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Grouping factors on multicore cables

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.

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0 David Stone over 4 years ago

Thanks Mike, you saved me some typing! Another way you could look at this Jason is as follows:

As the design current is low, put all the current in one of the cables, 4*0.62A (I may have misunderstood the exact current, but if a bit higher the method is fine. This is 6 circuits of 2.48 amps in one cable. I am assuming the cables for this example are 1.5mm² cores. Now to table 4C1. 6 circuits have a group factor of 0.57, and the cable rating is probably about 10A (I have no manufacturers data), so 5.7A per circuit is safe, in fact, you have a good margin if someone plugs in a multiple lead of some kind. The manufacturer will give a circuit rating with all cores loaded too I expect, I have been very pessimistic. Now you see that even if one cable carries all the current it is adequately rated, and as spread across 3 cables will be even safer, as the dissipation ability and surface area will be greater.

Feel free to ring

Regards

David
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0 David Stone over 4 years ago

Thanks Mike, you saved me some typing! Another way you could look at this Jason is as follows:

As the design current is low, put all the current in one of the cables, 4*0.62A (I may have misunderstood the exact current, but if a bit higher the method is fine. This is 6 circuits of 2.48 amps in one cable. I am assuming the cables for this example are 1.5mm² cores. Now to table 4C1. 6 circuits have a group factor of 0.57, and the cable rating is probably about 10A (I have no manufacturers data), so 5.7A per circuit is safe, in fact, you have a good margin if someone plugs in a multiple lead of some kind. The manufacturer will give a circuit rating with all cores loaded too I expect, I have been very pessimistic. Now you see that even if one cable carries all the current it is adequately rated, and as spread across 3 cables will be even safer, as the dissipation ability and surface area will be greater.

Feel free to ring

Regards

David
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Grouping factors on multicore cables

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