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
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  • Table 4C1 applies to Table 4D2A amongst others.

    Note 1: "These factors are applicable to uniform groups of cables, equally loaded." (My emphasis)

    Note 6: Rating factors have been averaged.

    The safest approach is to count the number of cables rather than circuits.

    Now suppose that you have a radial circuit formed from singles in conduit in a workshop. A circuit runs around the room above the doors. Drops, at intervals, go down to sockets. Is that one circuit or two in the vertical portions?

  • Thanks,

    Well its one circuit, but you have double the cables in the drops, so in that section the cables will have an influence on each other, and insulation may get  compromised.  
    How much of  concern that is difficult to assess, if you do not know whats going to be plugged in. Though you may possibly get an idea from its expected usage. But that can change. 

    So where do you go with your factors, Domestic generally has fewer options for containment as people don't want to see any sign of that, or cable.
    So it's in the wall. And to protect it from other trades it is put in capping or oval or such. And then you are starting to derate.
    I do get the general message though, about judgement


  • I do get the general message though, about judgement

    Splendid!

    Back to my singles in conduit. If the end two sockets were in full use, you might have 26 A up and down in the same tube, but in reality, it is my workshop so I can only use one machine at one point at a time. Granted, that may be, for example, a planer and a dust collector (so 2 machines), but the use will be intermittent and not prolonged. So my "domestic" workshop is not the same as a factory.

    I might add that domestic is not the defining characteristic. A joiner with an assistant, and perhaps an apprentice, in a small industrial estate is unlikely to consume much leccy for long.

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  • I do get the general message though, about judgement

    Splendid!

    Back to my singles in conduit. If the end two sockets were in full use, you might have 26 A up and down in the same tube, but in reality, it is my workshop so I can only use one machine at one point at a time. Granted, that may be, for example, a planer and a dust collector (so 2 machines), but the use will be intermittent and not prolonged. So my "domestic" workshop is not the same as a factory.

    I might add that domestic is not the defining characteristic. A joiner with an assistant, and perhaps an apprentice, in a small industrial estate is unlikely to consume much leccy for long.

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