Underground SWA Advice

[jestor_01]

Hi All, I'd like some thoughts on the cable size to an outbuilding on a large-ish domestic dwelling, pls.

Scenario: Three outbuildings to this property all have separate cu's running t+e socket and lighting circuits. Building 1 has SWA from an MCB on its cu to feed building 2's cu and then building 2's cu has SWA from an MCB feeding building 3's cu. Building 2 and 3 are mainly storage and so only have a couple of sockets and some floodlights / suspended fluorescent lights and so the SWA feeding buildings 2 & 3 (and relevant MCBs) sa you can imagine are decreasing (from what I can see the last cable run from building 2 to 3 is 3 core 4mm)

So thats the background (thanks for reading this far!) The advice I'm after is around what cable should be used from the house to feed the CU in building 1? If I add EVERYTHING I can see up (including full loads of buildings 2 & 3) I get around 12000 watts / 52Amps (80% would be lighting as there's a lot of 60w fluorescents / 500 & 150 PIR floods) There's also a welder & compressor in building 1. There's a fair amount of diversity as nowhere near all of this would be on at the same time.
Should 16mm or 25mm SWA be used between house and building 1? The run from house CU to first building is 80m and is to be buried (in ducting)
Also, 16 or 25mm in to the outbuilding CU is easy enough but as I'm fairly new to this so any advice / experiences on terminating that size of cable at the house end would be appreciated (25mm on to a MCB / buzz bar??!)

Tried to include all relevant info without waffling (too much) but if you need to know anything just let me know - and I appreciate any help.

cheers,
Jest.

 

[Spoon]

Hi Jest. First, welcome to the forum.
Are you an Electrician or DIYer? The reason I ask is if you are DIY then we have a DIY section and can get the post moved to there.
That is running from the house CU to building 2 CU at the moment?
You say that the total load will be about 52A (with everything running). What are your intensions for connecting back at the main house and what size is the DNO fuse?

 

[jestor_01]

Hi Spoon, thanks for the welcome. I'm a spark but fairly new and not hugely experienced which is why I came looking for some advice..
As it stands from house cu to outbuilding 1 cu is armoured but only about 8mm and approx 30 years old which is why it needs replacing.
The intention is to replace the 30 year old ~8mm cable with one of the right capacity (16 or 25mm?) and it will be re-buried in ducting. DNO fuse is 80Amps
Also, would you recommend connecting the new SWA direct to the house cu on a 40 /60A MCB (correctly cleated & glanded) - my lack of experience is causing doubts regards cable diameter on to the MCB / neutral / earth bar.
Thanks for your reply.

 

[HandySparks]

Welcome to the forum.

You haven't said what the earthing system is.

You need to make an assessment of what the likely maximum load will be. Even 25mm² won't quite do, for 3% VD over that distance at 52A. And, of course, you need to allow for VD in the final circuits. Changing the 500W floods for 50 or 60W LED would make quite a difference if there are enough of them, at a considerable cost.

Modern MCBs will take 25mm², not sure about neutral and earth bars. Best to take a sample of cable and try it in the CU. You could always terminate the SWA into a galv box adjacent to the CU and run a short length of, say, 10mm² into the CU. If the CU is plastic, then terminate the armour into a galv box or similar anyway for a cable of that size.

I happen to like these, especially for joining dissimilar cables, where the joint is accessible.
Buy Connection & Termination Blocks Z25-1 1pole connection/termination block RS Z25-1 online from RS for next day delivery.

 

[Spoon]

Hi Jestor. If you are just starting out then we are happy to help.
As you are new and wanting to progress then I am presuming that you have a copy of the Big Green Book... Look at pages 338 & 339 to see what size cable you might need. Page 339 will give you the figure to work out your voltage drop.
If you have the Big Red Book then the pages are 280 & 281.
You need to do a bit of work yourself to learn what to do. You will need to know the max amps you will be using. You said about 52 in your first post, but you need to make sure. Then you pick which breaker you want, then the cable.
Of course there is a lot more to it but lets see what you come back with and why you have explain why. If you try to learn then you will get on here.

 

[ipf]

Hi Jest. Earthing system first, as above.

I'll not get into cable ratings as I'm sure others will be commenting.

Regarding the outgoing supply, I'd be looking at splitting the supply, Henlys if it's a domestic situation and room available, as a starting point. Metal clad switched fuse/cb feeding outbuildings.

Load wise, plenty unknowns, especially with a workshop as indicated. A bit of work and thinking required from your side. Good luck.

 

[telectrix]

as above. i'd definitely take the feed form henleys, before the house CU. 100A D/P isolator from meter, then into henleys. tails to house CU and tails from henleys into a 60A SW. Fuse/MCB then 16mm is fine for current. you then may need to up it to 25mm to get acceptable VD. from what you say in the OP, diversity will bring down your design current to around 30A ( rough guesstimate).