Why there is a limit at all
Two reasons, and only one of them is about heat.
The first is pulling. Cables packed tight in a conduit jam, and the tension you need to get them moving is exactly the tension that strips insulation off a conductor against the edge of a bend. The damage happens inside the wall where nobody sees it, and it turns up years later as a fault nobody can locate.
The second is heat. Cables bunched together cannot shed heat the way a single cable in free air can, so they carry less current safely. That is a derating problem as well as a fill problem, and the two are separate calculations — passing the fill check does not mean the cables still carry their full rating.
AS/NZS 3000 handles the first by limiting how much of the conduit's cross-section the cables may occupy. That proportion is the space factor.
The space factor depends on how many cables
| Cables in the conduit | Space factor commonly applied |
|---|---|
| One | 53% |
| Two | 31% |
| Three or more | 40% |
Two cables getting less than three looks like a misprint the first time you see it. It is not. It is packing geometry — two round cables sitting side by side in a round bore leave big unusable crescents at the top and bottom, so a lower proportion of the area is genuinely available. Three or more nest better. One on its own can be pushed hardest of all.
A worked example
Six cables, each 10mm overall diameter, going down a wall:
- Area of one cable: π × 5² = 78.5 mm²
- Six of them: 6 × 78.5 = 471 mm²
- Six cables is "three or more", so the limit is 40%
- A 32mm medium-duty conduit has a 27.8mm bore: π × 13.9² = 607 mm². Fill is 471 ÷ 607 = 77.6% — way over
- A 40mm has a 35.4mm bore = 984 mm². Fill is 47.9% — still over
- A 50mm has a 44.6mm bore = 1562 mm². Fill is 30.2% — under, so 50mm is the size
Note how fast it moves. Going 32mm to 40mm only bought 30 percentage points, and it was still not enough. Conduit area grows with the square of the bore, so one size up is a much bigger jump than the label suggests — which cuts both ways when you are guessing on site.
How many 10mm cables fit at 40%
| Conduit | Internal bore | Cross-section | 10mm cables at 40% |
|---|---|---|---|
| 20mm medium duty | 17.0 mm | 227 mm² | 1 |
| 25mm medium duty | 21.4 mm | 360 mm² | 1 |
| 32mm medium duty | 27.8 mm | 607 mm² | 3 |
| 40mm medium duty | 35.4 mm | 984 mm² | 5 |
| 50mm medium duty | 44.6 mm | 1562 mm² | 7 |
Bores are typical Australian medium-duty figures. Heavy duty has thicker walls and a smaller bore for the same nominal size, so swapping medium for heavy on site can push a run over the limit without anything looking different from the outside. Check the bore of the product you actually have.
Fill is the arithmetic. Bends are the reality.
A conduit at 39% fill on paper can still be unpullable, and usually it is the bends that did it. Every 90° bend multiplies the tension needed to drag the bundle through, and the effect compounds — two bends are much worse than twice one bend.
- Keep it to two 90° bends between draw points where you can, three at the absolute outside. Beyond that, put in a junction box.
- Wide radius beats tight. A sweep bend pulls far easier than a solvent-weld elbow, and it is kinder to the insulation.
- Draw wire goes in first, every time. Retrofitting one into a buried conduit with a bend in it is a bad afternoon.
- Lubricant is not cheating. Cable pulling compound drops tension dramatically on a long run.
- Underground runs move. Water, silt and a bit of settlement all make a buried conduit harder to pull than the same conduit on a wall.
Do not forget the derating
Fill and current-carrying capacity are two different checks. A conduit that comes in under the space factor can still be carrying more circuits bunched together than the cable ratings assume, and each of those circuits has to be derated for the grouping, for the ambient temperature, and for the installation method. In a hot Australian roof space that combination bites hard.
Work the fill out here, then go and check the cable ratings against the grouping factors before you commit. If the derating pushes a cable a size up, the fill sum changes too — the loop is worth closing before the conduit goes in the wall.
Leave room for the next bloke
Filling a conduit to exactly the limit means every future addition is a new conduit. On a domestic rewire or a commercial fitout, going one size up on the main vertical runs costs very little at rough-in and saves an entire return visit later. Customers do not thank you for it, but the next sparkie does.
For licensed electricians
Electrical installation work in Australia must be carried out by a licensed electrician, and the certificate of compliance is theirs to issue. This is an estimating aid — it gets you to the right conduit size on site so you can order the right lengths and price the job.
Sized the conduit. Now price the run.
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Try Yamate free → Free to try · no card · sign in with a link, no password to rememberConduit fill FAQ
What is the maximum conduit fill in Australia?
The commonly applied space factors are 53% for a single cable, 31% for two cables and 40% for three or more. They come from the AS/NZS 3000 wiring enclosure provisions and are about being able to pull cables in without damaging them.
Why do two cables get a lower fill than three?
Packing geometry. Two round cables side by side inside a round bore leave large unusable gaps above and below them, so less of the cross-section is genuinely available. Three or more nest together far better.
How many cables fit in a 25mm conduit?
At 40% fill, a 25mm medium-duty conduit with a 21.4mm bore has about 360 mm² of cross-section, which is one 10mm cable. Thinner cables fit more — enter your actual cable diameter above.
How do you calculate conduit fill percentage?
Work out the total cross-sectional area of the cables — number of cables times pi times the radius squared — then divide by the internal area of the conduit and multiply by 100. Use the internal bore, not the nominal size on the outside.
Is heavy duty conduit the same size inside as medium duty?
No. Heavy duty has thicker walls, so the bore is smaller for the same nominal size. Swapping one for the other on site can take a run over the space factor without anything looking different.
How many bends can a conduit run have?
Aim for two 90 degree bends between draw points, three at the outside. Bends multiply pulling tension and it compounds, so a run that passes the fill check on paper can still be unpullable in practice.
Does staying under the fill limit mean the cables carry full current?
No. Fill and current-carrying capacity are separate checks. Cables bunched in a conduit shed heat poorly and have to be derated for grouping, ambient temperature and installation method. Do both.
Other calculators you'll want
General information for Australian trades, and an estimate only — not tax, financial or legal advice. It is not engineering, building or design advice either: a calculator works out a quantity from the figures you enter, and anything that needs sign-off still needs your engineer, certifier or building surveyor. Space factor limits follow published Australian guidance on the AS/NZS 3000 wiring enclosure provisions and can change between editions. Conduit internal diameters are typical Australian medium-duty figures and vary by manufacturer and duty rating — confirm the bore of the product you are using. This calculator does not derate cables for grouping, ambient temperature or installation method, which are separate checks that can change the cable size. Electrical installation work in Australia must be carried out by a licensed electrician; this is an estimating aid, not a design or a sign-off.