What is the maximum conduit fill ratio in Australia under AS/NZS 3000?

AS/NZS 3000:2018 Appendix C provides count-based selection in Tables C10, C11 and C12. For mixed cable bundles the calculator uses an equivalent area-based rule of thumb: 50% for a single cable, 33% for two cables, and 40% for three or more cables. This is conservative against the AS/NZS count tables for almost every cable family.

How do I size conduit for mixed cable sizes?

Sum the cross-sectional area of all cables (use π × OD² / 4 per cable based on the manufacturer outer diameter, not conductor size) and select the smallest conduit whose internal area gives a fill ratio at or below the AS/NZS 3000 Appendix C limit for that cable count. The ElecAS conduit sizing calculator does this automatically and handles mixed Prysmian and Olex catalogue products.

Does AS/NZS 3000:2018 require derating for cables in conduit?

Yes. AS/NZS 3000 Appendix C governs the geometric fill check; AS/NZS 3008.1.1:2025 Tables 22–24 require an additional grouping derating factor for cables enclosed in conduit because heat dissipation is reduced. Typical derating factors are 0.8–0.9 for two enclosed circuits and 0.7 or lower for six or more. The ElecAS cable selection calculator applies the derating side automatically.

What conduit size do I need for 4 × 25 mm² 4-core-and-earth XLPE cables?

A single 100 mm HD-PVC at roughly 21% fill, or a 125 mm at roughly 14% fill, will accommodate 4 × XLPE 4C+E 25 mm² Cu multicore (outer diameter ~23 mm per cable) under the 40% area limit for 3+ cables. Use the ElecAS conduit sizing calculator to confirm against your specific cable catalogue (SWA armoured variants are significantly larger).

Why are flat TPS and round multicore the same spec but different conduit fill?

A 2C+E 1.5 mm² CU PVC flat twin-and-earth has an outer cross-section of about 10.1 × 4.6 mm. The equivalent round multicore is 10.1 mm circular. The flat cable occupies less conduit area but more conduit width — both matter for pulling tolerance, and the calculator uses the largest dimension to stay conservative.

Does the ElecAS calculator support steel-wire armoured (SWA) cables?

Yes. The Prysmian and Olex SWA multicore families (2C+E, 3C+E, 4C+E in PVC and XLPE) are included with their published outer diameters. SWA armoured cables of the same nominal conductor size have outer diameters 40–60% larger than the unarmoured equivalent, so the resulting conduit recommendation often steps up one or two nominal sizes.

Is the conduit sizing PDF report compliant with AS/NZS 3000:2018?

The PDF report cites AS/NZS 3000:2018 Appendix C, shows the applied fill limit, lists the cables packed into each run with manufacturer catalogue reference, and includes a to-scale cross-sectional sketch. The report is a documented design check intended to support the responsible electrical engineer; final verification and field tolerance remain with the certifying engineer.

Is this conduit sizing calculator free to use?

Yes. The full conduit sizing calculator is free to use online with no sign-up required. The branded PDF report (with company logo, designer name and accent colour) is included in the free tier. Cloud project sync and team workspaces are paid Pro / Team features.

ElecAS

Conduit Sizing Calculator

Why this page matters

Free Australian conduit sizing calculator. Pick from real Prysmian and Olex cable catalogue data, mix cores and sizes, and get the smallest compliant HD-PVC conduit with parallel-run packing and a branded PDF report. This static content is published so the canonical route has meaningful crawlable HTML even before the interactive application hydrates.

Who this page is for

Electrical engineers, contractors, designers and installers sizing HD-PVC, medium-duty and heavy-duty conduits for AS/NZS 3000:2018 compliance in Australian and New Zealand projects.

Relevant standards

AS/NZS 3000:2018 — Electrical installations (Wiring Rules), Appendix C / Tables C10–C12
AS/NZS 3008.1.1:2025 — Electrical installations, selection of cables, Tables 22–24 (grouping factors)
AS/NZS 5000.1 — Electric cables — Polymeric insulated, voltages up to and including 0.6/1 kV
IEC 60364-5-52 — Selection and erection of wiring systems (equivalent fill guidance)
NEC Chapter 9 Table 1 — fill-percentage source for 1/2/3+ cable rule of thumb

