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.