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, with mixed cable types and sizes drawn from your chosen catalogue (Prysmian or Olex/Nexans, selected per calculation).

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 Table 3.33 applies a grouping derating for circuits bunched or enclosed in a conduit, because heat dissipation is reduced. The factor is 0.80 for two circuits, 0.70 for three, 0.57 for six, and falls to about 0.38 at twenty. 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?

Four 25 mm² 4C+E XLPE copper multicores are 24.9 mm outer diameter each, so they fit a single 100 mm heavy-duty PVC conduit at about 29% fill, or a 125 mm at about 19%. An 80 mm conduit is too small: it would sit at 48%, over the 40% limit. 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?

Where the catalogue lists a flat cable as two dimensions, such as about 10.4 × 5.6 mm for a 1.5 mm² flat twin-and-earth, the calculator takes the larger dimension and treats the cable as a circle of that diameter. That overstates the true area a little, which keeps the recommendation on the safe side for pulling tolerance.

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 to calculate. The branded PDF report (with company logo, designer name and accent colour) is included in the free tier; downloading it requires only a free account. Cloud project sync and project workspaces are paid Pro features.

ElecAS

Conduit Sizing Calculator: AS/NZS 3000 Conduit Fill and Space Factor for Australia

Free Australian conduit sizing and conduit fill calculator. Pick from real Prysmian and Olex cable catalogue data, mix cores and sizes, and get the smallest heavy-duty conduit with parallel-run packing and a branded PDF report referencing AS/NZS 3000:2018 Appendix C.

Conduit sizing - conduit fill, or conduit infill as it is often called on site - is a geometry problem, not an electrical one: total the cross-sectional area of every cable going into the enclosure and pick the smallest conduit that keeps the bundle under the allowed fill. This AS3000 conduit calculator works from manufacturer-published outer diameters rather than nominal conductor size, mixes cable types and sizes in a single run, splits the bundle across parallel runs when no single conduit fits, and exports a PDF that cites AS/NZS 3000:2018 Appendix C.

Conduit sizing at a glance

AS/NZS 3000:2018 Appendix C Tables C10, C11 and C12 are the Wiring Rules guides to the maximum number of cables in a conduit. They cover single-core sheathed, two-core-and-earth and four-core-and-earth cables only.
The 50% / 33% / 40% fill limits this calculator applies are a conservative industry rule of thumb, not a figure published in AS/NZS 3000: the term "space factor" does not appear anywhere in the Standard.
Fill is computed from manufacturer-published outer diameters, not nominal conductor size. An armoured 25 mm² cable can occupy more than twice the conduit area of the unarmoured equivalent.
Mixed cable groups are supported: actual cross-sectional areas are summed against the permitted fill, which the count-based Appendix C tables cannot do.
Sizes run 20 mm to 150 mm heavy-duty rigid UPVC to AS/NZS 2053.2, using published dimensions rather than the nominal number: above 63 mm heavy duty conduit follows Series 1 pressure pipe, so a 100 mm conduit is 114 mm across the outside with a 102.5 mm bore.
Where no single conduit fits, the bundle is spread evenly across parallel runs, and the space factor for each run is set by the number of cables in that run - so three runs of two cables are each held to the two-cable limit, not the six-cable one.
Fill is a containment check only. Cables enclosed in a conduit are separately derated for grouping under AS/NZS 3008.1.1:2025 Table 3.33.

Who this page is for

Electrical engineers, contractors, designers, estimators and electricians working out conduit fill (conduit infill) and the smallest compliant conduit size for AS/NZS 3000:2018 installations in Australia and New Zealand.

Relevant standards

AS/NZS 3000:2018: Electrical installations (Wiring Rules), Appendix C Tables C10–C12 (guides to the maximum number of cables installed in conduit)
AS/NZS 2053: Conduits and fittings for electrical installations, the source of the medium-duty and heavy-duty rigid UPVC internal diameters
AS/NZS 3008.1.1:2025: Electrical installations, selection of cables, Tables 3.33 to 3.43 (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 the 1/2/3+ cable rule of thumb

What this tool helps with

Auto fill limit: 50% for 1 cable, 33% for 2 cables, 40% for 3 or more. A conservative industry rule of thumb read alongside the AS/NZS 3000:2018 Appendix C Tables C10–C12 count guides, not a percentage published by the Standard.
Real product catalogue: 380+ Prysmian Australia and Olex (Nexans) 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.
Heavy-duty rigid UPVC nominal sizes to AS/NZS 2053: 20, 25, 32, 40, 50, 63, 80, 100, 125 and 150 mm. Medium-duty and corrugated conduit hold more at the same nominal size, so a heavy-duty result is the conservative one.
Catalogue references (Prysmian SKUs and Olex codes) embedded in the run schedule so the PDF is unambiguous for procurement.

