Advice on Shed Wiring

[thedawnbefore]

Hi,

I'm looking to get the lionshare of a small shed wired before a friendly electrician does the needful. I'm a confident DIYer, but I would appreciate any more experienced view to check my logic.

My house was professionally rewired in 2022, but I would like to connect a small 8x6ft shed to a secondary CU that's close by. Connecting to the main CU would require too much upheaval.

The shed is small, but I'm planning to install a few sockets for charging power tools and occasional use of garden tools - plus an internal and external light. No big loads.

Obviously, it's a very simple issue, but I'd appreciate a more experienced view on any issue with my connecting to the secondary CU. The 12m SWA run is clipped externally but fiddly with tight access, so keeping that deliberately light also. Just keen to avoid anything daft before my friend sees it!

Many thanks in advance.

 

[HappyHippyDad]

This is why I am a staunch supporter of the 'no advice to DIYers'. Unless someone is prepared to spend many hours on line with @thedawnbefore , going through the job at each stage, making sure it is done correctly, then the job is not going to be done properly or to the regulations (as it wont be notified or certified). This chap or lady may well be a thoroughly decent fellow with very good DIY skills, but he or she clearly doesn't understand electrics enough to be doing this.

There are so, so many factors involved here, from the depth of the buried cable (including cable warning tape) to Zs testing etc etc etc etc.

I think the only advice is to leave it fully to the electrician.

However, this is just my opinion and I am very open to fact I may be wrong. Perhaps it's best to give some basic info to DIYers. Perhaps this will decrease the chance of a dangerous situation. Personally, I think it is more likely to cause longer term problems. The DIYer gets more confidence, does more jobs, starts helping neighbours with electrics etc.

 

[thedawnbefore]

What a brilliant group, thank you for the interest in my issue. I’ve learnt a lot already from the feedback.

A 20A radial then from the second CU then. The SWA is clipped above ground so no issue there. It’s a short but awkward run hence my inclination to go light.

Reading between the lines, I should get my electrician friend to pop over first.

Thanks all

 

[davesparks]

It’s a short but awkward run hence my inclination to go light.

What do you mean by go light?

Are you suggesting you would install and undersized cable in order to make the job easier?

 

[thedawnbefore]

What do you mean by go light?

Are you suggesting you would install and undersized cable in order to make the job easier?

That was my inclination yes - I had understood from several calculators and manufacturer specs that 2.5mm 3-core clipped to external wall would be adequate for my couple of sockets to my shed? It would also be fused down appropriately.

While my main board is new and the secondary CU is on a 40A RCBO, would you put in another rcd at the shed end for good measure or is this pointless? My secondary CU which the shed is all mcbs and given this runs my heating, would be more than inconvenient if the shed sockets were to blow.


I will endeavour to consult my electrician friend in the design as you suggest. He struggles with English a bit though.

 

[DPG]

That was my inclination yes - I had understood from several calculators and manufacturer specs that 2.5mm 3-core clipped to external wall would be adequate for my couple of sockets to my shed? It would also be fused down appropriately.

While my main board is new and the secondary CU is on a 40A RCBO, would you put in another rcd at the shed end for good measure or is this pointless? My secondary CU which the shed is all mcbs and given this runs my heating, would be more than inconvenient if the shed sockets were to blow.


I will endeavour to consult my electrician friend in the design as you suggest. He struggles with English a bit though.

He struggles with English? My advice is pick a different one then!

 

[davesparks]

That was my inclination yes - I had understood from several calculators and manufacturer specs that 2.5mm 3-core clipped to external wall would be adequate for my couple of sockets to my shed?

It would likely be fine for a couple of sockets yes, it is definitely not fine for a 32A circuit though.

While my main board is new and the secondary CU is on a 40A RCBO, would you put in another rcd at the shed end for good measure or is this pointless?

No I wouldn't install another RCD as that would not comply with the wiring regulations

My secondary CU which the shed is all mcbs and given this runs my heating, would be more than inconvenient if the shed sockets were to blow.

