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?