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Grouping factors on multicore cables

jbrameld
Good afternoon learned colleagues.


Following on from my previous post about the use of 'YY' type cables, our esteemed client has stated that BS7671 does not have grouping factors for multicore cables.  Personally I beg to differ in that table 4C1 states that it is 'rating factors for....a group of circuits or a group of multicore cables....'  What's the hive mind opinion on that statement?


In our case, we have 4 x multicore cables, each carrying 3 circuits each so a total of 12 circuits.  Should we use the figures from the column headed '4' or headed '12'? 12 would give us the worst case, and in fact we are well inside even this figure by our calculations.


Also, the design current for these circuits is only 1.24A, so it is well under the figue of 0.3 x Cg x It at which point under Note 9 arguably all of these circuits could be ignored for grouping purposes anyway.  Do you agree with the vaidity of that argument?


We can park the 'YY' element of the argument for this discussion as the question would equally well apply to the use of a multicore SWA or Harmonised type cable - would you just use the total number of circuits involved from Table 4C1 or is there another method that should be used?


Thanks in advance.


Jason.


Parents
  • 0
    The fact that certain types of cables are mentioned in the regs does not mean you cannot use others, if you know what you are doing. This is now a well trod path for you...

    The same applies to the de-rating. Just because there is not a table for  some particular installation method or arrangement of very many circuits, does not mean it cannot be made to work safely and well, if you have designed it properly.


    The actual requirement  is that at full load it does not reach a copper temperature exceeding 70C in the most heated core (which may not be the one with most current, it may be the one with most hot neighbours.)

    Given it is in actually wiring in place, and no longer a paper design, you could just do that. There are stickers that indicate a 'high tide' temperature that we often fit inside kit we think the customer is abusing - and if we open it up and the sticker has over heated, we know they have. These also have a place as part of inspections to see if sub mains overheat and so on, and are a lot cheaper than hiring logging equipment for months.

    The other approach is to say what is the total current limit - for example a house may have a 60A company fuse, so even if there are 3 30 amp rings, a 40A cooker and shower cable, there is no need to consider them all under load at once when looking at cable grouping in the bundle going to the fuseboard - only combinations that add to less than the company fuse can occur for significant lengths of time.

     

    But here, I think you have a lot of margin - consider that figure of  current is 30% means the heating is 0.3*0.3 or 0.09. of the full load heating, so that 40C rise from 30 to 70 becomes a 4 degree rise from 30 to errr, well about 34 degrees actually. So the assumption that can be ignored stems from that. It is not a safe assumption when you get cylindrical bundles of cables ten cables in radius,  but that is rare.

    Given the total copper resistance and the currents, the dissipation in watts per meter of length can be deduced, and close behind that  and knowing the surface area for cooling is the core temp.

    For the individual cables, look at the datesheets.


    Mike.

Reply
  • 0
    The fact that certain types of cables are mentioned in the regs does not mean you cannot use others, if you know what you are doing. This is now a well trod path for you...

    The same applies to the de-rating. Just because there is not a table for  some particular installation method or arrangement of very many circuits, does not mean it cannot be made to work safely and well, if you have designed it properly.


    The actual requirement  is that at full load it does not reach a copper temperature exceeding 70C in the most heated core (which may not be the one with most current, it may be the one with most hot neighbours.)

    Given it is in actually wiring in place, and no longer a paper design, you could just do that. There are stickers that indicate a 'high tide' temperature that we often fit inside kit we think the customer is abusing - and if we open it up and the sticker has over heated, we know they have. These also have a place as part of inspections to see if sub mains overheat and so on, and are a lot cheaper than hiring logging equipment for months.

    The other approach is to say what is the total current limit - for example a house may have a 60A company fuse, so even if there are 3 30 amp rings, a 40A cooker and shower cable, there is no need to consider them all under load at once when looking at cable grouping in the bundle going to the fuseboard - only combinations that add to less than the company fuse can occur for significant lengths of time.

     

    But here, I think you have a lot of margin - consider that figure of  current is 30% means the heating is 0.3*0.3 or 0.09. of the full load heating, so that 40C rise from 30 to 70 becomes a 4 degree rise from 30 to errr, well about 34 degrees actually. So the assumption that can be ignored stems from that. It is not a safe assumption when you get cylindrical bundles of cables ten cables in radius,  but that is rare.

    Given the total copper resistance and the currents, the dissipation in watts per meter of length can be deduced, and close behind that  and knowing the surface area for cooling is the core temp.

    For the individual cables, look at the datesheets.


    Mike.

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