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Discuss Power to outbuilding- Consequences of extraneous-conductive-parts on choice of SWA size in the DIY Electrical Forum area at ElectrciansForums.co.uk.

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  1. Daniel Pooley
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    Daniel Pooley EF Member

    Location:
    Oxford
    I realise this is a much-discussed topic, I have certainly read a lot on the forums already! However I wish to ask about the subtitles of earthling the outbuilding and the cost/practicality consequences.

    The outbuilding (a DIY’s workshop) will have one socket ring and two lighting radials. I wish to install a garage CU, this one probably: British General 5-Module 3-Way Populated Garage Consumer Unit - https://www.screwfix.com/p/british-general-5-module-3-way-populated-garage-consumer-unit/1926g

    The shed has some metal parts (sheets of powder-coated steel cladding) but 1) I cannot foresee any situation where I could be touching exposed metal and 2) they are not at earth, being on a timber frame. So, I think it qualifies as “no extraneous-conductive-parts”.

    This being the case, and because I am TNC-S, I would rather run power out on SWA from my house, using a spare way on the house’s CU. I think I am on TN-S. (I can clearly see the earth bonded to the sheath/armour of the incoming supply).

    The outbuilding is 25m away, so to comply with voltage drop regs for the lighting ring I was going to use 3-core, 6mm^2. I.E bring the protective conductor to the outbuilding on 6mm^2.

    6mm^2 is certainly adequate current wise.

    But what about the earthling? If in the future I decide to bond the cladding I need to be at 10mm^2 to comply. To help the cost I have the option of bring the SWA into the house at a different point (via a rated junction box) and go 10 meters inside, under the floorboards. But I cannot use 10mm^2 ‘twin and earth’ because the earth is always smaller than the earth. At the point I have to use 10mm T&E and run 10mm green/yellow cable too. I can do it but it’s become a little bit complicated and very expensive.

    So I think I have these choices.

    1) Treat the install as no extraneous-conductive-parts, run my 3-core 6mm SWA all the way from house CU to outbuilding.

    2) Treat it as having extraneous-conductive-parts and run 10mm^2 SWA (with some extra 10mm plus T&E in the house)

    3) Use and earthling rod and make a TT system at the outbuilding. Almost all the forums I have read seem pretty negative about this, citing changeable ground conditions and high impedance.

    I think there my choice all hangs on the no extraneous-conductive-parts definition, and if yes there is a substantial increase in cost and complexity.

    I have read a lot of the forum debate on this, and I don’t want to open a can of worms. I read the guide by John Ware (2005) and enjoyed sinewave’s “The Ultimate PME/TNS Exportation! by Sinewave“. (A good guide For 'exported PME' and outbuildings! - https://www.electriciansforums.co.uk/threads/a-good-guide-for-exported-pme-and-outbuildings.28159/page-2) . I also watched most of John Ward’s stuff but am still not 100% clear.

    Please, any/all advice appreciated. I think I ‘know the answer – I have to 10mm everything to be safe.


    Dan
     
  2. buzzlightyear
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    buzzlightyear Electrician's Arms

    Location:
    star command
  3. SparkyChick
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    SparkyChick Making a banana smoothy for my fave gorilla Electrician's Arms

    Location:
    South Wales
    Business Name:
    SparkyChick
    Hi Dan,

    First up, I'm guessing that as you've got no electrical qualifications listed in your profile you're not an electrician, in which case some of what I'm about to advise may be of no use.

    Guidance Note 8 from the IET covers earthing and bonding, along with the tests you should carry out to establish whether or not a piece of metal work should be considered an extraneous conductive part or not.

    If you're not a spark, you're not going to have the kit to properly establish this and given your life (and potentially the lives of others) may depend on this, you shouldn't be making assumptions about whether an item needs to be bonded or not.

    As you're in England, you're subject to the 2013 edition of Part P of the building regs, which allows for Third Party Certification. This means you can engage the services of a Third Party Certifier who will design the installation, instruct you how to install it (and check it as you progress) and then they can certify it and sign it off. This approach may save you money, but ultimately my advice is engage the services of a qualified electrician sooner rather than later.
     
    • Agree Agree x 3
  4. Daniel Pooley
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    Daniel Pooley EF Member

    Location:
    Oxford
    Hi SparkyChick, buzzlightyear,


    Thank you for taking time to help me, it is appreciated! I thought placing this in the DIY thread was the right place, sorry if I wasn’t clear on my lack of skill. No training, (electronics doesn’t count here !), just a little too ambitious and hoping for some help.

    I like the idea of the third part certification. I should say that it is urgent for me to decide the SWA (and therefore the earthing system) as the groundwork is being done now. Getting an electrician to connect the CU’s will happen sometime next year. I think this might open a can of forum worms, so let me be specific.


    Am I right in thinking the choice of CSA (be it 10mm or 6mm or whatever is adequate for the intended load) is critically dependant on if you have to bond extraneous conductive part or not.


    The 2nd question, if you are using 3-core at 10mm, and want to take this indoors, what do you use as twin&earth is not big enough.


    Really appreciate all advice and please take it as 100% registered that I need a proper electrician else it’s a real risk.


