How do I calculate conduit fill for mixed cable sizes?

Add up the area of every cable from its outer diameter (π × OD² ÷ 4, using the manufacturer’s published diameter, not the conductor size), divide by the conduit’s internal area, and check the result against the AS/NZS 3000 Appendix C space factor for that number of cables: 50% for one, 33% for two, 40% for three or more. The ScaleSet conduit sizing calculator does this for any mix of cable types and sizes from the Prysmian or Olex catalogue, picks the smallest heavy-duty conduit that complies, and splits the bundle evenly across parallel runs when no single conduit up to your maximum size can take it.

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

It depends on the cable’s outer diameter, not its conductor size. At the 40% space factor a 20 mm heavy-duty conduit (15.3 mm bore) takes six 2.5 mm² V-90 single-insulated building wires at 3.7 mm, four 4 mm² or three 6 mm², and a 25 mm conduit (19.9 mm bore) takes eleven, seven or five. A 2.5 mm² circular two-core-and-earth at 11.3 mm does not fit a 20 mm conduit at all, and only one fits a 25 mm, because two cables may fill only 33%. Pick your cable from the catalogue and the calculator gives the count for it.

What is the internal diameter of heavy-duty conduit?

The bores this calculator uses for heavy-duty rigid UPVC conduit to AS/NZS 2053.2 are 15.3 mm for 20 mm, 19.9 mm for 25 mm, 26.5 mm for 32 mm, 33.7 mm for 40 mm, 43.1 mm for 50 mm and 55.1 mm for 63 mm. Above 63 mm the nominal size is no longer the outside diameter: an 80 mm conduit is 89 mm outside with a 79.7 mm bore, 100 mm is 114 mm with 102.5 mm, 125 mm is 140 mm with 125.8 mm and 150 mm is 160 mm with 143.7 mm. Medium-duty rigid conduit has a larger bore at the same size and medium-duty corrugated a smaller one, so size those against their own bore.

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 ScaleSet cable selection calculator applies the derating side automatically.

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

An 80 mm heavy-duty conduit. Four 25 mm² 4C+E XLPE copper multicores at 24.9 mm outer diameter take up 1948 mm², which is 39% of an 80 mm conduit’s 79.7 mm bore, just inside the 40% limit for three or more cables. A 63 mm conduit (55.1 mm bore) would be 82% full, and a 100 mm conduit (102.5 mm bore) at 24% leaves spare room. Steel-wire armoured versions are larger, so pick the exact cable from the catalogue and let the calculator confirm it.

Does the ScaleSet 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 to 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 conduit sizing calculator is free to use online, with no sign-up to calculate. Downloading the PDF report needs a free account, and saving the job to your device as a project file needs none. A custom logo and accent colour on the report, cloud save and project workspaces are Pro features.

ScaleSet

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 PDF report referencing AS/NZS 3000:2018 Appendix C (downloading it needs a free account; your own logo and accent colour on it are Pro).

Overview

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 the space factors AS/NZS 3000:2018 Appendix C Paragraph C6.2 publishes for one, two, and three or more cables in a circular enclosure; AS/NZS 3000 Tables C10 to C12 are that equation worked for common cable and conduit combinations.
Fill is computed from manufacturer-published outer diameters, not nominal conductor size. An armoured cable takes far more of the conduit than the unarmoured cable of the same conductor size.
Mixed cable groups are supported: actual cross-sectional areas are summed against the permitted fill, which the count-based AS/NZS 3000 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: AS/NZS 3000 Paragraph C6.3 says the count does not consider grouping. The circuits sharing a conduit are bunched, and their cables take the AS/NZS 3008.1.1:2025 Table 3.33 grouping factor for that number of circuits.
The result is for heavy duty rigid conduit. Medium duty corrugated conduit has a smaller bore at the same nominal size: AS/NZS 3000 Table C10 fits four 1 mm² sheathed singles in a 20 mm corrugated conduit against five in heavy duty, so a corrugated run is sized against its own bore.

