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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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    Chris - are you designing a modular system in general terms (as if you were the manufacturer of the system, which could be then installed anywhere) or checking what's needed for a specific installation?


    If the latter, could you use some expected load characteristics (e.g. W/m²) to put some sensible limits on the total load - and hence exclude the equivalent of some of the circuits from the calculation - perhaps as already suggested, enforced, by some upstream overload protection.


    If the former it seems trickier - but are there any "manufacturer's instructions" that put any limits on how the system might be utilized (e.g. recommended limits on floor area served, total load and so on)?


    I presume the system is something like this - https://www.theiscraft.co.uk/solution/modular-wiring/ (random Google find) - which begs the question is there any mileage in using a larger number of MDBs each with fewer 'ways' to reduce the grouping in each conduit run?


       - Andy.
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  • 0
    Chris - are you designing a modular system in general terms (as if you were the manufacturer of the system, which could be then installed anywhere) or checking what's needed for a specific installation?


    If the latter, could you use some expected load characteristics (e.g. W/m²) to put some sensible limits on the total load - and hence exclude the equivalent of some of the circuits from the calculation - perhaps as already suggested, enforced, by some upstream overload protection.


    If the former it seems trickier - but are there any "manufacturer's instructions" that put any limits on how the system might be utilized (e.g. recommended limits on floor area served, total load and so on)?


    I presume the system is something like this - https://www.theiscraft.co.uk/solution/modular-wiring/ (random Google find) - which begs the question is there any mileage in using a larger number of MDBs each with fewer 'ways' to reduce the grouping in each conduit run?


       - Andy.
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