    Best


    Dan
     
  5. aesmith
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    aesmith DIY Member DIY

    Location:
    Aberdeenshire
    Hi, fellow DIYer here who did something similar a few years ago. In my case the electrician selected and supplied the SWA, then was happy for me to do the excavation and burial. Alternatively if you can't engage your electrician first, how about burying a duct with draw cord just now, then the correct spec cable can be pulled in when the time comes.
     
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  6. Murdoch
    Offline

    Murdoch Electrician's Arms

    Location:
    Woking
    I would find your friendly spark now .............. get him involved and get the cable in

    Or simply bury some duct with strong cord through it or 2.5mm single cable ............... string is not strong enough and WILL perish quite quickly.
     
  7. Strima
    Online

    Strima Electrician's Arms

    Location:
    St Neots
    He's on holiday...
     
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  8. Murdoch
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    Murdoch Electrician's Arms

    Location:
    Woking
    Its threads like this that confirm to me why I'll never get involved in 3rd party certification.

    Best you get LABC involved, they will relieve you of a couple of hundred, or more and should inspect at each stage. You would still need to accept responsibility for the design, they won't advise, and if you get it wrong, you'll have to do it again.
     
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  9. richy3333
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    richy3333 Trusted Advisor

    Location:
    UK
    Best thing the niceic did was not to get involved in that nonsense in England
     
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  10. spinlondon
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    spinlondon Trusted Advisor

    Location:
    Harlow Essex
    I ‘d price up 6mm2 SWA and 10mm2 SWA first.
     
  11. PEG
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    PEG Trusted Advisor

    Location:
    Manchester
    If it is urgent,and you have not involved anyone who could specify a suitable cable on site,i would put a duct in. It will be considerably cheaper than the wrong choice of cable.

    Your installation needs assessing properly,as the description has two different earthling arrangements,in one paragraph. ;)
     
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  12. Sparkingmad
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    Sparkingmad Regular EF Member

    Location:
    Dorset
    I would definitely seek expert advice from an electrician. You want to run swa 25mtrs in the garden how far is the mains from point of termination of swa? I would run a 50mm duct and make sure it’s smooth bore inside with a nylon drawstring. Then I would attach a draw rope and pull that through so you can pull what ever cable your sparks calculates and designs. Also consider if you need phone, internet, cctv or alarm cables taken up to the workshop? If you do it might be wise to lay a separate smaller duct in at the same time.
     
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  13. Daniel Pooley
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    Daniel Pooley EF Member

    Location:
    Oxford
    Hello everyone, thank you for all the replies. I am running a duct in anyway as (like you guys) I think it a good idea for the future. Annoyingly this duct will have several tight bends in it so I really want to get the SWA in. This is due to the nature of my property, which is terraced into a hill.

    @spinlondon@spinlondon - price aside, I want to conform to regs, and I guess another ~40quid to go from 6mm to 10mm is okay.


    Thank you everyone for the advice. I wish someone had of adressed the questions but okay, you have my safety at heat.

    I am going to push ahead and put in 10mm, 3-core, SWA from the house CU to the workshop. I *think* this way I coverall bases as a future sparky could:
    1. Bond all extraneous-conductive-part
    2. Make a TT system with earth rod
    3. Or, if the metallic items are not considered extraneous-conductive-part, just have a very nice fat earth from the house CU.
    Advice (constructive if possible) still appreciated

    dan
     
  14. Murdoch
    Offline

    Murdoch Electrician's Arms

    Location:
    Woking
    Apart from the fact you should get a spark involved, I would say the bigger cable 3 core then you have flexibility.

    And sea can't go round tight corners so you may need to reconsider your route
     
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  15. Daniel Pooley
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    Daniel Pooley EF Member

    Location:
    Oxford
    @Murdoch@Murdoch - thank you for the advice. I think my cable route is okay, laying it in the trench, I just wouldn't fancy pulling it through a duct.

    Where I go into the building I need a junction box anyway.
     
  16. aesmith
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    aesmith DIY Member DIY

    Location:
    Aberdeenshire
    Make sure you leave enough cable sticking out at both ends, particularly if you don't know the exact route the electrician will take at the house end.
     
  17. Murdoch
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    Murdoch Electrician's Arms

    Location:
    Woking
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  18. Daniel Pooley
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    Daniel Pooley EF Member

    Location:
    Oxford
    @Murdoch@Murdoch is there any way you can advise me. I understand talking to someone like me is frustrating so at the same time, please do not feel obliged.

    best

    dan
     
  19. Murdoch
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    Murdoch Electrician's Arms

    Location:
    Woking
    Consult a local spark is my advice ........................
     
  20. Amp David
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    Amp David Trusted Advisor

    Location:
    Glossop
    No way, just an earthling
     
  21. johnduffell
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    johnduffell Active EF Member

    Location:
    uk
    Take your swa to the cu if possible, no point adding extra joints if not needed.
     
  22. Spoon
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    Spoon Trusted Advisor

    Location:
    Lancashire
    Hello OP. I understand you want to do this yourself and I'm glad you think your cable route is ok.
    When the electrician comes to wiring up the cable, if he's not happy with the route/installation I'm sure you will be happy take up the cable and do it to the regs.... if it's wrong...
     
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