Yes that will be inconvenient but with an RCBO at the supply end you are stuck with the risk of that happening, even if you put another RCD in the 40A RCBO will almost certainly trip first in a fault.

will endeavour to consult my electrician friend in the design as you suggest. He struggles with English a bit though.

You may need to get someone else to help with the design of the job then, understanding the wiring regulations can be difficult even for some people who don't struggle with English.

 

[DefyG]

My secondary CU which the shed is all mcbs and given this runs my heating,

Heating ?? - Not an Off Peak supply is it by any chance as you could be back to square one!!

 

[thedawnbefore]

It would likely be fine for a couple of sockets yes, it is definitely not fine for a 32A circuit though.

No I wouldn't install another RCD as that would not comply with the wiring regulations



Yes that will be inconvenient but with an RCBO at the supply end you are stuck with the risk of that happening, even if you put another RCD in the 40A RCBO will almost certainly trip first in a fault.



You may need to get someone else to help with the design of the job then, understanding the wiring regulations can be difficult even for some people who don't struggle with English.

So helpful and sincerely appreciated. Thank you.

Rest assured, I will get this checked and notified as part of a more complex battery install i'm having done professionally next month. I am a stickler for paperwork, and this is as much about my curiosity and wanting to learn and impress my electrician friend. Where I have done similar DIY in the past, qualified electricians have told me my work is perfect and have been happy to do the final CU termination and notify - but in hindsight, this was probably more luck than judgment as admittedly I had not involved them in the design. I respect the 2.5mm is controversial and not how others may do it. I've got to get the SWA round some very tight spaces though. I just want to avoid anybody finding an obvious 'bodge' or non-compliance.

I think this is the last question —would this new design be more appropriate? Should I put a corresponding MCB at the shed end in a waterproof DIN enclosure? A full CU seems overkill now, but from other installs I've observed, there should at least be a switch over there? I've updated my design below.

Thank you again and excuse my ignorance.

 

[thedawnbefore]

Heating ?? - Not an Off Peak supply is it by any chance as you could be back to square one!!

Good call out thanks. This CU is actually for an Air Source Heat Pump.

The contractor (not the guys who rewired the rest of the house) put in a great big 10 way CU for 3 ASHP MCBs. It's only on a 45A RCBO but the ASHP/Immersion never draws more than 15A. Obviously, it's less than ideal if it shuts my heating down, but I guess I can isolate the non-critical shed in the event there's a fault with it. It's only running a garden vacuum/pressure hose and some cordless chargers.

 

[davesparks]

. I respect the 2.5mm is controversial and not how others may do it.

It is not controversial, nor is it a situation of being 'not how others would do it'. If you had followed your original design of a 32A MCB protecting the 2.5mm cable it would have been non-compliant and potentially dangerous.
However following your new design of a 20A MCB protecting the 2.5mm it is likely to be compliant and safe subject to confirmation.

I've got to get the SWA round some very tight spaces though.

Is there any particular reason it needs to be SWA?

Sould I put a corresponding MCB at the shed end in a waterproof DIN enclosure?

No, There is no point having a 20A MCB in series with another 20A MCB.
Why waterproof? If water is getting in to the shed then you should fix that before installing the electrics.

A full CU seems overkill now, but from other installs I've observed, there should at least be a switch over there?

You'll probably want a light switch for the light but other than that no.
You will need to fuse down for the light but this could be done via a fused connection unit.

 

[thedawnbefore]

e similar DIY in the past, qualified electricians have told me my work is perfect and have been happy to do the final CU termination and notify - but in hindsight, this was probably more luck than judgment as admittedly I had not involved them in the design. I respect the 2.5mm is controversial and not how o

Thank you for another very helpful response. Just to assure you we are saying the same thing, I was referring to the 2.5 on 20A radial which I now understand to be technically compliant but still seems to be drawing criticism from some folks here. I respect it leaves little flexibility for future. I’m starting to think just put in 4mm

Is there any particular reason it needs to be SWA?

Actually no, come to think of it. It’s clipped to a wall/fence away from any mechanical interference. Squirrels perhaps.

Would you recommend an alternative e.g NYY-J? I’m not sure hard conduit will work and the flexible stuff would be unsightly for the neighbours