Conduit Fill Calculator
What size conduit do these cables need? Add your cables and get the smallest size that fits.
Data: Cable sizes from Nexans Olex product data (checked Oct 2026). Fill ratios: AS/NZS 3000 edition not confirmed. About standard editions
For reference only. Always verify with the current published standard and consult a licensed electrical engineer for critical installations.
Cable List
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About This Conduit Fill Calculator
This calculator tells you the minimum conduit size for a group of cables based on the space factor rules. Built for Australian electricians running cables through conduit on commercial and industrial jobs.
Use it when planning conduit runs with mixed cable sizes. Add your cables, and it works out the total cross-sectional area and the smallest conduit that meets the fill limit.
Cable sizes come from Nexans Olex product data and conduit bores from the Clipsal rigid conduit catalogue (outside diameter less two walls). The space factors (50%, 33%, 40%) are the ones commonly quoted from AS/NZS 3000 Appendix C. Check against the current edition of AS/NZS 3000 for your installation.
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Questions
- How many cables fit in 25mm conduit?
- It depends on the cable type and size, not just the count. What matters is the combined cross-section of the cables against the bore of the conduit. Single core building wire is small: 2.5mm² is about 3.6mm across, so a 25mm medium duty conduit (21.4mm bore) takes a lot of it. Flat TPS is much bigger: 2.5mm² TPS is 12.2mm x 5.4mm, so four of them already need a 40mm medium duty conduit on this calculator. Enter your cables above and it tells you whether 25mm clears the fill limit.
- What is the maximum conduit fill percentage in Australia?
- The figure commonly quoted from AS/NZS 3000 is 40% of the conduit's internal cross-section for three or more cables. This calculator uses that figure. Check it against the current edition of AS/NZS 3000 before you rely on it.
- How do I calculate conduit fill for a group of cables?
- Add up the overall cross-section of each cable (including insulation and sheath), then find the smallest conduit whose internal area is at least 2.5 times that total (the 40% rule for three or more cables). This calculator does it for single core building wire and flat TPS, using Nexans Olex cable sizes and Clipsal conduit bores.
- Does conduit fill affect cable derating?
- Yes. Cables grouped in conduit generate heat that reduces their current-carrying capacity. AS/NZS 3008.1.1 provides grouping derating factors that must be applied on top of the conduit fill check.
- What size conduit do I need for 4 x 2.5mm² TPS cables?
- Nexans Olex lists 2.5mm² flat TPS at 12.2mm x 5.4mm, so this calculator counts each cable as 65.9mm² and four as 263.5mm². At 40% fill you need a conduit with at least 659mm² of bore, about 29mm across. A 32mm medium duty conduit has a 27.8mm bore (607mm²), which is 43% full, so it fails. A 40mm medium duty conduit (35.2mm bore, 973mm²) is 27% full and passes. So the answer is 40mm.
- Can I run different cable sizes in the same conduit?
- Yes. The fill check works on the total cross-section of all the cables, whatever their sizes. You still need to apply the grouping factors from AS/NZS 3008.1.1 for the number of circuits. This calculator lets you add several cable groups of different types and sizes and works out the combined fill.
- What is the space factor rule for conduit fill in Australia?
- This calculator uses the space factors commonly quoted from AS/NZS 3000 Appendix C: 50% for one cable, 33% for two and 40% for three or more. They leave room to draw the cables in without damage. We have not yet checked them against a copy of the current edition, so confirm them in your copy.