Conduit fill and space factor — how many cables actually fit.
Pick a conduit, add your cables, get the space factor against AS/NZS 3000. Space factor is total cable area ÷ conduit internal area — keep it at or below your limit (40% is the common working figure) so cables draw in and heat gets away.
Conduit internal area & usable space (medium-duty PVC)
| Conduit | Internal Ø | Internal area | Usable @ 40% | 2.5mm² singles @ 40% |
|---|
Internal diameters are typical medium-duty rigid PVC; light-duty is larger inside, heavy-duty smaller. "2.5 mm² singles" assumes 3.6 mm overall diameter per conductor — always leave room for the earth and for drawing in.
Conduit fill, plainly.
What's the maximum conduit fill in Australia?
AS/NZS 3000 controls cables in an enclosure by space factor — total cable area ÷ conduit internal area — not by a single fixed percentage like the US NEC. A 40% space factor is the common working figure so cables draw in and heat escapes; grouped runs go lower. Follow the conduit maker's fill data and derate for grouping.
How do I calculate conduit fill?
Add the overall area of every cable (area = π ÷ 4 × overall ز, using the insulated outer diameter, not the copper size), divide by the conduit's internal area, ×100 for the percentage. Keep it at or below your limit.
How many 2.5 mm² cables fit in 20 mm conduit?
A 20 mm MD conduit is ~16.5 mm inside (~214 mm²). At 40% that's ~86 mm² usable, and a 2.5 mm² single is ~10.2 mm² — so about 8, fewer once you count the earth and easy drawing.
Planning a bigger job?
Sizing the cables themselves? Use the cable size & voltage drop calculator. Need it installed? We match Adelaide electricians who quote before they start.