conduit Internal Area= 346.36 mm2
Assumptions and limits
- The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Conduit Fill calculated?
Fill = total conductor area ÷ conduit internal area; the limit is 40 per cent above two conductors Fill limits exist so conductors can be pulled without damage and so heat can escape. The limit tightens for two conductors because that arrangement leaves the least usable space.
Why does Conduit Fill matter?
Overfilled conduit is a common inspection failure, and it also forces ampacity derating that can undo the point of the larger cable.
What values do I need to enter?
This calculator takes 3 inputs: Conduit internal diameter, Number of conductors, Conductor outer diameter. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.