Lollipop final circuit

[ban-all-sheds]

BAS Towers may need to have it's CU moved, with circuits extended using DIN rail terminal blocks.

Now - I know I said this: //www.diynot.com/forums/viewtopic.php?p=165038#165038

but I'm sitting here thinking yes, it's unconventional, and yes it makes testing more awkward, but is there really any great merit in extending ring final line conductors with 2 terminals and 2 x 2.5mm² cables into the CU rather than putting both legs into one terminal and running a single 4mm² to the breaker?

 

[ericmark]

I had quite a heated argument with my son over this. 434.2.1 is the regulation he quoted and I said putting two 2.5mm cables into one 6mm cable was not really a reduction in the current-carrying capacity of the conductors so it was permitted.

We had to agree to differ and since it was his job two times 2.5mm was used.

 

[PrenticeBoyofDerry]

I would like to know what went down on the cert with regards to the circuit and conductor size?

 

[ricicle]

BS7671 is only a minimum standard. If you can state that you believe the set - up is as safe and can still be tested then where's the problem.
Personally I have no problem with it. I have some old Federal DBs at work where there is no RCD protection on the socket circuits. I have bolted an enclosure next to them and managed to get the 2.5s through to the RCDs. I have then fed the RCDs via 6mm 6491Xs from the 30A MCBs.
So that set up is safer than before because an RCD is now incorporated.

 

[JohnW2]

BAS Towers may need to have it's CU moved, with circuits extended using DIN rail terminal blocks.
Now - I know I said this: //www.diynot.com/forums/viewtopic.php?p=165038#165038
but I'm sitting here thinking yes, it's unconventional, and yes it makes testing more awkward, but is there really any great merit in extending ring final line conductors with 2 terminals and 2 x 2.5mm² cables into the CU rather than putting both legs into one terminal and running a single 4mm² to the breaker?

I think that you (with your old thinking!) and I also crossed swords over this one in the past, but I agree with you (new thinking!) and Eric that there seems to be nothing intrinsically wrong with doing as you suggest. There would obviously be less scope for argument if, as Eric suggests, you used 6mm², rather than 4mm², cable - since (although one can stage some sort of argument by analogy with a 32A radial final) there is otherwise obviously the scopr for people to start fussing about the 'CSA reduction' from 2x2.5mm² to 4mm².

In fact, I inherited a situation like this in my house, and happily lived with it for several years. A 6mm² (well, imperial equivalent) cable which appeared to have originally been supplying a second cooker circuit in the (large) kitchen had been diverted into the (large) cellar and used as the start/finish points for a 2.5mm² (imperial equivalent) ring final circuit. As I say, I happily lived with that for a good few years, until I eventually treated the cellar to a sub-main and its own CU.

As you know, I do my best to play at Devil's Advocate, but I find it hard to come up with any 'sensible' objections that anyone could raise, particularly if you used 6mm² cable. You should, of course, label your CU appropriately, so that anyone coming across it knows what is going on, and knows where to find the ends of the ring for testing etc.

Kind Regards, John.

 

[ban-all-sheds]

Hmmm.

IMO that reg can't apply if the CPD is already rated for the circuit beyond where the conductor size changes.

Reason being that 434.2 talks about ... the point where a reduction in cross-sectional area or other change causes a reduction in the current carrying capacity of the conductors ...

So if you'd installed a 433.1.5 compliant 32A ring final using 6mm² cable because in places it runs through insulation, how would you comply with 434.2 if you thought it applied? How would you install a lower rating breaker part way around a ring? What rating breaker would you use? If the circuit ran in a ceiling void, and then dropped down inside insulated walls, meaning that you had 2 x 2m runs of Ref Method 103 for each of several sockets, how many such breakers would you install, given that there would be dozens of places where an other change causes a reduction in the current carrying capacity of the conductors ?

 

[JohnW2]

....I do my best to play at Devil's Advocate, but I find it hard to come up with any 'sensible' objections that anyone could raise, particularly if you used 6mm² cable.

The aforementioned Devil's Advocate has come up with one little thing that nit-pickers might think of. Even with 6mm² cable (CPC CSA = 2.5mm, I think), you'd have slightly less total CPC CSA (2.5mm² vs. 2x1.5mm²) between CU and ring. If you wanted to pre-empt even that, I suppose you could run an additional modest G/Y alongside the cable.

Kind Regards, John.

 

[ban-all-sheds]

Well now there's a thing.

I'm also minded not to extend each N and E conductor, but instead to have N & E bars in the enclosure with the DIN rail terminals and just run longer neutrals from the CU incomer/RCD and a longer earth conductor from the MET...

 

[EFLImpudence]

start fussing about the 'CSA reduction' from 2x2.5mm² to 4mm².

It wouldn't be a reduction - you're going the wrong way.

 

[OwainDIYer]

Well now there's a thing.

I'm also minded not to extend each N and E conductor, but instead to have N & E bars in the enclosure with the DIN rail terminals and just run longer neutrals from the CU incomer/RCD and a longer earth conductor from the MET...

But then you decide you want one circuit on an RCBO ...

More to the point if you common the Ns then they are not separate circuits are they?

and you probably have oodles of spare 2.5mm etc to hand, but not the larger sizes?

 

[JohnW2]

start fussing about the 'CSA reduction' from 2x2.5mm² to 4mm².

It wouldn't be a reduction - you're going the wrong way.

Whoops - well spotted That being the case, 4mm² cable would be fine, so long as it was effectively 'Method C' - provided (in some people's minds) that he wasn't relying only on its CPC (1.5mm² vs.2x1.5mm²).

Kind Regards, John.

 

[EFLImpudence]

Well now there's a thing.
I'm also minded not to extend each N and E conductor, but instead to have N & E bars in the enclosure with the DIN rail terminals and just run longer neutrals from the CU incomer/RCD and a longer earth conductor from the MET...

But then you decide you want one circuit on an RCBO ...

It's only a Wylex with four 3036s in shed with a light, kettle and pc.

 

[securespark]

I'm not sure I like this idea.

From csa to the 6 milli joining up with the 2.5's, it's not a standard circuit arrangement.

Secure not happy!

 

[JohnW2]

I'm not sure I like this idea. From csa to the 6 milli joining up with the 2.5's, it's not a standard circuit arrangement. Secure not happy!

That's surely the point of this thread, isn't it? If it were a 'standard circuit', there would be no need for a discussion. There is no compulsion to use 'standard circuits', provided one can demonstrate that one's design is no less safe/satisfactory than a 'standard circuit' - and I don't think anyone has yet come up with anything that challenges such a view (apart, possibly, from the CPC CSA issue - which is easily sorted).

Is Secure not happy with anything which is not a 'standard circuit' per the BRB/BGB?

Kind Regards, John.

 

[Lectrician]

Preety sure there was article in the NIC connections magazine on this, and they outcome was it is fine.