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.

 

[westward10]

Why aren't you consulting the electrician who is doing the needful ?
Is the 2.5 SWA okay for 32A?
Do you have selectivity between protective devices?
You have no selectivity between RCDs.

 

[DefyG]

The 'needful' starts at the design stage.

 

[thedawnbefore]

Thanks very much @DefyG and @westward10, very helpful

 

[DPG]

Thanks very much @DefyG and @westward10, very helpful

It was indeed helpful and accurate advice.

 

[Mikegh]

Am I seeing 3 RCDs in series there

 

[thedawnbefore]

Am I seeing 3 RCDs in series there

Quite right - seems this was a diagrammatic error upon checking, thank you. On checking the secondary CU does not contain an RCD. I'm now thinking this is overengineered for my purposes and that I'll just put in some SWA, and my friend can hook up a simple radial circuit when he gets time. Much thanks.

 

[DPG]

Quite right - seems this was a diagrammatic error upon checking, thank you. On checking the secondary CU does not contain an RCD. I'm now thinking this is overengineered for my purposes and that I'll just put in some SWA, and my friend can hook up a simple radial circuit when he gets time. Much thanks.

Have you done the calculations on the 2.5mm SWA?

 

[thedawnbefore]

Have you done the calculations on the 2.5mm SWA?

Thanks, @DPG; yes, I did but will check with a pro. Ta.

 

[davesparks]

Thanks, @DPG; yes, I did but will check with a pro. Ta.

I think you have made a mistake in those calculations as 2.5mm SWA is not going to be suitable for a 32A circuit.
Do you actually need 32A for the shed, could you not just install a 20A radial and fuse down from that for the lights?

 

[MDJ]

what else is on the secondary DB?
How far is the shed from the Secondary DB?
2.5mm2? nope not big enough, in fact allow for the future, get a decent size cable in there, again though what is on the Secondary DB?

 

[DefyG]

Table 4E4 states greater than 32A for reference D, but that's only part of the story! just a starting point.
Its no wonder some people assume 2.5 will be fine on a 32A mcb.

 

[westward10]

You need to use Table 4D4A.

 

[DefyG]

PVC/SWA cable is quite rare now locally but can get it to order so now we just use xlpe/swa cable as standard.
But my point is a DIYer even a confident DIYer might not know the difference and assume any 2.5 cable is capable of greater than 32A when buried without considering other factors!

 

[westward10]

Unless terminations are suitably rated for the temperature rating of the cable such as XLPE use Table 4D4A.

 

[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!