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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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    I think that I can see the point of this: 3 circuits on each phase and the modular construction is rather like the wiring loom of a modern motor car.


    Given that the total supply is 125 A (presumably with an OCPD somewhere) that is an average of 14 A per circuit. If some of them are rather lower because they are lighting circuits, they may be ignored - see Note 9 to Table 4C1.


    So if that leaves 6 circuits in 4 mm² singles, you have a CCC of 32 A (RM B) x 0.57 = 18 A each. Allowing for diversity, perhaps the MCBs should be downgraded to 20 A?


    Then what is realistically the current consumption of each circuit? A few PCs on desks? Surely not electric space heating!
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  • 0
    I think that I can see the point of this: 3 circuits on each phase and the modular construction is rather like the wiring loom of a modern motor car.


    Given that the total supply is 125 A (presumably with an OCPD somewhere) that is an average of 14 A per circuit. If some of them are rather lower because they are lighting circuits, they may be ignored - see Note 9 to Table 4C1.


    So if that leaves 6 circuits in 4 mm² singles, you have a CCC of 32 A (RM B) x 0.57 = 18 A each. Allowing for diversity, perhaps the MCBs should be downgraded to 20 A?


    Then what is realistically the current consumption of each circuit? A few PCs on desks? Surely not electric space heating!
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