How to calculate conduit fill and size a conduit to AS/NZS 3000:2018

Pick the cable manufacturer: Choose Prysmian Australia or Olex (Nexans) 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 and conduit fill under AS/NZS 3000:2018

Conduit fill, conduit infill and space factor: three names for one check

Australian sites use conduit fill, conduit infill and space factor more or less interchangeably, and all three describe the same arithmetic: the total cross-sectional area of the cables divided by the internal cross-sectional area of the conduit, as a percentage. Whether you went looking for an AS3000 conduit calculator, a conduit infill calculator or a conduit fill calculator, this is the same job.

Only one of the three is standard terminology, and it is not the one most specifications use. Cable manufacturers say fill. Australian specifications usually say space factor, which is the wording on this calculator, but that phrase does not appear anywhere in AS/NZS 3000. Infill is site usage borrowed from the building trades. None of it changes the calculation, but on a drawing "conduit fill percentage" is the least ambiguous of the three.

What matters more than the name is what the percentage is measured against. This calculator measures against the internal area of heavy-duty rigid UPVC conduit to AS/NZS 2053, using the cable maker’s published outer diameter. A fill percentage quoted without both of those is not checkable.

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. It is a practical industry rule of thumb, not a figure from the Standard, and for single-core sheathed cables it allows noticeably more cables than the count guides in Appendix C Tables C10 to C12: so check those tables as well for building-wire installations.

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.

Conduit internal diameter and usable area by nominal size

A conduit is sold by its nominal size, which approximates the outside diameter. Fill is governed by the inside, and the wall thickness that separates the two depends on the duty rating: heavy-duty rigid UPVC has thicker walls than medium duty, so a 25 mm heavy-duty conduit has a smaller internal diameter, and holds fewer cables, than a 25 mm medium-duty one. The figures below are the heavy-duty dimensions this calculator uses.

Heavy-duty rigid UPVC conduit to AS/NZS 2053: internal area, and the area left usable at each fill limit this calculator applies
Nominal size	Internal diameter	Internal area	Usable at 50% (1 cable)	Usable at 33% (2 cables)	Usable at 40% (3+ cables)
20 mm	16.6 mm	216 mm²	108 mm²	71 mm²	87 mm²
25 mm	21.2 mm	353 mm²	176 mm²	116 mm²	141 mm²
32 mm	28.0 mm	616 mm²	308 mm²	203 mm²	246 mm²
40 mm	35.0 mm	962 mm²	481 mm²	317 mm²	385 mm²
50 mm	44.4 mm	1548 mm²	774 mm²	511 mm²	619 mm²
63 mm	56.4 mm	2498 mm²	1249 mm²	824 mm²	999 mm²
80 mm	71.9 mm	4060 mm²	2030 mm²	1340 mm²	1624 mm²
100 mm	92.0 mm	6648 mm²	3324 mm²	2194 mm²	2659 mm²
125 mm	114.6 mm	10315 mm²	5157 mm²	3404 mm²	4126 mm²
150 mm	137.9 mm	14935 mm²	7468 mm²	4929 mm²	5974 mm²

How many cables fit in a 20 mm, 25 mm or 32 mm conduit?

The honest answer is that it depends on the cable’s outer diameter, not its conductor size, so the question only has an answer once the cable is named. The table below is the answer for some common Australian cables, produced by running this calculator at a 40% fill limit and reading off the largest quantity that still fits one conduit. A dash means not even one cable of that type fits inside that nominal size at 40%.

Maximum number of identical cables in a single heavy-duty conduit at a 40% fill limit, as calculated by ElecAS
Cable	Outer diameter	20 mm	25 mm	32 mm	40 mm	50 mm	63 mm	80 mm	100 mm
2.5 mm² single-core V-90	3.7 mm	8	13	22	35	57	92	151	247
4 mm² single-core V-90	4.6 mm	5	8	14	23	37	60	97	159
6 mm² single-core V-90	5.2 mm	4	6	11	18	29	47	76	125
16 mm² single-core V-90	7.3 mm	2	3	5	9	14	23	38	63
25 mm² single-core V-90	8.9 mm	1	2	3	6	9	16	26	42
2.5 mm² 2C+E circular PVC	11.3 mm	—	1	2	3	6	9	16	26
25 mm² 4C+E circular XLPE	24.9 mm	—	—	—	—	1	2	3	5

Where the area method and the Appendix C count tables disagree

The counts above come from the area method, which is the only method that can handle a mixed bundle. AS/NZS 3000:2018 Appendix C answers the same question a different way, by tabulating cable counts directly: Table C10 for single-core sheathed cables, Table C11 for two-core-and-earth and Table C12 for four-core-and-earth, each broken out for medium-duty rigid UPVC, medium-duty corrugated and heavy-duty rigid UPVC conduit.

For single-core sheathed building wire the two methods do not agree, and the count tables are the stricter of the two: the area method permits noticeably more cables than Appendix C guides. That gap is real and it is not resolved by adjusting the percentage. Where your installation is single-core sheathed cable in standard conduit, treat the Appendix C count as the governing figure and use this calculator to check the geometry, the mixed-bundle case and the parallel-run split.

