Wiring shed from ring main

[PeterDawes]

Hi Guys. Great site.

I need power in the garden shed. The load will only be lighting and a couple of sockets for hedge trimmer, saw etc. Length of run is 45 metres. The plan is to use 2.5mm SWA buried in a trench. Questions.

1. Consumer unit is at the far side of the house, awkward to reach, so I'd like to take it off the downstairs ring main from the back of a socket on the garden side. Is this OK?

2. How deep should the trench be? (We're having a digger to level the garden, so this can be done at the same time)

3. I guess we'll need it RCD protected. Should this go at the house end or in the shed, with an FCU in the house? Or what?

4. Will the shed need its own MCB unit or can the wiring just be hard-wired into the supply?

5. Should I drop an earthing rod at the shed end, or use the household earth?

6. Should I earth the SWA shield?

7. What's the best way of bringing the SWA through the house brickwork? You can't just shove it through a hole as SWA doesn't bend enough. Can't see a junction box for that purpose in Screwfix.

I'm heavily into electronics and a keen DIYer but not up to speed with modern wiring practice. Looked here for an answer, can't find this exact situation as I don't keep koi carp! Can anyone advise?

Thanks
Pete

 

[leeco]

45 meters

they will be some vollage drop there you will need bigger swa and not from your ring main

 

[PeterDawes]

Thanks, leeco.

Given that it's unlikely to need more than 13 amps, I calculate the voltage drop to be 4.2 volts with 2.5mm cable. Is this worth worrying about, and why not take off the ring main for such light loads?

Can anyone answer the other questions?

 

[ban-all-sheds]

I'm heavily into electronics and a keen DIYer but not up to speed with modern wiring practice. Looked here for an answer, can't find this exact situation as I don't keep koi carp! Can anyone advise?

My advice is that you should take the time to properly learn about the things you need to know, as this is not the sort of job you should be doing on the basis of just asking the questions that happen to occur to you.

What if you don't think to ask about something important because you don't even realise it exists and that you don't know it? For example you haven't asked about, or given any indication that you've considered, what your EFLI value would be at the end of 45m of 2.5mm².

Or what if you think you know something, but you're wrong, as you are with your volt-drop calculation, for example?

And do you plan to notify?

 

[Grizzly]

Or what if you think you know something, but you're wrong, as you are with your volt-drop calculation, for example?

Indeed. I get the voltage drop to be nearer to 10.5V - 11V (I think that's about right), rather than the OPs calculation of 4.2V
This is higher than the normally allowed maximum VD of 4% of 230V (9.2V)

 

[ban-all-sheds]

4% is no longer the limit - see Appendix 12.

 

[PeterDawes]

Thank you, Grizzly. I found another calculator which agrees with your 10.5V. The cable manufacturer shows this on the just-acceptable side of 2.5mm / 4mm.

> And do you plan to notify?

I was just asking some exploratory questions before ordering parts. The plan is to do most of it myself and get a qualified electrician friend to test and approve it. Ban-all-sheds, I have to say your sneering comments are not very helpful.

 

[ban-all-sheds]

Thank you, Grizzly. I found another calculator which agrees with your 10.5V.

You should also be able to work it out yourself from data tables and first principles.

The cable manufacturer shows this on the just-acceptable side of 2.5mm / 4mm.

10.5V is not acceptable.

The plan is to do most of it myself and get a qualified electrician friend to test and approve it.

Is he registered to self-certify Building Regulations compliance? Are you absolutely sure that your friend is going to lie for you? If he were genuinely acting as designer, and supervising and directing your installation work you wouldn't need to be asking any of these questions.

Ban-all-sheds, I have to say your sneering comments are not very helpful.

I wasn't sneering - I was simply advising you that you don't know enough to be doing this job - that's abundantly clear from the questions you were asking. There's nothing wrong with asking questions, but it is not a substitute for ensuring that you have the right level of knowledge before you start, and jumping in at the deep end with a job like this because you think you've asked all the questions you need is not to be advised.

Can anyone advise?

Yes - don't ask for advice if you are going to reject that which you don't like.

 

[PeterDawes]

Is he registered to self-certify Building Regulations compliance?

