# Grouping Factors

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.

Parents
• This comes up quite a lot, most recently here:  Cables in ducts - Derating factor for cable grouping

Overall, whilst it might be reasonable to go down the track you have, it's not covered by BS 7671 tables. I would guess it will be over-engineered and you would be recommended to adopt another approach following relevant parts of BS IEC 60287 series, or ERA guidance.

2x4C conductors

Not sure what that means? Do you mean 2 no. 4C cables, or 2 x 4 x 1C (= 8 x 1C) cables ?

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)?

That depends on whether the line conductors are fully loaded at the same time as the neutral. 3-phase current-carrying-capacity (3 or 4 single-core cables, or a 3 or 4-core cable) covers you for all line conductors fully loaded, and at the same time neutral up to 30 % .,. or say two line conductors and a neutral fully loaded.

• Hi,

I was referring to multicore cables, specifically 2no. 4-core cables.

at one point, I was asking about if I was to go with single core conductors.

• Hi,

would you be able to recommend which part of BS IEC 60287 can be used to determine a more precise appliable grouping factor?

thanks