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power supply to shed in garden

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dave321

from United Kingdom

Joined: 27 Aug 2008
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Location: United Kingdom

PostPosted: Wed Aug 27, 2008 5:28 pm Reply with quote

my son wants to put an electric supply to his shed in the garden.
this is what we propose using a spur

1)from a ring double socket in the house, using 2.5sqmm ring mains cable to a waterproof junction box on external wall

2) using a 30amp joint block connect to 2.5sqmm armoured 3 core cable and run to shed < 10m from house

3)armoured cable to waterproof junction box on exterior of shed connect using 30amp joint block to normal 2.5sqmm ring mains cable into shed

4) connect to 2way rcb "garage" consumer unit

5) connect 16amp mcb to one double socket and 6 amp mcb to one 3ft fluorescent light

Questions :-
1)is 2.5sqmm armoured cable ok ? or does it need to be thicker ?
2)can the armoured cable go straight into the shed and the consumer unit
(and do away with the waterproof junction box and normal ring mains cable to conect to consumer unit) ?

how does this sound ? how much to expect for checking and issuing relevant certification ?

appreciate any help and advice, do not want to compromise safety

thanks...........dave
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ban-all-sheds

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PostPosted: Wed Aug 27, 2008 5:56 pm Reply with quote

dave321 wrote:
how does this sound ?

Terrible.

You can't supply a CU from a ring final, you can't have more than 1 socket on an unfused spur, and increasing the size of the cables will not change any of that.


Quote:
how much to expect for checking and issuing relevant certification ?

And certification doesn't work like that - read the Wiki on Part P.
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ericmark

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PostPosted: Wed Aug 27, 2008 6:08 pm Reply with quote

It needs to be thicker not because of current used but to exceed the 16mm² earth requirement as 6mm² is required.
Yes it can go direct, as BAS says supplying from ring main really not on.
Paper work at £75 hire of test equipment and £75 to LABC is it worth it? I can direct to IET web site and what is required and Part P web site by why as either you will take not a blind bit of notice or you will realise it’s cheaper to get a registered electrician to do the job.
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electronicsuk

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PostPosted: Wed Aug 27, 2008 6:09 pm Reply with quote

ban-all-sheds wrote:
You can't supply a CU from a ring final


Perhaps what Ban should have said is that you can't supply a CU from a ring final without fusing it down via an FCU. It isn't a particularly elegant solution and doesn't leave a lot of room for future proofing, but if you only intend on a small load (perhaps a light and a socket) both now and in the future then it should do the job.

You'd need to be sure that you wont overload the ring final with the addition of the extra load of the shed, and make sure all necessary RCD protection is in place, the earthing arrangements are correct, and more besides. The shed question has been asked so many times before that I wont waste time going into specifics, try a search for shed electrics or outbuilding electrics.
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electronicsuk

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PostPosted: Wed Aug 27, 2008 6:10 pm Reply with quote

ericmark wrote:
It needs to be thicker not because of current used but to exceed the 16mm² earth requirement as 6mm² is required.


How do you know this is a requirement from the information provided by the OP?
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ban-all-sheds

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PostPosted: Wed Aug 27, 2008 6:18 pm Reply with quote

And which?

16mm² or 6mm²?
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ban-all-sheds

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PostPosted: Wed Aug 27, 2008 6:20 pm Reply with quote

electronicsuk wrote:
Perhaps what Ban should have said is that you can't supply a CU from a ring final without fusing it down via an FCU.

But then what's the point of the CU?
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electronicsuk

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PostPosted: Wed Aug 27, 2008 6:21 pm Reply with quote

ban-all-sheds wrote:
electronicsuk wrote:
Perhaps what Ban should have said is that you can't supply a CU from a ring final without fusing it down via an FCU.

But then what's the point of the CU?


