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Modular wiring home run cable sizing

Frankie 223
Hello, I am trying to confirm the appropriate cable size for a home run cable in a modular wiring system. There is a 9-port MDB that supplies  radial circuits with socket outlets. As per BS7671, the appropriate grouping factor to be applied is either 0.5 (table 4C1) or 0.45 (table 4B5) assuming all circuits are carrying more than 30% of the current capacity of the home run cable. This ends up in having to use 6mm2 or even 10mm2 cables instead of 4mm2 that would be expected for radial circuits in 20A MCBs.


What are your thoughts?


Any comments would be appreciated.


Having said that and looking at regulation 2.3.3.1 from appendix 4 I have the feeling that based on the BS even when we are supplying lighting and power circuits in trunking, we would have to oversize all circuits to comply with the grouping factors. So how is it justified when we are installing a power and lighting DB with all these outgoing circuits in different sizes and loads in 2.5 (or even 1.5) for lighting and 4 for power?
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    Andy, thank you for your comments. While I do agree in principle with your views about the cable sizes, the point is how to prove compliance with BS 7671 through valid electrical calculations. In general I understand that modular wiring system‘s main advantage is that it can significantly reduce the installation time as everything would be prefabricated and it would be like a ‘plug and play’  system. On the other hand, examples like this could maybe show that there are some disadvantages as well?


    The home run cable (which would house the 9 circuits in singles and thus it would be 27 core) will be around 40m. I believe that voltage drop is not a problem. How to apply a reasonable grouping factor is and I feel the way BS 7671 is written, I end up with having to oversize the cabling.
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    Andy, thank you for your comments. While I do agree in principle with your views about the cable sizes, the point is how to prove compliance with BS 7671 through valid electrical calculations. In general I understand that modular wiring system‘s main advantage is that it can significantly reduce the installation time as everything would be prefabricated and it would be like a ‘plug and play’  system. On the other hand, examples like this could maybe show that there are some disadvantages as well?


    The home run cable (which would house the 9 circuits in singles and thus it would be 27 core) will be around 40m. I believe that voltage drop is not a problem. How to apply a reasonable grouping factor is and I feel the way BS 7671 is written, I end up with having to oversize the cabling.
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