Cramming too many cables into a conduit traps heat and makes them hard to pull. Check the fill percentage against the recommended space factor before you run them.
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Start free trialFill is the total cross-sectional area of all the cables divided by the internal area of the conduit, as a percentage.
To allow for heat dissipation and pulling cables through, a maximum fill (space factor) applies — commonly 40% for three or more cables.
If the fill exceeds the limit, use a larger conduit or split the cables across more runs so the cables aren’t crammed and overheating.
Cabling standards apply a maximum “space factor” so cables aren’t packed too tightly. Common figures are 53% for a single cable, 31% for two cables, and 40% for three or more. These allow heat to dissipate and make cables easier to draw in without damage.
Overfilled conduit traps heat, which derates the cables’ current-carrying capacity and can damage insulation over time. It also makes cables very hard to pull in and can damage them during installation. Staying within the space factor protects both safety and workmanship.
Use the actual internal bore, not the nominal size. Nominal conduit sizes refer to the outside diameter, so a “20 mm” conduit has a smaller internal diameter depending on wall thickness. Check the manufacturer’s data for the true internal diameter.
No. It’s a quick space-factor check. Actual conduit and cable selection must consider derating for grouping and temperature, bend allowances and the requirements of AS/NZS 3000 and the wiring rules, verified by a licensed electrician.
This calculator is an indicative space-factor check assuming equal-diameter cables and a circular bore. It does not account for cable derating from grouping, bends or temperature. Conduit and cable selection must comply with AS/NZS 3000 and be verified by a licensed electrician.
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