Someone orders the conduit for a tenancy submain before the cable goes in. Often that someone works for another trade. A trench crew lays what the drawing says. This page works one common case and shows the margin the figure leaves.
The inputs
The submain runs four 35 mm² single core cables. The conduit is 50 mm. That is the size most draft drawings carry for a submain of this rating. It is worth a check before the trench closes.
Single core is the case that matters here. The core type sets the overall diameter the tool reads. The four singles give a cable area of 277.59 mm². One 35 mm² multicore cable gives 475.29 mm² on its own. Enter the core type that will go in the duct.
The clause
Paragraph C6.2 of Appendix C in AS/NZS 3000:2018 sets the space factor method. Tables C10 to C12 of AS/NZS 3000:2018 give a maximum cable count instead. That is a different route to the same decision. The engine uses the fill percentage method and states that choice on every run.
The steps
The engine takes the internal area of the conduit. It sums the overall area of the cables. It then divides one by the other.
- Conduit internal area: 1,425.31 mm²
- Cable area: 277.59 mm²
- Conduit fill: 19.48 %
- Permitted fill: 40 %
- Recommended conduit: 40 mm
The result
The four cables fill 19.48 % of the 50 mm conduit. The permitted figure is 40 %. The run passes with a wide margin.
The recommended conduit is 40 mm. The engine returns the smallest conduit that keeps the fill inside the limit for this cable set. On a long trench that one step down is a real line item. The drawing is often the only thing standing in its way.
Read the margin before the order changes. Fill percentage is a geometric check, not a pulling check. It counts area, and it ignores friction, cable stiffness and pulling tension. A long run with bends needs more room than the space factor alone suggests. So does a run that has to take a fifth cable later. Two more cables of the same size still pass in 50 mm. They do not pass in 40 mm.
The cable area figure depends on the overall diameter. That diameter varies with the insulation and the sheath. The engine uses a representative overall diameter for a 35 mm² single. Check the cable data sheet where the cable is already chosen, because a heavier sheath moves the fill.
Bends matter for a second reason. A duct with several sets of bends needs a draw pit. The pit spacing belongs on the same drawing as the conduit size. Settle both before the civil work starts.
Run the same conduit with the cable count the drawing shows. Include any earth or control cable in the same duct. Then check the current carrying capacity separately. Cables in a buried conduit derate for soil and depth, and that check sizes the conductor, not the duct.
This page is a design aid. Verify every value against the current edition of the standard.
See also: Conduit fill: 4 x 6 mm² singles in 25 mm, AS/NZS 3000 Appendix C: conduit space factor and Conduit fill and installation methods.