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 Paragraph C6.2 (space factor 0.5 / 0.33 / 0.4) and Tables C10 to 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)

What this tool helps with

Auto fill limit: 50% for 1 cable, 33% for 2 cables, 40% for 3 or more. These are the space factors AS/NZS 3000:2018 Appendix C Paragraph C6.2 publishes, and the AS/NZS 3000 Tables C10 to C12 count guides are that same equation worked for common cables and conduits.
Real product catalogue: 380+ Prysmian Australia and Olex (Nexans) cables with manufacturer-published outer diameter and mass.
Distinguishes round multicore, XLPE multicore, SDI single-core and SWA steel-wire-armoured cables: same nominal spec but different OD and fill.
Mixed-cable packing, largest cable first into the least-filled run, spread evenly over the fewest parallel runs when no single conduit fits.
Cross-sectional sketches to scale and a PDF report citing AS/NZS 3000:2018 Appendix C for every run (downloading it needs a free account; your own logo and accent colour on it are Pro).
Heavy-duty rigid UPVC nominal sizes to AS/NZS 2053: 20, 25, 32, 40, 50, 63, 80, 100, 125 and 150 mm. Medium-duty rigid conduit has a larger bore at the same nominal size and medium-duty corrugated a smaller one (AS/NZS 3000 Table C10), so size a corrugated run against its own bore: this calculator works heavy-duty rigid conduit.
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 3000 Paragraph C6.2 space factors (50% / 33% / 40%), or switch to manual to hold a lower figure a specification calls for.
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. Download the PDF report citing AS/NZS 3000:2018 Appendix C, with catalogue references for procurement; downloading needs a free account, and your own logo and accent colour on it are Pro.

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 the Standard’s own term. AS/NZS 3000:2018 Appendix C Paragraph C6.2 says space factor, which is the wording on this calculator and in most Australian specifications. Cable manufacturers say fill. 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 Paragraph C6.2 states the basis directly: the number of cables that can be installed in a circular conduit is the space factor multiplied by the ratio of the internal cross-sectional area of the enclosure to the cross-sectional area of the cable, where the space factor is 0.5 for one cable in the enclosure, 0.33 for two cables and 0.4 for three or more. The space factor recognises the space lost to the circular geometry of cables and enclosures.

AS/NZS 3000 Tables C10, C11 and C12 are that equation worked for the common cases: the maximum number of single-core sheathed, two-core-and-earth and four-core-and-earth cables in medium-duty and heavy-duty UPVC conduit, using manufacturer catalogue dimensions for cables to AS/NZS 5000.1 and 5000.2 and nominal bores from AS/NZS 2053. AS/NZS 3000 Paragraph C6.3 adds that the count assumes a relatively short, straight enclosure with few bends, and should be reduced where that is not the case.

The ScaleSet conduit sizing calculator uses the area-based approach so it can handle any mix of cable types (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 2053.2: the dimensions this calculator uses, the internal area, and the area usable at each space factor
Nominal size	Outside diameter	Internal diameter	Internal area	Usable at 50% (1 cable)	Usable at 33% (2 cables)	Usable at 40% (3+ cables)
20 mm	20 mm	15.3 mm	184 mm²	92 mm²	61 mm²	74 mm²
25 mm	25 mm	19.9 mm	311 mm²	156 mm²	103 mm²	124 mm²
32 mm	32 mm	26.5 mm	552 mm²	276 mm²	182 mm²	221 mm²
40 mm	40 mm	33.7 mm	892 mm²	446 mm²	294 mm²	357 mm²
50 mm	50 mm	43.1 mm	1459 mm²	729 mm²	481 mm²	584 mm²
63 mm	63 mm	55.1 mm	2384 mm²	1192 mm²	787 mm²	954 mm²
80 mm	89 mm	79.7 mm	4989 mm²	2494 mm²	1646 mm²	1996 mm²
100 mm	114 mm	102.5 mm	8252 mm²	4126 mm²	2723 mm²	3301 mm²
125 mm	140 mm	125.8 mm	12429 mm²	6215 mm²	4102 mm²	4972 mm²
150 mm	160 mm	143.7 mm	16218 mm²	8109 mm²	5352 mm²	6487 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 with its automatic space factor (50% for one cable, 33% for two, 40% for three or more) and reading off the largest quantity that still fits one conduit. A 0 means not even one cable of that type fits that size.

