Table 4C1 and the ring final circuit

Hello

My question is regarding table 4C1 in BS7671
Grouping factors.
A ring final circuit is this a circuit or two multi core cables?

So a ring final has
Two multicore cable (legs) which will be 2.5mm2 T&E cable.
And..
It is one circuit, usually 32A MCB, what is the arrangement

So table 4C1
If we used item 1 bunched in air, on a surface, etc.. would the ring final be classed as arrangement 1 (Factor 1.00)  one circuit
Or arrangement 2  (2 multicore cables factor  0.80)
If a ring final is two multi core cables and not one circuit, that would follow that 2 ring final circuits (4 multicore) would have a 
factor of 0.65 which is quite some derating.
Many thanks
John
Parents
  • Rings are an odd one. The tables are really intended for cables that can carry their full load - but with a ring you've got two 20A(+) cables that can only carry 32A between them (e.g. one at 20A and the other at 12A or both at 16A, or any other combination you choose), so from the physics point of view, it's really just one-and-a-bit circuits.

    That said, in domestics, most circuits almost never carry anything like their full rated current and even when one or two circuits occasionally do, the rest almost certainly won't (with a few notable exceptions, like a house full of night storage heaters) - so often grouping factors are downplayed to almost the point of being ignored in domestics (even DNOs reckon on each house drawing no more than 10A on average). I think one version of the OSG effectively said don't worry about grouping for up to 6 normal domestic circuits bunched together (high power, long duration, loads like storage heaters and immersions excepted). EVs and heatpumps might shift the goalposts a bit - but still the majority of domestic circuits carry negligible loads most of the time.

    Things can be quite different in some commercial or industrial situations where several rings might all be carrying close to full load for much of each day.

    The multi-core cable vs circuit debate is an old one - originally it was circuits so you could use the same tables whether you had singles in conduit or multi-core cables, but neither seem that clear where you can have conductors in parallel, including multi-core cables in parallel, or situations where conductors go down and back up the same path (e.g. looping into switches or sockets). Really you've looking for how many times as much heat will be created by the cables in your particular situation, compared with the reference installation method - so some judgement is needed. (Remember appendix 4 is only informative, you don't have to follow it to comply with BS 7671).

       - Andy.

  • Thank you also , you reply has covered a lot of my questions. 

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