Appendix C of AS/NZS 3000:2018 covers the space a cable bundle may take inside a wiring enclosure. Two routes sit in that Appendix, and they answer the same question in different units.
The question
A run needs four circuits in one conduit, each a 6 mm² multicore cable. The drawing shows a 50 mm conduit. Does it hold them, and which number decides that?
The inputs
- Conduit: 50 mm nominal
- Cables: four 6 mm² multicore cables
- Method: fill percentage
The conduit fill tool takes those two rows and nothing else. It reads the nominal conduit size for the internal area. It reads the conductor size and the core count for each cable diameter.
The clause and the tables
Para C6.2 of AS/NZS 3000:2018 sets the space factor. The space factor is the share of the internal cross section that the cables may occupy. The share changes with the cable count. The tool applies 50 % for one cable, 33 % for two and 40 % for three or more.
Tables C10 to C12 of the same Appendix answer the question a second way. They give a maximum cable count for a conduit size and a conductor size. That route needs no area arithmetic. It also covers fewer cable types than a fill calculation does.
The conduit fill tool uses the Para C6.2 fill percentage route. It reports the Tables C10 to C12 route as a note, so a design can be checked both ways.
The steps
The tool works in three moves.
First it converts the nominal conduit size into an internal area. A 50 mm conduit gives 1,425.31 mm².
Second it adds the overall area of every cable in the bundle. Four 6 mm² multicore cables give 754.77 mm². Overall diameter drives this figure, not conductor area, so insulation and sheath both count.
Third it divides one by the other. 754.77 mm² over 1,425.31 mm² gives 52.95 %.
The result
The bundle fails. Fill reaches 52.95 %, and a bundle of four may take 40 %. The tool reports the failure against Para C6.2 and names the next conduit that works: 63 mm. In a 63 mm conduit the same four cables give 32.35 %.
The four cables would go into the 50 mm bore. They still take more than the permitted share of it. That gap between "goes in" and "passes" is the point of the example.
Four 6 mm² cables look modest on a schedule. Area grows with the square of the overall diameter, so the bundle is larger than it looks. In a 25 mm conduit the same four cables give 250.16 %. Their area is more than twice the bore.
Two habits follow from that arithmetic. Size the conduit from the overall diameters, not from the conductor sizes. Then check the count route in Tables C10 to C12 as well. A bundle that passes on area can still exceed the tabulated count.
The core arrangement matters as much as the conductor size. A single core cable and a multicore cable of the same conductor size have different overall diameters. Changing the core count in the tool therefore moves the fill percent. The conductor size on the schedule does not change with it. Four single core cables and one four core cable are different bundles, and the tool treats them so.
The permitted share depends on the cable count as well, not on the conduit size. Two of these cables in a 40 mm conduit give 43.07 %. That fails the 33 % share for two cables. It would fail the 40 % share for three or more as well. Adding a circuit can therefore move the limit and the cable area in the same calculation.
Overall diameters vary between manufacturers. The tool derives them from typical data, so a borderline result deserves a check against the cable maker's own figures.
This page is a design aid. Verify every value against the current edition of the standard.