Maximum number of identical cables in one heavy-duty conduit with the AS/NZS 3000 Paragraph C6.2 space factors, as this calculator works them
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, unsheathed	3.7 mm	6	11	20	33	54	88	185	306
4 mm² single-core V-90, unsheathed	4.6 mm	4	7	13	21	35	57	120	198
6 mm² single-core V-90, unsheathed	5.2 mm	3	5	10	16	27	44	93	155
16 mm² single-core V-90, unsheathed	7.3 mm	1	2	5	8	13	22	47	78
25 mm² single-core V-90, unsheathed	8.9 mm	1	1	3	5	9	15	32	53
2.5 mm² 2C+E circular PVC	11.3 mm	0	1	1	3	5	9	19	32
25 mm² 4C+E circular XLPE	24.9 mm	0	0	0	0	1	1	4	6

Where this calculator and the AS/NZS 3000 Appendix C count tables differ

The counts above are the AS/NZS 3000 Paragraph C6.2 equation, and AS/NZS 3000 Tables C10, C11 and C12 are the same equation worked for common cables: AS/NZS 3000 Table C10 for single-core sheathed cables, Table C11 for two-core-and-earth and Table C12 for four-core-and-earth, each for heavy-duty rigid UPVC, medium-duty corrugated and medium-duty rigid UPVC conduit. For the same cable the two agree: a 2.5 mm² circular two-core-and-earth fits none in a 20 mm heavy-duty conduit, one in 25 mm or 32 mm, three in 40 mm, five in 50 mm and nine in 63 mm, here and in AS/NZS 3000 Table C11, and a 25 mm² four-core-and-earth XLPE fits none up to 40 mm and one in 50 mm or 63 mm, here and in AS/NZS 3000 Table C12. Where they differ, by a cable, the difference is the cable’s diameter, not the method or the bore: the AS/NZS 3000 Table C12 row for 25 mm² four-core-and-earth XLPE fits a cable about 25.2 to 26.2 mm across, the catalogue cable here is 24.9 mm, and so an 80 mm conduit takes four of them here and three in AS/NZS 3000 Table C12.

The single-core rows differ, and the difference is the cable, not the method. AS/NZS 3000 Table C10 counts sheathed single-core cables. The single-core rows above are unsheathed V-90 building wire, which is smaller: AS/NZS 3000 Table C10 puts three 2.5 mm² sheathed singles in a 20 mm heavy-duty conduit, where six of the 3.7 mm unsheathed wire fit. Pick the construction actually being installed; a sheathed single-core cable is counted at its own, larger diameter.

Three-core-and-earth cable has no AS/NZS 3000 Appendix C table at all, and neither do armoured, flexible or halogen-free constructions or a mixed bundle. The equation covers every one of them, which is why this calculator works it directly rather than reading the tables.

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. The armoured cable has a much larger catalogue OD, 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 the round multicore and SWA variants of the same 2C+E 1.5 mm² copper PVC cable (10.1 mm and 15.2 mm in the Prysmian catalogue) resolve to different fill outcomes.

How the area-based packing algorithm works

Cables are flattened from quantities into individual items and sorted by outer diameter, largest first. Each is placed in the least-filled conduit run, so the runs come out balanced, and where that quick pass cannot settle a bundle an exhaustive search tries every other arrangement. When no single conduit up to your maximum size can take the bundle, the calculator finds the fewest runs any allowed size achieves and then the smallest conduit that achieves them. Each run is held to the space factor for the number of cables it ends up with, so two large cables at 36% of a conduit are allowed once a third cable makes it a three-cable run within 40%.

The space factor is not a packing limit: AS/NZS 3000 Paragraph C6.2 says it recognises the space lost to the circular geometry of the cables and the enclosure, and AS/NZS 3000 Paragraph C6.3 says the count assumes a short run, clear of obstructions, with few bends. Where the run is long or has several bends, use fewer cables than the count and check the pulling tension and bending radius.

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 circuits bunched or enclosed together, AS/NZS 3008.1.1:2025 Table 3.33 gives 0.80 for two circuits, 0.70 for three, 0.57 for six and 0.38 for twenty or more. The ScaleSet 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 allowance AS/NZS 3000:2018 Appendix C Paragraph C6.2 makes for the gaps between round cables in a round enclosure: 0.5 for one cable, 0.33 for two and 0.4 for three or more. The number of cables is the space factor times the conduit’s internal area divided by one cable’s area, so as a percentage it is the fill limit. AS/NZS 3000 Tables C10 to C12 are that equation worked for common cables and conduits.
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.
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, which can push the conduit recommendation up a nominal size or more.
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 ScaleSet 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.

