Table 4C1 and the ring final circuit

Question

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

mapj1 · Accepted Answer

This question leads to the best practice of not putting the two legs of the ring down the wall to the socket under common capping or trunking, rather two pieces of trunking spaced slightly apart ;-) . 
 The way the regs are worded, one may argue that the two limbs of a ring or whatever (it could just as easily be a radial with a looped-back section) are one circuit, and grouping does not apply. However, while meeting the letter of the rule as written, this is not sensible, as one can easily consider a case where both cables are carrying significant current and the heat from one cable also elevates the temperature of the other. The reason we have the grouping factors is not so that we can argue what constitutes a separate circuit, but rather that cables in the middle of bundles where all cables are loaded, do not overheat. 
 Now, having said that there is so much slack in the average domestic ring, in the sense that the loading over more than a few minutes is a lot less than the full 32A, that in fact there are very few cases of cables showing heat damage, and practice it may well not be a real issue. However, it may be unwise to push this with circuits that supply areas of heavy load - thinking kithchens laundries etc. 
 Mike

AJJewsbury · Answer

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.

mapj1 · Answer

not sure that rings are going anywhere soon ;-) Good for IT and high integrity earthing, and of course domestics. Rubbish for long thin shaped buildings of course. Realise that as the designer, you can over-ride the default cable choices, and indeed use different grouping factors, if you know enough about the circuit and it's load use pattern to do so . Rather like the cable ratings for various installation methods that are not the same as the ones in the annex, what suits your specific situation is where a mixture of reasoning and experience comes to the rescue. (' in this case more than any two can't ever be fully loaded as the incomer is only 60A... vs ' I've seen loads like this and it's never an issue.') 
 Mike

Chris Pearson · Answer

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?

Table 4C1 and the ring final circuit

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