SWA for new shed

Hi all,

We are in the middle of having all sorts of landscaping and groundworks completed around our house. Currently, we have the rear patio dug up ready for a new one to go down. I've also demolished the old shed and lifted the slab base it was on.

Before our tradesmen start putting the new patio down, I want to make preparations for an SWA cable to go in from our main CU to a secondary CU that will go in the new shed - a 25m reel of SWA will give about 2m spare each end, as the accurate distance is closer to 21m once all of the bends are accounted for.

I'm trying to understand for my own peace of mind what size SWA cable should be appropriate and what its route should be so I can dig the right trench in the right place. I'm thinking the best approach will be to simply install conduit and worry about the cable later but Friday afternoon curiosity has got the better of me...

The plan is to run the SWA straight out the wall from the CU and along the side of the house (clipped to the brickwork) for 11m then down under the new patio, the existing garden and coming up at the site of the new shed another 10m away - so 11m clipped in air & 10m buried underground. I plan initially to have just a simple light circuit in the shed and provision for a couple of sockets in the shed, plus a couple outside for garden tools, however I want to put some future-proofing in to save me digging stuff up or buying new cable! For the same reason I'm going for 4-core SWA (just in case!)

Using TLC's voltage drop calculator I get this:
View media item 80497
From everything I've studied, 2.5mm just seems small. I can't help think I'm missing something and 4mm will be better? I've allowed for 5kW in the above calculation which should give me a fair bit of headroom.

What do people think?

Any advice on conduit to use would be good too - I need about 12m in one run if possible.

Many years since I went to college but seem to remember the volt drop should not be more than 2.5% Your table shows 3.5% so maybe 4mm would be better.
Don't forget to allow big enough diameter conduit to allow for air circulation as some people think its ok being a tight fit so long as it goes in.

A sparky will be along soon to correct me and give you the right info.

The volt drop limits are 3% for lighting (if material) and 5% for power.

Why do you want to use 4 core?

There are a lot of variables involved with outside electrics so I would advise you to discuss the installation with an electrician as someone will have to satisfy the Local Authority Building Control and issue the relevant certificates and schedules.

As you're coming from the consumer unit, presumably with a separate breaker for the new circuit (?), I'd go for 6mm and to hell with it - should be loads of capacity for whatever you want to do in the future. And instead of "a 25m reel" get a measured length from a wholesaler. 6mm won't cost significantly more than 4mm, and buying from a wholesaler you'll get more for your money than a precut reel from one of the sheds.

If you're certain of the start and end points, lay the SWA underground rather than putting in conduit and hoping to pull it through - armoured is difficult to pull through.

pj

Firstly you will only need 3 core SWA, secondly I assume some lighting will be wanted, so belt/braces I would factor in that for VD.

Then it would all be down to what load is required, even if you wanted a simple setup of a light and a double socket, you could be looking at 6kW demand. So that would automatically put you on the 4.00m2 sizing, anything much more then your on 6.00mm plus.
Thirdly and most importantly! I would strongly suggest that you employed the services of a qualified/competent electrician. Then they can design, install and commission for you, with all the relevant documentation being issued.

Installing a new CU is notifiable - if you don't use a registered electrician you'll need to apply for Building Regulations approval in advance, and describe how you'll ensure compliance with Part P.

ban-all-sheds said:

Installing a new CU is notifiable

Click to expand...

As you know, strictly speaking is is only replacing a CU that is notifiable - and I don't think that anyone could argue that installing a new one (in the shed) where there had not been one previously was 'replacing'.

It would, in any event, be a bit silly for installation of a mini-CU in a shed to be notifiable, whereas to achieve much the same functionally with a collection of FCUs (or even MCBs etc. in enclosures) would not be notifiable!

However, most of that is probably moot, since it sounds very much as if what the OP is proposing would be a 'new circuit' - and hence would be notifiable, anyway, new CU or no new CU!!

Kind Regards, John

I assume if you are installing a new consumer unit, that it is highly likely one or two new circuits would be installed onto it

PrenticeBoyofDerry said:

I assume if you are installing a new consumer unit, that it is highly likely one or two new circuits would be installed onto it

Click to expand...

Well, that's all down to the usual debate about 'what is a new circuit?'. If it were done with FCUs, rather than MCBs within something called a 'CU', would you still call everything downstream of the FCUs 'a new curcuit'? I don't know what they intended, but if everything connected via an FCU is to be regarded as 'a new circuit' then a lot more is still notifiable (in England) than we generally think!

However, as I just wrote, I strongly suspect that the feed TO the shed mini-CU (from the main house CU) will be, fairly non-contentiously, a 'new circuit', so that would be notifiable, anyway, even if there were FCUs, rather than a mini-CU, connected to it, and regardless of what was happening downstream of the mini-CU (or FCUs).

Kind Regards, John

Anyway a CU is not required in the shed. Just run straight to the sockets and a FCU for the lights.

winston1 said:

Anyway a CU is not required in the shed.

Click to expand...

It's the easiest way to get RCD protection for the sockets.

winston1 said:

I assumed that there is RCD protection on the main CU in the house.

Click to expand...

"Assumed". What a dangerous little word that is….

Hi everyone, thanks for all the replies. It's funny, I was in two minds as to whether to post or not as assumed it would turn into the usual "make sure you use a proper spark" and "do you need to notify" type debates...

Anyway, picking between all this I've got a better idea now.

@EFLImpudence - I'd considered 4-core as convinced I'd read somewhere having the extra core could enable remote 2-way switching, to say activate shed lights from the house for instance. Can't really see a use for this (and can't actually figure out in my head how it could work with just one core!) but was purely thinking about future-proofing - happy to go to normal 3-core instead. Yes, understand I will get a proper spark in but needed to get a trench dug before the patio went down so didn't have the luxury of time on this! As I was considering using conduit I wanted to have an idea of SWA size, so I could size the conduit accordingly...

@pjcomp - The current main CU has a breaker already in place and labeled up for the "Shed", next to the one for the "Garage" (which has already been completed with SWA and a mini CU.) Back to the conduit, I have pretty much given up on this idea and just dug a 550mm+ trench under the lawn ready for whatever SWA goes in - agree that I may as well just go 6mm, as for the sake of a small price difference its not worth choosing 4mm. Again, due to time and convenience the 25m reel from Screwfix felt like the easiest route - I'll probably just let the spark bring their own.

Regarding whether to have a mini CU in the shed or not - for me it makes more sense to have that level of control, for the sake of a bit more money. Everything is being done from scratch and personally, I'd prefer to have CU isolation at the shed, fed from an MCB in the main CU. It just seems like a neater, tidier approach and should give me more options in the future if I wanted to add outside lights or similar. I'll need to check if the current Shed MCB is on the RCD side of the main CU...

Does it matter if I've got RCD protection at the main CU AND the mini-CU?


Anyway, irrespective of all of the above, the trench is built, I'll probably go with 6mm SWA and I'm waiting for a spark to call me back to come and do a proper design - I just really needed to understand what I was digging up and where!

wizbongre said:

Does it matter if I've got RCD protection at the main CU AND the mini-CU?

Click to expand...

When it's possible, it's better to have RCD protection at the outhouse but not at the house CU (particularly if the RCD at the house CU covers multiple circuits) - since that avoids faults in the sheds taking out several of the house circuits. However, you can't do that if there is any T+E wiring between CU and SWA which is buried <50mm deep in walls - since that would need RCD protection.

KInd Regards, John