Three-core-and-earth cable has no Appendix C table at all, and neither do armoured, flexible or halogen-free constructions. For those the area method is the available check, which is the reason this calculator uses it.

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 Table 3.33 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 (Nexans) 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.

Key terms

Conduit fill (conduit infill): The proportion of a conduit’s internal cross-sectional area taken up by the cables inside it. "Conduit infill" is the same check under a name commonly used on Australian sites; the Wiring Rules and the cable catalogues both call it fill.
Space factor: The industry name for the maximum permitted fill ratio, expressed as a percentage. It is not AS/NZS 3000 terminology: the Standard gives count-based guides in Appendix C Tables C10 to C12 rather than a published percentage.
Outer diameter (OD): The overall diameter of the finished cable including insulation, bedding, armour and sheath. Conduit fill is driven by OD, not by the nominal conductor size, because the area a cable occupies scales with the square of its outer diameter.
Heavy duty rigid UPVC conduit: The orange rigid conduit specified to AS/NZS 2053 for general Australian installation work. Thicker walls than medium duty, so a heavy duty conduit of the same nominal size has a smaller internal diameter and holds fewer cables.
Medium duty conduit: A thinner-walled rigid UPVC conduit to AS/NZS 2053 with a larger internal diameter than heavy duty at the same nominal size. AS/NZS 3000 Tables C10 to C12 tabulate medium duty, medium duty corrugated and heavy duty separately for that reason.
Nominal size: The size a conduit is sold and specified by (20, 25, 32, 40, 50, 63, 80, 100, 125 or 150 mm). Up to 63 mm it is the outside diameter, so the bore that actually determines fill is smaller and depends on the duty rating. From 80 mm up, heavy duty conduit uses Series 1 pressure pipe sizes and the nominal number is neither: an 80 mm conduit is 89 mm outside with a 79.7 mm bore, and a 100 mm is 114 mm outside with a 102.5 mm bore.
TPS: Thermoplastic-sheathed flat building wire (typically twin-and-earth). Because it is flat rather than round, this calculator treats it as a circle of its larger cross-sectional dimension, which slightly overstates the area it occupies and keeps the recommendation on the safe side.
SDI: Single-core double-insulated cable: one conductor with an insulation layer and a separate outer sheath. Smaller outer diameter than an equivalent multicore, so more cables fit a given conduit.
SWA: Steel-wire armoured cable. The armour bedding and wires add substantially to outer diameter, typically 40 to 60 per cent over the unarmoured equivalent, which often pushes the conduit recommendation up one or two nominal sizes.
Parallel runs: Two or more conduits carrying one cable group because no single conduit of the permitted maximum size can take the bundle within the fill limit. The calculator balances the cables across the minimum number of runs rather than filling one and leaving the last nearly empty, then picks the smallest conduit that still achieves that number of runs.
Grouping derating: The reduction in a cable’s current-carrying capacity when it shares an enclosure with other loaded cables, because each cable heats its neighbours. Set by AS/NZS 3008.1.1:2025 Table 3.33 for cables bunched or enclosed in conduit, and applied by the ElecAS cable selection calculator, not by the fill check.
AS/NZS 2053: The Australian and New Zealand standard for conduits and fittings for electrical installations. It defines the dimensions, including internal diameter, for medium duty and heavy duty rigid UPVC conduit in each nominal size.

Reviewed by

Wisam Tozah: Associate Electrical Engineer. B.Eng (Electrical), MIEAust, CPEng, NER, NSW DBP, NSW PRE, APEC, IntPE(Aus). See how these calculations are verified. 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 a practical rule of thumb rather than a figure from the Standard: for single-core sheathed cables it permits more cables than the Appendix C count tables, so check those as well.
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, with mixed cable types and sizes drawn from your chosen catalogue (Prysmian or Olex/Nexans, selected per calculation).
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 Table 3.33 applies a grouping derating for circuits bunched or enclosed in a conduit, because heat dissipation is reduced. The factor is 0.80 for two circuits, 0.70 for three, 0.57 for six, and falls to about 0.38 at twenty. 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?: Four 25 mm² 4C+E XLPE copper multicores are 24.9 mm outer diameter each, so they fit a single 100 mm heavy-duty PVC conduit at about 29% fill, or a 125 mm at about 19%. An 80 mm conduit is too small: it would sit at 48%, over the 40% limit. 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?: Where the catalogue lists a flat cable as two dimensions, such as about 10.4 × 5.6 mm for a 1.5 mm² flat twin-and-earth, the calculator takes the larger dimension and treats the cable as a circle of that diameter. That overstates the true area a little, which keeps the recommendation on the safe side for pulling tolerance.
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 to calculate. The branded PDF report (with company logo, designer name and accent colour) is included in the free tier; downloading it requires only a free account. Cloud project sync and project workspaces are paid Pro features.