Double socket on 32A MCB


Postby ericmark » Fri Apr 04, 2008 9:14 pm

I think I am losing logical thinking. Or am I? Asked to put a radial from a consumer unit in 2.5mm² to one double socket protected by 32 amp MCB.
My answer was MCB too big should be 20 amp
Reply:- “Your allowed an unfused spur on a ring main which may be connected to the origin of the circuit in the distribution board on a 32 amp MCB, all you are doing is same thing without the ring main.â€
ericmark

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Postby kuzz » Fri Apr 04, 2008 9:40 pm

I've pondered a similar thing before. Consider 30A cooker circuit 6mm cable. When connecting an oven and a hob to a cooker circuit, hob in 6mm without question but does the 2kw oven really need to be 6mm too? If you use 2.5 how different to spurring off the ring main is it?
I'll look forward to someone going through the regs book on my behalf.
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Postby kuzz » Fri Apr 04, 2008 9:43 pm

I've pondered a similar thing before. Consider 30A cooker circuit 6mm cable. When connecting an oven and a hob to a cooker circuit, hob in 6mm without question but does the 2kw oven really need to be 6mm too? If you use 2.5 how different to spurring off the ring main is it?
I'll look forward to someone going through the regs book on my behalf.
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Postby kuzz » Fri Apr 04, 2008 9:56 pm

And when inside the oven it isn't like the internal wiring is done in 6mm is it? far from it, it's all 1.5mm, not fused down from that original 30A at any point.
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Postby sparx » Fri Apr 04, 2008 10:30 pm

Hi Eric, an oft argued point,
The regs are not absolutes of course but a guide in the same way as the highway code interpretates the motoring law for us mere mortals.
We are all quite at liberty to design any circuit set up we want as long as we are prepared to show by calcs. how they are safe.
To save us having to do constant designs for basic, oft used circuits, the IET have published 3 domestic only standard circuits called A1, A2, & A3 types which we know as a Ring, & 2 types of radial. These circuits have set parameters that are to be encompassed. They have calculated that if all the rules for these are followed to the letter then "no further calcs. are required".
This does not allow us to pick bits from it as we see fit, so by default the arrangement suggested DOES NOT COMPLY!
There is also no need to put the overload device at the origin of the circuit provided it is a short distance from source to protective device, no tees off it and unlikely to sustain mechanical damage, eg a panel meter reading bus-bar volts on a 1000A bar can and often is fed by a piece of 1.5mm2 without fusing at all until a panel mounted fuse holder next to the meter!
Another such example given at an IET Inspect/test course was a FCU fed from a socket on a ring to a spur of 1.5mm2 to a fixed appliance, the short length between socket and FCU can be in 1.5mm2 as long as short & unlikely to be damaged.
I must agree it is difficult to find such stated in the regs & in a quick check I can only find conditional ref. in a note to reg. 536.1 pg. 116
So you are correct and unless using 4mm2 on 32A, then 2.5mm2 should be classed as a type A3 radial, fused at 20A.
N. B. in red book appendix 15 (informative) page 363 there is a 'cut-&-paste' typo in top box, top line says 'A radial final circuit starts AND FINISHES at the dist bd.......
The capitalised words above should be deleted in your books.
Finally the supply to the house is outside the scope of the IET regs and is covered under the 'Electricity, Safety, Quality & Continuity Regulations 2002,
Get person insisting on you deviating from the regs. to sign the non-compliant installation Cert. as I wouldn't and it would fail a later P.I.
regards as ever SPARX
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Postby ericmark » Sat Apr 05, 2008 7:49 am

Thank you, Yes I had noticed the mistake on radial diagram. I think 536.1 would have fitted the bill spot on as the consumer unit was in the garage and fed from 32 amp ring main supply. I had scanned for overcurrent but once I saw your reply I scanned for protective and found 313.1 (vi) I now see does say origin. And 314.2 I think would mean the garage should not be fed from ring main. And 361.1 (ii) also shows discrimination. 433.2.2 Allows the protective device not to be at origin, but limits it to 3 meters. So re-reading it seems yes we can use 2.5mm² to feed a single, double or single socket but no more than 3 meters from ring. Unless we use 6242X and not the normal 6242Y. As to supply regulations although the BS 7671 says it does not cover supply the supply regulations themselves say BS 7671 should be followed?
ericmark


Postby ericmark » Sat Apr 05, 2008 1:04 pm

Hello Kuzz as to supply to an appliance which plugs in, this should be protected within the appliance. This is to harmonize with Europe who have no provision for fuses within the plug. With a fixed appliance the manufactures instructions should be followed, and these will normally tell you what protection is required. Often single ovens are designed to plug in but are of a built in design. Here because they are built in they don’t have to protect within the appliance and can stipulate what size protection device is required, as really they should be connected to fused spur unit.
As to the splitting of a supply 433.2.1 comes into play and where a reduction occurs you must protect for overload. So reading 433.2.2 if the oven is within 3 meters of the cooker switch then wiring in 2.5mm² and terminating in a fused spur unit would be OK. But if more than 3 meters then the fusing would have to be within the 3 meters of oven switch, so if more, you would also need a connection unit or cable outlet. Most consumer units are surface mount and would not look good in a kitchen, but there are some flush mounted shower units which contain 1 RCD and 1 MCB an example it the Proteus PSU95FL which to date is the neatest way I have found to put mini consumer units into a kitchen. I had seen the units in Porta Cabins but took me ages to find what they were called as I was looking in catalogues under consumer units not shower units.
ericmark


Postby 4213paul » Sat Apr 05, 2008 10:48 pm

Fitted lots of these always use 2.5 t+e radial 16amp mcb.
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