Hi all, I’ve noticed quite a bit of difference in opinion when it comes to grouping factors and I’m hoping this community can confirm the correct procedure and considerations. This is for academic purposes.

Say I have 6 TP-N final circuits that are fed from the one distribution board. Each final circuit requires 2x4C conductors. The installation method is Method D No. 70 – In underground ducts. 6 ducts are installed side by side with a duct-to-duct separation distance of 150mm, each duct contains 2x4C conductors and a CPC.

How would I go about correctly applying the grouping factors required? From my point of view, Table 4C3 only considers single multicore cables in underground ducts. But using Table 4C1, I can correctly account for the number of circuits per duct.

So, my understanding is you’d get the factor for 2 circuits per duct (since it’s two multicore cables) from Table 4C1 and then you would multiply this with the factor for 6 ducts with a separation distance of Nil from Table 4C3. This would be 0.80 * 0.60 = 0.48

Additionally, as a sidebar, say I used single core cables, I would say the number of circuits per duct would be 6/3 (2) as per note 5 of table 4C1. If the neutral conductor is carrying more than 30% of the grouped rating and I only used the one neutral, would the number of grouped circuits per duct be 7/3 (rounded up to 3)?

I’m conscious of the fact that this is probably erring on the side of caution, but I’ve been told on several occasions that it's only one circuit per duct, 2xD applies and that my grouping factor is ridiculous.