It's a bit unfair having a go at the electrician when it was my decision - it's fairly obvious that I don't know much about electrics, but just out of interest, what are the advantages of radial circuits? Other than saving the hassle of fitting the return cable, which isn't a problem in a renovation/new build like ours.
Alternative to fused switched sockets above kitchen worktops
- Thread starter fuzzytrifle
- Start date
It's a bit unfair having a go at the electrician when it was my decision - it's fairly obvious that I don't know much about electrics, but just out of interest, what are the advantages of radial circuits? Other than saving the hassle of fitting the return cable, which isn't a problem in a renovation/new build like ours.
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It's more the other way round.
Depending on the layout there may be no advantage having a ring circuit.
If the accessories are situated on a circular route then a ring can use smaller conductors for a given loading.
Indeed.
....and even then, the 'advantage' of a ring is not necessarily as straightforward as it sounds. If one simply 'cut' the ring at its mid-point and ran each of the resultant radials from a 20A MCB (assuming 2.5mm² cables), then one would get a bit more potential total loading (40A, rather than 32A) from exactly the same lengths (actually fractionally less) of the same size cables. The 'catch', of course, is that one would then theoretically have 20A available in each of those circuits, which offers slightly less flexibility. Having said that, the ring only really offers an advantage in that situation if large loads (approaching, in total, the maximum capacity of the circuit) are all put on the same 'side' of the ring - which is something one is not really meant to do, anyway!
It's probably even more difficult to think of any major advantages of a 2.5mm² ring over a 4mm² radial, other than the somewhat easier installation of 2.5mm² cable. Indeed, with most layouts, it would even use a bit less copper.
I personally still nearly always use ring finals, but probably only because of historical 'habit'; I couldn't produce much of an argument in their favour (although Bernard's point about making it more likely that a functional CPC will exist at most/all sockets, even if something becomes disconnected or broken, is valid).
Kind Regards, John
It could be 25A without being theoretical.
Two 4mm² radials at 32A would be enough for most properties.
I don't think 4mm² is any less 'easy'.
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It could, indeed, if the cable were 'clipped direct' etc. - but I'm not sure I've ever seen a 2.5mm² radial on a 25A MCB (if you could find one - does anyone include them in their standard ranges?). Is that just my lack of exposure/experience? My use of the word "theoretical" was a reference to the fact that, in practice, people could overload a circuit on a 20A breaker, up to 30A or more for short periods.
As I said, I would find it difficult to argue in favour or 32A rings over 32A radials.
Nor do I, but I think some people do! I'm not sure that all sockets (or other accessories) are rated for 3x4mm² conductors, are they? - which could be a problem if one wanted 'branches'.
Kind Regards, John
Maybe not all but I think it's more of a stockist problem.
Is that thinking not applicable to all socket circuits?
That's not a valid arguement.
Does that mean it's not allowed?
After all, two 2.5mm² doubled is 10mm², three 4mm² is only 12mm² and stranded.
It is, indeed - which is why socket circuits are not really precisely designable in the same way as are fixed-load circuits. The designer has to apply his/her crystal ball in order to predict/guess what the likely loading will be. However, it remains true that, 'theoretically', the load should not exceed the design load (which, of course, in turn must not exceed the In of the CPD). In practice, however, the design may poorly predict the load, or the user may exceed the load that the designer reasonably predicted - so that much higher loads (than 'should' arise) may arise, and be 'tolerated' (by CPD). A B20 will allow 40A to flow for about 15 minutes - that's plenty of time for someone to simultaneously boil a kettle and make some toast (and goodness knows what else) whilst the dryer is running.
You tell me! The common interpretation of the regs seems to be that the word of MIs always has to be religiously complied with 'to the letter'!
Eh? That's not really a valid comparison, is it?! Why "two 2.5mm² doubled"? The terminals of a socket on a ring which gave rise to a spur would each have to accommodate 3x2.5mm² conductors, i.e. 7.5mm² total - a fair bit less than 3x4mm² (radial with a branch), namely 12mm² total. Am I missing something?
Kind Regards, John.
Yes.
And there are twice as many failure scenarios of an identical nature which leave the circuit inadequately protected.
Are you assuming that the single CPC on one side of an interuption would not have a low enough resistance to afford adequate protection? I would have thought that it very commonly would be adequate.
The trouble with CPC faults is, of course, that they are usually 'silent'.
Kind Regards, John
Surely saying, for example, '3 x 2.5mm²' is only information on the size.
What difference can it make?
When I have two 2.5mm² conductors to fit in a socket terminal I double the ends over - that is 10mm² of solid cores in the terminal.
There isn't room for three doubled so with spurs they are connected straight.
So, three 4mm² is 12mm² not much more than 10mm² with the easier task of fitting twenty-one strands.
No.
I'm assuming that one would not normally consider a 2.5mm² conductor to be adequately protected by a 32A breaker.
Again, you tell me! Yes, I agree that they should be talking simply about the capacity of the terminal to acommodate a certain total CSA of conductors. There are, of course, some subtleties here since, for example (because of the geometry) four 2.5mm² solid conductors would probably require a bigger 'hole' than would a single 10mm conductor. However, that subtlety aside, consider the MI for MK Logic Plus sockets, which say:
In terms just of total CSA, that obvioulsy makes no sense. With 4mm² or 6mm² conductors, you can have a total CSA of 12mm² (which would correspond to over 4, nearly 5, conductors of 2.5mm²). However, the MI only allows 3x2.5mm² conductors. Is that perhaps because 4mm² and 6mm² are stranded, thereby probably reducing the 'hole size' they require for a given total CSA? In passing, it sounds as if your 'doubling' (per below) of 2.5mm² conductors may well be in violation of the MIs (hence arguably non-compliant with BS7671!), certainly if you were using a Logic Plus socket!
Ah, right. As above, that may be non-compliant
Kind Regards, John
Oh, right. As we know only too well that is the fundamental 'Ring Finals Discussion'!
I'm not taking sides here since, as I've said, I find it hard to put up a particularly rational argument in favour of ring finals - but the reality, of course, is that one does have to have pretty dramatically asymettrical (and very high) loading of a ring final for the cable to become 'inadequately protected'. I realise that one has to be able to justify the design right down to a CCC of 20A (and right up to a load of 32A), but if (as I imagine is common) the wiring is 'clipped direct', then one would be pretty hard-pressed to load the circuit so asymettrically, and so highly, that the the current in one arm exceeded the cable's CCC.
Kind Regards, John
Quite.
So, if there is capacity for 3 x 4 or 2 x 6 then there is undoubtedly capacity for (almost) 5 x 2.5.
ca·pac·i·ty
1.
the ability to receive or contain: This hotel has a large capacity.
2.
the maximum amount or number that can be received or contained; cubic contents; volume: The inn is filled to capacity. The gasoline tank has a capacity of 20 gallons.
Then the MI should say something other than 'capacity'?
I don't think I shall loose too much sleep over this query.
If a live conductor drops out (which is twice as likely as a cpc), then you no longer have a ring.
Nothing to do with asymmetrical loading.
Everything to do with a 2.5mm² "radial" (for L or N) on a 32A breaker.
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