Methods reviewed by

Wisam Tozah (CPEng, NER, MIEAust) for ScaleSet Pty Ltd. The methods are tested against the standards' published tables and worked examples before every release. See how they are verified. ScaleSet is a design aid: your inputs and results are not reviewed or certified, and responsibility for the design stays with the engineer who issues it. LinkedIn. Updated 29 October 2026.

Frequently asked questions

What is the maximum conduit fill percentage (space factor) under AS/NZS 3000?: AS/NZS 3000:2018 Appendix C Paragraph C6.2 sets a space factor of 0.5 for one cable, 0.33 for two cables and 0.4 for three or more, so the cables may take up 50%, 33% or 40% of the conduit’s internal cross-sectional area. AS/NZS 3000 Tables C10, C11 and C12 are that equation worked for single-core sheathed, two-core-and-earth and four-core-and-earth cables, and AS/NZS 3000 Paragraph C6.3 asks for fewer cables in a long run or one with several bends. On auto, this calculator applies the three factors to each conduit by the number of cables in it, for any mix of cables; switch to manual to hold a lower figure a specification calls for.
How do I calculate conduit fill for mixed cable sizes?: Add up the area of every cable from its outer diameter (π × OD² ÷ 4, using the manufacturer’s published diameter, not the conductor size), divide by the conduit’s internal area, and check the result against the AS/NZS 3000 Appendix C space factor for that number of cables: 50% for one, 33% for two, 40% for three or more. The ScaleSet conduit sizing calculator does this for any mix of cable types and sizes from the Prysmian or Olex catalogue, picks the smallest heavy-duty conduit that complies, and splits the bundle evenly across parallel runs when no single conduit up to your maximum size can take it.
How many cables fit in a 20 mm or 25 mm conduit?: It depends on the cable’s outer diameter, not its conductor size. At the 40% space factor a 20 mm heavy-duty conduit (15.3 mm bore) takes six 2.5 mm² V-90 single-insulated building wires at 3.7 mm, four 4 mm² or three 6 mm², and a 25 mm conduit (19.9 mm bore) takes eleven, seven or five. A 2.5 mm² circular two-core-and-earth at 11.3 mm does not fit a 20 mm conduit at all, and only one fits a 25 mm, because two cables may fill only 33%. Pick your cable from the catalogue and the calculator gives the count for it.
What is the internal diameter of heavy-duty conduit?: The bores this calculator uses for heavy-duty rigid UPVC conduit to AS/NZS 2053.2 are 15.3 mm for 20 mm, 19.9 mm for 25 mm, 26.5 mm for 32 mm, 33.7 mm for 40 mm, 43.1 mm for 50 mm and 55.1 mm for 63 mm. Above 63 mm the nominal size is no longer the outside diameter: an 80 mm conduit is 89 mm outside with a 79.7 mm bore, 100 mm is 114 mm with 102.5 mm, 125 mm is 140 mm with 125.8 mm and 150 mm is 160 mm with 143.7 mm. Medium-duty rigid conduit has a larger bore at the same size and medium-duty corrugated a smaller one, so size those against their own bore.
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 ScaleSet cable selection calculator applies the derating side automatically.
What conduit size do I need for 4 × 25 mm² 4-core-and-earth XLPE cables?: An 80 mm heavy-duty conduit. Four 25 mm² 4C+E XLPE copper multicores at 24.9 mm outer diameter take up 1948 mm², which is 39% of an 80 mm conduit’s 79.7 mm bore, just inside the 40% limit for three or more cables. A 63 mm conduit (55.1 mm bore) would be 82% full, and a 100 mm conduit (102.5 mm bore) at 24% leaves spare room. Steel-wire armoured versions are larger, so pick the exact cable from the catalogue and let the calculator confirm it.
Does the ScaleSet 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 to 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 conduit sizing calculator is free to use online, with no sign-up to calculate. Downloading the PDF report needs a free account, and saving the job to your device as a project file needs none. A custom logo and accent colour on the report, cloud save and project workspaces are Pro features.