Very little. Doesn't mean you can't do it, though icon_wink.gif
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ericmark

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PostPosted: Wed Aug 27, 2008 6:34 pm Reply with quote

OK good point maybe he can get away with it but also maybe he only intends to use two 6 amp RCD's in consumer unit. So we both jumped to conclusions and just as you say needs to return to CU and I agree where unsure of earthing arrangements it needs to be equivalent to 16mm copper earth which in turn means 6mm SWA. OK if no external metal parts and never going to plug lawn mower into shed supply maybe not required. Easy way is ban all sheds of course!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
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GaryMo

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PostPosted: Wed Aug 27, 2008 6:44 pm Reply with quote

Eric, why does the CPC to the shed need to be 16mm?

I see it needing to be 10mm at the most assuming extraneous-conductive-parts exist in the shed.
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dave321

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PostPosted: Wed Aug 27, 2008 6:48 pm Reply with quote

guys,

i appreciate all yor replies ! looks like a sparky job

dave
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ban-all-sheds

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PostPosted: Wed Aug 27, 2008 7:30 pm Reply with quote

ericmark wrote:
maybe he only intends to use two 6 amp RCD's in consumer unit.

That's not what he said, and that's not what those 2-way CUs come with as standard.


Quote:
where unsure of earthing arrangements it needs to be equivalent to 16mm copper earth which in turn means 6mm SWA.

In what way is 6mm² SWA equivalent to 16mm² copper?


Quote:
OK if no external metal parts and never going to plug lawn mower into shed supply maybe not required.

And what does plugging the mower in have to do with it?
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ericmark

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PostPosted: Thu Aug 28, 2008 12:23 am Reply with quote

There are three ways to earth an out building.
1) Take the earth from the house
2) Use an earth rod
3) Use both house and rod earths
If the out building has no external metal work, and also is not going to be used with any earthed equipment in the garden, then taking an earth from the house is most likely OK. But where there is external metal work or the possible use of any Class I equipment in the garden, then most likely it would require and earth rod. But also the house is likely to have some external metalwork like a soil pipe “411.3.1.2 (v) Exposed metallic structural parts of the building” so using just a TT supply will also likely cause a problem as both earthing system may not be the same voltage so only safe method where all facts are non known is duel method. “411.3.1.1 Exposed-conductive-parts shall be connected to a protective conductor” Table 54.8 gives earth as 10mm² sorry thought it was 16mm² my mistake assuming cable buried in ground 522.8.10 means SWA is required and for rodents 522.10.1 must be considered and also 610.4, 610.6 and 631.1. My thanks to John Ware and his excellent article. John Peckham also did a good article on the use of SWA as an earth where he refers to table 54G now 54.7 but first we need to work out k1/k2xS where k1 = 115 table 43.1 and k2 = 51 table 54.4 and S = 10 table 54.8 so csa of SWA needs to be 22.55mm² and Mr Peckham tables tells us 6mm² two core has 24mm² of steel wire armouring and all smaller cable falls below the 22.55mm² limit so that’s why I have always considered using 6mm² SWA is safest bet and if in middle of a housing estate so all surrounding houses will ensure there is not a voltage slope often I will not bother with rod but if on edge of housing estate and shed is a good distance from house then I will use rod. Since I am not a member of registered body and I don’t want a fail if uncertain I stick one in anyway. I do apologise for the mistake where I said 16 instead of 10 but calculations where still valid for saying use 6mm² SWA of course he may have a TT supply to his house and then I am wrong anyway but still think better to play safe and say use 6mm² where one does not have information to show a smaller cable can be used as if the guy digs the trench lies his cable then gets someone in to connect up and they say no I would think he would be a bit niffed to say the least. Sorry so long to answer took some time to cross ref between old 16th which was in force when the John’s wrote their article to the 17th which we are using now.
Eric
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ban-all-sheds

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PostPosted: Thu Aug 28, 2008 8:37 am Reply with quote

Eric - apologies if the answer is in there somewhere, but your great splodge of unpunctuated text is just too much effort to plough through...

ericmark wrote:
But where there is ... the possible use of any Class I equipment in the garden, then most likely it would require and earth rod.

Why does plugging a mower into a socket in the shed mean that the shed has to have a TT supply but plugging it into a socket in the house does not mean that the house has to have one?
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ericmark

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PostPosted: Thu Aug 28, 2008 10:19 am Reply with quote

Why does a caravan require a TT supply when house is TN-C-S?
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