Conduit Fill Calculator
Determine the minimum conduit size for your cable group based on the 40% maximum fill rule per AS/NZS 3000.
For reference only — always verify with the current published standard and consult a licensed electrical engineer for critical installations.
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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.
Fill percentages follow AS/NZS 3000 Appendix C6. The 40% rule applies for three or more cables. This is a reference tool. Verify against the current edition of the standard for your installation.
It is free with no signup — check a conduit run from your phone before you order materials.
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Conduit Fill Calculator FAQs
How many cables fit in 25mm conduit?
It depends entirely on the cable type and size you are running, not just the conduit size — a 25mm conduit might comfortably take four small 1.5mm² TPS cables but struggle with two much larger ones, because what matters is the combined cross-sectional area of the cables against the conduit's internal area, not a cable count. Enter your cable sizes and quantities into the calculator above and it will tell you straight away whether 25mm clears the AS/NZS 3000 fill limit or whether you need to step up.
What is the maximum conduit fill percentage in Australia?
AS/NZS 3000 limits conduit fill to 40% of the conduit's internal cross-sectional area. This ensures cables can be pulled through without damage and allows adequate heat dissipation.
How do I calculate conduit fill for a group of cables?
Sum the overall cross-sectional area of each cable (including insulation), then find the smallest conduit whose internal area is at least 2.5 times that total. This calculator does it automatically for common Australian cable and conduit sizes.
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?
Four 2.5mm² flat TPS cables (each approximately 11.5mm x 7.5mm, overall area around 86mm²) have a combined area of about 344mm². Applying the 40% fill rule per AS/NZS 3000, you need a conduit with at least 860mm² internal area, which means a minimum 25mm heavy-duty conduit (internal area approximately 380mm² won't work) — you'll typically need 32mm conduit. This calculator handles the exact areas for all common Australian cable and conduit sizes automatically.
Can I run different cable sizes in the same conduit?
Yes. AS/NZS 3000 allows mixing different cable sizes in the same conduit as long as the total cross-sectional area of all cables does not exceed 40% of the conduit's internal area. You still need to apply the correct grouping derating factors from AS/NZS 3008.1.1 for the number of circuits. This calculator lets you add multiple cable groups of different sizes and computes the combined fill percentage.
What is the space factor rule for conduit fill in Australia?
AS/NZS 3000 Appendix C6 sets the maximum conduit fill at 40% of the conduit's internal cross-sectional area when three or more cables are installed. For a single cable the limit is 50%, and for two cables it is 33%. These space factors ensure cables can be drawn in without damage and allow enough air circulation for heat dissipation. This calculator applies the correct space factor based on the number of cables you enter.
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