What this tool helps with

Auto AS/NZS-aligned space factor: 50% for 1 cable, 33% for 2 cables, 40% for 3+ cables — conservative industry rule of thumb against AS/NZS 3000:2018 Appendix C Tables C10–C12.
Real product catalogue: 380+ Prysmian Australia and Olex (Prysmian Group) cables with manufacturer-published outer diameter and mass.
Distinguishes flat TPS, round multicore, XLPE multicore, SDI single-core and SWA steel-wire-armoured cables — same nominal spec but different OD and fill.
Mixed-cable bin packing (best-fit decreasing) with automatic parallel-run distribution when no single conduit fits.
Cross-sectional sketches to scale and a branded PDF report citing AS/NZS 3000:2018 Appendix C for every run.
Coverage for 20 mm, 25 mm, 32 mm, 40 mm, 50 mm, 63 mm, 80 mm, 100 mm, 125 mm and 150 mm HD-PVC conduit nominal sizes.
Catalogue references (Prysmian SKUs and Olex codes) embedded in the run schedule so the PDF is unambiguous for procurement.

How to size a conduit for AS/NZS 3000:2018

Pick the cable manufacturer — Choose Prysmian Australia or Olex (Prysmian Group) so the calculator filters to that catalogue. Both are tagged with manufacturer in the product database.
Choose a cable family — Select the construction family — PVC Insulated single core, PVC Multicore Circular, XLPE Multicore Circular, PVC/XLPE SWA armoured, SDI single-core, Versolex flex or Envirolex halogen-free. The family determines outer diameter and mass.
Pick the cable size and quantity — Choose the nominal conductor area (1, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300, 400, 500, 630 mm²) and how many cables of that type are running through the conduit.
Set the space factor — Leave auto on to apply the AS/NZS-aligned 50% / 33% / 40% rule, or override manually if your specification requires a different fill (some clients require 35% maximum, particularly for harsh-environment runs).
Set the maximum enclosure size — Cap the largest HD-PVC nominal you are willing to install (e.g. 100 mm). The calculator falls back to parallel runs of this maximum size when a single conduit cannot fit the bundle.
Review the recommendation and export the PDF — The output shows the chosen conduit nominal, average fill percentage, run schedule and to-scale cross-section. Export a branded AS/NZS 3000:2018 Appendix C compliance PDF with catalogue references for procurement.

Complete guide to conduit sizing under AS/NZS 3000:2018

What does AS/NZS 3000:2018 Appendix C say about conduit fill?

AS/NZS 3000:2018 Appendix C provides count-based selection tables (Tables C10, C11 and C12) for the maximum number of single-core, two-core-and-earth and four-core-and-earth cables permitted in standard medium-duty and heavy-duty UPVC conduit nominal sizes. The tables embed a space factor implicitly and account for cable pulling friction, conductor expansion under load and field installation tolerances.

For mixed bundles, designers fall back to an area-based rule of thumb recognised in IEC 60364-5-52 and common Australian engineering practice: 50% maximum fill for a single cable, 33% for two cables, and 40% for three or more cables. This rule is conservative against the AS/NZS count tables for almost every common cable family.

The ElecAS conduit sizing calculator uses the area-based approach so it can handle any mix of cable types — flat TPS, round multicore, XLPE single-core SDI, steel-wire armoured (SWA), flexible Versolex and halogen-free Envirolex — that the AS/NZS count tables do not directly cover.

Why outer diameter (and not conductor size) drives conduit selection

Conduit fill is a geometric problem, not an electrical one. A 25 mm² copper conductor inside a thin V-90 PVC sheath has a very different outer diameter to the same conductor inside an XLPE/SWA/PVC armoured construction. Catalogue OD for the latter can be 50% larger, and the area inside the conduit it occupies scales with diameter squared.

The calculator reads outer diameter directly from the manufacturer catalogue (Prysmian Australia Technical Cable Guide and Olex Cable Handbook), so flat TPS, round multicore and SWA variants of the same 2C+E 1.5 mm² CU PVC nominal correctly resolve to three different fill outcomes.

How the area-based packing algorithm works

Cables are flattened from quantities into individual items, sorted by outer diameter descending, and packed bin-by-bin using a best-fit-decreasing heuristic. Each cable is assigned to the least-filled conduit run that still has room within the fill limit. When a single conduit cannot fit the bundle, the algorithm switches to parallel runs of the user-selected maximum nominal size and distributes the cables evenly.

The area-based check is conservative against geometric circle packing: the jamming limit for identical circles inside a circular container is approximately 78%, and even with mixed sizes the practical achievable packing rarely exceeds 60%. Fill limits of 40% therefore leave significant geometric headroom for real-world pulling tolerances and field bends.

When to apply grouping derating in addition to fill checks

Cables enclosed in conduit are derated for grouping under AS/NZS 3008.1.1:2025 Tables 22–24 in addition to satisfying the AS/NZS 3000 Appendix C fill check. Fill alone is a containment check; it does not address the reduced heat dissipation inside the conduit.