Yes

Are you absolutely sure that your friend is going to lie for you?

Who said anything about lying, for heaven's sake?

you wouldn't need to be asking any of these questions.

I don't need to ask any questions. I just like to know more about these things before anyone starts the job. I freely admit to not knowing EFLI values but have now learnt something by looking it up.

don't ask for advice if you are going to reject that which you don't like.

I haven't rejected a scrap of it. All advice is welcome, except for "don't worry your pretty little head".

 

[ban-all-sheds]

Who said anything about lying, for heaven's sake?

So what would you call it when he signs an official document to say that he did all the work?

I freely admit to not knowing EFLI values but have now learnt something by looking it up.

Excellent.

So I hope you now see why, when from your original post there was absolutely no indication that you weren't going to be doing it all yourself on the basis of what you gleaned from asking questions, I said what I did.

Because you not knowing about fault loop values, and how important they are, is a classic example of an "unknown unknown", and one which could have fatal results if you get it wrong. IMO it shows how inadvisable it is for people to do jobs like this when they think they've found out everything they need to know by asking questions that only cover their "known unknowns".

I haven't rejected a scrap of it.

So when you described important and relevant advice, sincerely given, as "sneering comments" you weren't rejecting it?

All advice is welcome, except for "don't worry your pretty little head".

I did not say or infer anything like that.

At a point when we'd heard nothing about you using an electrician in some way, and that you were only asking questions out of interest and so you could buy materials, I advised you that you didn't know enough, that there were areas where you didn't know there were gaps in your knowledge and there were areas where you thought you knew but you didn't.

I did not say or infer that any of this was beyond you, just that you should take the time to learn properly, and that trying to do it just by asking questions was not a proper method.

 

[PeterDawes]

OK Ban-all-sheds, I take your point. Just that I was hoping for more information than, in effect, "Take my advice and don't do it".

I'm not asking how to build an atomic particle accelerator, just what would be standard practice for wiring a shed 45 metres away, taking the supply from the ring main (or consumer unit if necessary). Most of it could be specified on paper without even seeing the job. The only variable I can see is the ring main itself - its length of run and its normal domestic loading. But what do I know?

Are you honestly saying he (the electrician) has to do ALL the work, including digging the trench and screwing light fittings to the shed wall, otherwise he's a criminal? Even if he trusts me to do it properly AND inspects the work? If so, the law is not doing much for human relations.

 

[ban-all-sheds]

I'm not asking how to build an atomic particle accelerator, just what would be standard practice for wiring a shed 45 metres away, taking the supply from the ring main (or consumer unit if necessary).

Not from a ring final. Either from the CU, or a separate feed split off from the tails before they enter the CU, via a switchfuse. Some people like to RCD protect the cable - IMO that has a number of disadvantages.

The cable should be sized, as with any cable, to carry the load required without excessive volt drop, without exceeding Zs for the breaker used, and without exceeding its capacity given any derating factors arising from its installation method etc and the temperature limits of anything to which it's connected or in proximity.

It should be installed in a way that will not damage it, or leave it at risk of damage due to external influences.

Each final circuit should be designed and installed with the same care, and in accordance with the regulations.

Are you honestly saying he (the electrician) has to do ALL the work, including digging the trench and screwing light fittings to the shed wall, otherwise he's a criminal?

In theory yes, as he'll be declaring that it was all done by him in accordance with the Wiring Regulations and the Building Regulations.

In practice no - I'm sure he wouldn't want to dig the trench himself, but he might well want to specify how deep it goes, what bedding is provided for the cable, and what warnings etc. He certainly won't appreciate the mystery of finding it all backfilled and just two ends sticking out.

Basically he must be the one to design everything, and he must be able to say, hand on heart, that everything you did was under his direction and supervision. So all your questions in the original post are for him to answer, as he is responsible for how it's done in each of those aspects.

 

[PeterDawes]

That's the stuff! Thanks, mate, it makes a lot of sense.

Without wishing to appear cocky, I do have a degree in physics and am a serious electronics enthusiast. I've built and designed electronic equipment of vastly greater complexity than a shed supply. But I defer respectfully to you guys like ban-all-sheds for expert advice on domestic wiring. Please don't think the worse of me for that.