Electrics in a shed - does it need to be a ring main?

[191]

Hello all.

Apologies for being a newbie and not having trawled through the posts to find an answer, but...

I'm wiring up a new shed and doing the first fix - I'll eventually be getting the final connections signed-off by a pro.

Got the SWA out to an IP 55 consumer unit, and some nicely placed 20 mm conduit in place for the lighting and three double metal clad sockets.

You're probably ahead of me when I tell you I can't get the 2.5 mm twin and earth through the 20 mm conduit!

I'll be ok if the three sockets are linked daisy chain fashion as a spur, but do I need the return from the last socket back to the CU to make a ring??

Thoughts... Can I strip the PVC sheathing from the twin and earth and place the core wires in the conduit instead?


All advice gratefully received!
Thanks
1-9-1

 

[PrenticeBoyofDerry]

I think you would want to look at your cable sizing, 2.5mm2 is a tad on the small side.
And you should get an electrician on board before you start installing!
With regards to circuit arrangement, I assume at the shed location a mini CU will be installed and provisions for RCD protection?
From there you can either construct a ring final or radial circuit but they must be designed correctly.

 

[JohnW2]

I think you would want to look at your cable sizing, 2.5mm2 is a tad on the small side.

I suspect that you're thinking about the SWA, but the OP doesn't tell us what size that is - and, in any event, it appears to be a 'fait accomplis'. The 2.5 mm² appears to refer to 'the lighting and three double sockets', for which I don't think you would probably regard it as being 'on the small side'. However, as you say, there should be an electrician on board, for several reasons,some of which you allude to.

Kind Regards, John

 

[PrenticeBoyofDerry]

I think I have got ahead of myself, so lets start again!
SWA cable from house to shed, is what CSA and the distance this cable is routed is?

 

[Taylortwocities]

What size MCB is in the "IP55" consumer unit for the socket circuit?

 

[JohnW2]

I'll be ok if the three sockets are linked daisy chain fashion as a spur, but do I need the return from the last socket back to the CU to make a ring??

As PBoD has indicated, there are a number of other issues which need to be addressed, and you really ought to have an electrician on board.

As regards the above specific question, it depends how much power you need, and what rating of MCB in the mini-CU you will use to protect the cable. If a total of 20A (across all three sockets) is enough for you, then you can just use a 2.5 mm² radial (i.e. 'daisy-chained', as you describe above), provided that the MCB protecting the cable is no more than 20A (16A if that would be enough for you). If you may need more than 20A, then you would need to 'complete the ring' back to the CU (as you also mention above), in which case you would use a 32A MCB.

Do I take it that your CU already has MCBs? If so, what ratings are they (maybe 6A and 16A, or 6A and 32A?) - and does it also have an RCD?

Kind Regards, John

 

[191]

Thanks for the responses guys.

To complete the picture the SWA from the house CU to the shed is about 20 metres. Its 4 mm (I think!?)

The IP-rated CU in the shed has an RCD, and there is a 6A MCB for the lights, and at the moment a 32A MCB for the sockets.

The sockets will be used for a trickle charger for a mobility scooter, and for occasionally running power tools such as drill and strimmers. There are no 'fixed' appliances like freezers or tumble dryers. That's not to say a future owner might want to do this though.

Hope this adds a bit more meat to the skeletal question.

Thanks again
1-9-1

 

[winston1]

Thanks for the responses guys.

To complete the picture the SWA from the house CU to the shed is about 20 metres. Its 4 mm (I think!?)

The IP-rated CU in the shed has an RCD, and there is a 6A MCB for the lights, and at the moment a 32A MCB for the sockets.

The sockets will be used for a trickle charger for a mobility scooter, and for occasionally running power tools such as drill and strimmers. There are no 'fixed' appliances like freezers or tumble dryers. That's not to say a future owner might want to do this though.

Hope this adds a bit more meat to the skeletal question.

Thanks again
1-9-1

So you have 2 RCDs protecting this circuit, one in the house and one in the shed? If so wrong.

 

[Taylortwocities]

OK, so change the 32A MCB to a 20A one and then you can 'daisy chain' the sockets (ie a radial circuit) instead of a ring.

As Winston says, you shouldnt have two RCDs.

 

[EFLImpudence]

There may well be but it has not been stated that there are two RCDs.

No mention of fault protection for the swa.

"4mm² (I think)" isn't really good enough.

 

[ericmark]

As said swap the B32 for a B20 MCB and no problem with daisy chain method.

As to RCD my Caravan has a 30ma RCD and the camp sites I use also have 30ma RCD's which is required, so there is no problem having a RCD in both the house and the shed.

All it means is the shed one is unlikely to ever trip the house one will likely trip first. Can't see point in swapping from RCD to Isolator you already have the RCD so just use it.

 

[Taylortwocities]

If you have two 30mA RCDs then the issue is that, in a fault situation, one of the RCDs may trip. There's no telling which one as it will dpend on which RCD has the faster trip time (they vary). Or both might trip.

Also pressing the test button on one of the RCDs is likely to trip the other one too.

Its not dangerous, but its poor circuit design.

 

[191]

Good morning everyone...

...and thanks for all of your responses overnight.

When I head back to my parents house this afternoon I'll double check the CSA of the SWA that's already in place. What needs to change in terms of MCBs and RCDs if it is only 2.5 mm? I guess with 2.5 mm SWA and a set of sockets on a radial 'daisy chain' the set up effectively becomes a glorified extension lead? The lights BTW are on a separate circuit of 1.0 mm twin and earth.

I'll leave the speccing and connecting of the MCB/RCDs to the electrician when he comes - I just want to get the conduit and cabling and lighting in place while I have a free weekend.

Thanks again
1-9-1

 

[PrenticeBoyofDerry]

For 20m run in SWA, a minimum 4mm2 csa cable would only really be suitable for a 20A socket radial and a 6A lighting circuit.
The SWA cable does not require to have RCD protection at the distribution side, but the socket circuit within the shed do and any cables buried within the fabric of the shed less than 50mm (unless mechanically) protected would.
As you only have a vague idea of the distribution cable size, I suspect not a great deal of thought has gone into the design side of this and you really require an electrician now, not later! Safety is a legal requirement!

 

[Taylortwocities]

If it is recent, the SWA will have the conductor sizes embossed on the outer sheath.