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DIY domestic wiring to outbuildings

Former Community Member
Former Community Member
Hello, I am new to the forums and would like to introduce myself. In the past I have undertaken a lot of domestic wiring as a DIYer. Most of this predated the Part P requirements. However, in 2013 I undertook a fairly major wiring project at my home that included two new consumer units and half a dozen or so circuits. I did this under building control and they arranged inspection and testing. I secured a full set of certificates to underpin my work. I am about to go through the same process to add in two outbuildings to my set up. I have submitted a building notice and am awaiting to hear from Building Control re appointment of a firm to undertake inspection and testing.

Whilst I gather most DIY work scares many of you you, worth pointing out that whilst I am not any form of electrical Engineer, I do have (from many years ago) a HND in Mechanical and Production Engineering and a B Eng (Hons) in Engineering Technology. Both of these included a significant element of electrical technology and electronics. I have also worked in engineering (aerospace) for much of of my early career and still work in a STEM environment. Further,  intend to do a short course on the latest regulations and may also undertake a C&G course on testing and inspection to possibly get to a point where I can register with one of the governing bodies.

I have come to this forum as I am designing my changes and preparing for the first fix. i have been seeking guidance on the internet and from the regulations themselves. I find the regulations open to many interpretations and the advice on many forums to be wildly different. I really want to get this installation right and will have many questions, most requiring practical experience that I simply do not have. I believe this forum will be able to guide me in the right direction when I need it.


My first question relates to burial of SWA. I had a digger in last week to put in a series of trenches for cables. I intend to do all of the external wiring in SWA and in an ideal world would have buried it at 600mm. However, I have places in the garden where it is not possible to get that deep. For an example, under one lawn I have an underground structure that limits the trench depth to 300mm. What would be a appropriate method for installing SWA at this depth? Do I need to add something to prevent a spade/fork penetrating the cable, if so, what would you recommend? I also need to watch my budget here so looking for cost effective views if possible.


There will be further questions. Thank you.


regards

  • 0

     If someone attacks with a spade the worst damage is a bit of a flash, and a repair. 




    Most inconvenient and perhaps expensive as well. Best leave some spare slack cable for that possibility then. 


    522.8.10 and 132.1 (ii) and 134.1.1.




    Z.

  • Former Community Member
    0 Former Community Member
    All


    Thank you, some great advice. I also managed to speak to the company that will be inspecting on behalf of BC on Friday and now am happy I have a solution. Not using a pipe, I have a lot of existing galvanised pipes, all redundant, in my garden already. I don't want any confusion. The company are content with marker tape, concrete slabs and sand. However. In looking into this installation, I now have concerns about some existing external wiring and would appreciate your views.


    My garden is a long narrow garden with lighting provided by aluminium lamp posts. These are connected in two circuits. The lighting to the front is fed by the house consumer unit in 1.5mm SWA protected by a 6A MCB in turn protected by the consumer unit 63A/30mA RCD. The rear in a similar manner to a CU in the rear garage (this is a third outbuilding and not one under previous discussion, with exported TNCS for which the installation was signed off by in 2013). My concern is that the earthing of all of these lights relies upon exported TNCS from the CU (s). This is via a 1.5mm core plus armour in the SWA. Is this sufficient?. The lights are fairly old and I believe designed to be earthed ( I do need to check this). Each one is bolted down to concrete in the ground. 


    Any thoughts on this appreciated.
  • Former Community Member
    0 Former Community Member
    I should add that there are some fairly long cable lengths in this. The furthest light has a cable length of approx 45m.
  • 0

    My concern is that the earthing of all of these lights relies upon exported TNCS from the CU (s). This is via a 1.5mm core plus armour in the SWA. Is this sufficient?. The lights are fairly old and I believe designed to be earthed ( I do need to check this). Each one is bolted down to concrete in the ground. 



    I practice it's probably absolutely fine.


    In theory the lighting columns should really be treated as extraneous-conductive-parts and so be main bonded - in that way you can ensure that the small c.p.c.s aren't obliged to carry large diverted N currents (or large earth fault currents from outside the installation) on their own. In practice the resistance to Earth of the lighting columns in likely to be significant - several tens if not hundreds of Ohms - and that alone will limit currents flowing through the c.p.c.s to Earth via the columns to something quite modest and within the capabilities of small c.p.c.s. even in the worst case conditions.


    BS 7671 doesn't really recognise the situation of extraneous-conductive-parts having a large resistance to Earth - the only concession (intended really for street lighting columns connected direct to the mains) is that the bonding size may be reduced from 10mm2 to 6mm2 - but even that's likely to be over-engineered for your situation.


    Generally it's acceptable for PME Earth to be used on outdoor lighting - even steel street lighting columns are normally PME'd - despite the recognised risks of a broken PEN.


       - Andy.
  • 0
    But when you put the hot tub in you may need to reconsider it ?
  • Former Community Member
    0 Former Community Member
    Thank you. That answers my question well and will stop me worrying and let me get on with the job in hand.


    In answer to some comments above, I am wiring up two outbuildings. Neither are habitable or have water supplies. The first is a garage. In the short term the requirement is for a simple 6A lighting circuit and a 16A ring. I do have future aspirations to include a 32amp EV charge point and possibly a door opener (x2), hence have specified the cable to support that (16mm2). The second building is for a small summerhouse. This will only have a 6A lighting circuit and a 16A ring. I have no further requirements that I know about today. However, am putting in 10mm2 in to allow for a little future expansion. 


    With regards to EV Chargers, in talking to my BC's electricians last week. I mentioned the potential EV charge point. He did say that I need to get my earth resistance,via my earth spike, to below 50 ohms as most of the EV charger manufacturers require that and any installer will not install without it. Has anybody come across that? 


    Regards
  • 0
    Installing a 16 amp socket ring circuit is pointless, consider making it a 20 amp 2.5 mm T&E radial or even a couple of them as that will still be less cable.


    Andy B.
  • Former Community Member
    0 Former Community Member
    Andy, that is a good point and given I just want a socket close to the CU and another at the other side, that would be easier and cheaper and still fit my needs. I had not even thought that way, that's why I joined this forum, thank you.

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