For 3-core-and-earth and 4-core-and-earth multicore cables drawing 80% or more of their rating, designers typically apply a grouping factor of 0.8–0.9 for two-circuit enclosed runs, dropping to 0.7 or lower for six or more circuits in the same conduit. The ElecAS cable selection calculator handles the derating side; the conduit sizing calculator handles the containment side, and the two are intended to be used together.

Manufacturer catalogue coverage

The calculator ships with 380+ cable products mapped from the Prysmian Australia Technical Cable Guide (October 2015 edition) and the Olex (Prysmian Group) Cable Handbook (2017 edition). Coverage includes V-90 PVC and X-90 XLPE single-core and multicore families, PVC-bedded steel-wire armoured (SWA) variants, single-core double-insulated (SDI), Versolex flex (XLPE/TPE) and Envirolex halogen-free (XLPE/HFS RE-110).

Every product carries its manufacturer catalogue reference (Prysmian SKU or Olex code), so the exported AS/NZS 3000:2018 Appendix C compliance PDF includes a procurement-ready run schedule with the exact cable model installed in each conduit.

Reviewed by

Wisam Tozah — Associate Electrical Engineer. B.Eng (Electrical), MIEAust, CPEng, NER, NSW DBP, NSW PRE, APEC, IntPE(Aus). LinkedIn.

Frequently asked questions

What is the maximum conduit fill ratio in Australia under AS/NZS 3000?: AS/NZS 3000:2018 Appendix C provides count-based selection in Tables C10, C11 and C12. For mixed cable bundles the calculator uses an equivalent area-based rule of thumb: 50% for a single cable, 33% for two cables, and 40% for three or more cables. This is conservative against the AS/NZS count tables for almost every cable family.
How do I size conduit for mixed cable sizes?: Sum the cross-sectional area of all cables (use π × OD² / 4 per cable based on the manufacturer outer diameter, not conductor size) and select the smallest conduit whose internal area gives a fill ratio at or below the AS/NZS 3000 Appendix C limit for that cable count. The ElecAS conduit sizing calculator does this automatically and handles mixed Prysmian and Olex catalogue products.
Does AS/NZS 3000:2018 require derating for cables in conduit?: Yes. AS/NZS 3000 Appendix C governs the geometric fill check; AS/NZS 3008.1.1:2025 Tables 22–24 require an additional grouping derating factor for cables enclosed in conduit because heat dissipation is reduced. Typical derating factors are 0.8–0.9 for two enclosed circuits and 0.7 or lower for six or more. The ElecAS cable selection calculator applies the derating side automatically.
What conduit size do I need for 4 × 25 mm² 4-core-and-earth XLPE cables?: A single 100 mm HD-PVC at roughly 21% fill, or a 125 mm at roughly 14% fill, will accommodate 4 × XLPE 4C+E 25 mm² Cu multicore (outer diameter ~23 mm per cable) under the 40% area limit for 3+ cables. Use the ElecAS conduit sizing calculator to confirm against your specific cable catalogue (SWA armoured variants are significantly larger).
Why are flat TPS and round multicore the same spec but different conduit fill?: A 2C+E 1.5 mm² CU PVC flat twin-and-earth has an outer cross-section of about 10.1 × 4.6 mm. The equivalent round multicore is 10.1 mm circular. The flat cable occupies less conduit area but more conduit width — both matter for pulling tolerance, and the calculator uses the largest dimension to stay conservative.
Does the ElecAS calculator support steel-wire armoured (SWA) cables?: Yes. The Prysmian and Olex SWA multicore families (2C+E, 3C+E, 4C+E in PVC and XLPE) are included with their published outer diameters. SWA armoured cables of the same nominal conductor size have outer diameters 40–60% larger than the unarmoured equivalent, so the resulting conduit recommendation often steps up one or two nominal sizes.
Is the conduit sizing PDF report compliant with AS/NZS 3000:2018?: The PDF report cites AS/NZS 3000:2018 Appendix C, shows the applied fill limit, lists the cables packed into each run with manufacturer catalogue reference, and includes a to-scale cross-sectional sketch. The report is a documented design check intended to support the responsible electrical engineer; final verification and field tolerance remain with the certifying engineer.
Is this conduit sizing calculator free to use?: Yes. The full conduit sizing calculator is free to use online with no sign-up required. The branded PDF report (with company logo, designer name and accent colour) is included in the free tier. Cloud project sync and team workspaces are paid Pro / Team features.