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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?
  • 0
    I don't quite understand the context but you mention a home.

    Take a look around a similar home and see what is running. My home ticks over and about 400W so 1 1/2 Amps total. Yes the microwave will draw 8A for a few minutes and the kettle 12A for a few more but nothing to worry group rating factors.

    Experience is your best guide.
  • 0
    but you mention a home

    I presumed by "home run" the OP was using the American term for the wiring from the DB to the first accessory on the circuit (in this case the Master Distribution Box).


       - Andy.
  • 0
    You are probably right, Andy, but if it is not a home then the context is everything.

    In some contexts each socket could be fully loaded up to the maximum the incoming supply so little diversity can be assumed and 6mm² cable for 20 A may be needed.
  • 0
    Chris 223:

    Chris, your description is more or less the scenario. Only difference is that there are separate MDBs for power and lighting and as such, the grouping factor would be 0.5 and end up with the rating of the home run to be 16A. Is it ok to use the estimated design current for sockets?


    I can not argue with those calculations. What you haven't mentioned (and may be a commercial confidence) is what will be plugged into the sockets. By their very nature, sockets imply diversity - they do not have the same 13 A load plugged in 24/7! As David has mentioned, the grouping factor is based upon fully loaded circuits. I think that diversity will allow you to get away with 20 A OCPDs or even 32 A ones.


    The reference is 311.1.


  • 0
    You really do need to know who will do what with your sockets.

    A number of years ago I was involved with a certain Opera House. At the end of the Commissioning of the systems we needed to test the air-conditioning at full load, i.e. simulate the heat given off by 2000+ seated people.

    We did this by having a large number of 3kW fan heaters as well as some strings of 100W lamps distributed around the auditorium.

    I suspect the diversity calculations didn't allow for all available ring mains to be fully loaded for several hours.
  • 0
    Harry Macdonald:

    You really do need to know who will do what with your sockets.

    A number of years ago I was involved with a certain Opera House. At the end of the Commissioning of the systems we needed to test the air-conditioning at full load, i.e. simulate the heat given off by 2000+ seated people.

    We did this by having a large number of 3kW fan heaters as well as some strings of 100W lamps distributed around the auditorium.

    I suspect the diversity calculations didn't allow for all available ring mains to be fully loaded for several hours.


    At the back of my mind is a figure of 100 W for a person, so that would be 200 kW. Wow!


  • 0
    My Dad can remember the balcony being load tested in the newly built cinema by putting a sack of sand under every seat.
  • 0
    No multiplexes in those days! I recall the cinemas in Hull being used for trade union meetings - they could hold 2000 people.
  • 0
    With gas emergency lights for many years past when the main lighting was electrified.

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