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

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 for the reply

mapj1 said:

However, while meeting the letter of the rule as written,

Thats all I am trying to understand, what is the rule. What Is written, it's quite difficult to decipher what they mean, and says two things in my opinion.

mapj1 said:

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.

Yes I don't want my cables to get hot, so I am concerned about understanding this table. 
I am a worst case  worrier, I guess.

Two radials would be two circuits.
Not really sure whats the difference (Its seems there isn't) between that and a ring final. You have two loaded legs, with some  (indirectly ??) proportional relationship.
The leg with the greater resistance, will carry less current, but its still a factor.

Rings are on  their way out i realise however.

You see all the time 50 x 50 trunking and often smaller, containing the majority of a domestics cables up to the board. It sounds like you are concerned about two legs of the same  circuit in capping, so that  makes me wonder about larger groupings of these circuits squeezed into trunking. 
.
And when this is discussed on forums, its alway, its only domestic, not fully loaded .....And that is very true. But the only domestic response, seems to be the end of the discussion.

I guess the regs can't really say in print, don't worry about it for domestic, this is more the case for environments that have heavy and constant loads.
So its just left hanging it seems.    Two ring finals and you get a derating of possibly 0.64. which would push up to 4mm, and for  main lightly loaded circuits. And also large areas taken up with lightly populated containment..



I have asked my CPS, and really searched and could not get an answer, to what appears to be a simple question .. is a ring, two multi core or one circuit.

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

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

depends for what purpose - in terms of the no of MCBs needed, a ring is one circuit. In terms of cable grouping, its just not really.

M.

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

JohnE said:

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.