What size SWA for mains supply

[xmgcx]

I am self-building a new house, I am just digging in the main sewers etc and whilst doing this I really want to bury a suitable SWA cable which runs from my meter box in the garage to area of the consumer unit in the house, this is the main supply cable to the house.

I have not yet got to the stage of locating an electrician but whilst I have a jcb on site I would like to get the main cable buried so its all ready for an electrician.

Total length of this run will be 25 metres.

My question is what size SWA cable should I use, looking at the specs of these cable I am fairly certain I need to use 35mm ?
Secondly how many cores should I use, do I need 2 core, 3 or 4 core?
Is it best to use 3 core so I can have one for the earth? 3 core colours do not seem to include a blue wire just brown, grey and black is that still okay ?
Also is it best to run a seperate earth wire alongside the swa?

Just really want to do what ever is best, so when I later get a sparky in I have buried the right cables.

Appreciate any advice.

Cheers

 

[Adam_151]

The best thing to do would be to probably put in civil engineering duct with a strong rope through it, then the sparky can pull in whatever cable he wants.

Assumming reference method 3 then 25mm² would be sufficent, but 35mm² passes by a greater margin and leaves more volt drop for final circuits; 25mm² drops 4.375v at 25m, 100A. 35mm² drops 3.125v (the macimum volt drop in total must be no more than 9.2v [4% of 230v])

I'd use 3 core, it'll lower the Z's at the board which is always a good thing (the armour on 25mm² SWA adds approx 0.1 ohm over 25m, using a core gives about a 5th of that fiqure, and using the armour and core in parrallel drops it a little more)

As you can see, its quite well linked with the design of the final circuits, so no one can give you a definative answer, I would put my neck out and say that I cannot envisasge a situation where 35mm² 3 core would not leave enough 'scope' (unless the supply was 3 phase, or over 100A), however 25mm² core might be appropiate, so with all that in mind, I'd stress that the duct would be the best plan, of course it should be burried at the correct depth (deep enough not to be in the way of any likely disturbance of the ground, recommended 500mm) also I'm unsure at what depth (someone else adviose please!) but the yellow tape that warns of cable above needs to be burried above it, take 'in progress' pics of this to show to the sparky

3Core SWA is coloured for 3 phase and no neutral use, however it can and is often used on single phase with the ends being oversleeved with different colours

 

[xmgcx]

Cheers Adam, thanks very much for that, I think I will either put a duct in as you suggest a go for 3 core 35mm2

regards
Mark

 

[scousespark]

I'd agree with Adam and put a rope in the duct. The spark will then use that to draw the cable, after properly specifying what you need.

 

[JohnD]

It seems to me that:

- you are going to need a duct anyway

- you have the digger on site

- you don't know for sure yet what cable your electrician will recommend

- you aren't ready to use the cable yet

So from my point of view, this is a good time to dig the trench and lay the duct, but no need to buy the cable yet, especially as you may overspend and buy something needlessly big (or underspend and buy something too small). Even if you buy the ideal size, the cashflow has taken an unnecessarily early hit.

I agree about laying the duct and putting a rope in it.

While you're trenching, you might thing about an additional duct or two for e.g. telecomms; remote switching of garage lighting or electric gates; CCTV cables; who knows what might be handy later?

 

[xmgcx]

Cheers John,

Yes good idea, I will put an additional smaller duct in so that I can use it for alarms & stuff.

I have already put a duct the other side of the meter as requested by the local electric board for the supply from the mains to the meter box at the garage so i'll do the same thing from the meter box to the house.

I have loads of the yellow warning tape left over so i will stick that a little bit above the duct, so should I or anyone accidently dig near it hopefully we will see the tape before its too late!

I'll also take several digi photos of it all as I am with the sewers to aid locating them in the future.

Thanks again for you help, much appreciated.

regards
Mark

 

[Lectrician]

Where in the house is the CU going? An armoured cable will need to terminate into a metal clad CU, and the cable will need to be surface.....not ideal in a hall or WC!

 

[xmgcx]

Consumer Unit will go in utility room and might/could easily placed in a cupboard so hopefully will be okay.

Kind Regards
Mark

 

[Adam_151]

Maybe someone like Lec can answer this, but how difficult is 25/35 mm² SWA to terminate, I imagine that it'd be difficult to manover into the right place to mark it to the right length etc, then to do the actual terminating, is it pretty arkward, or is it easier than you'd imagine?

 

[RF Lighting]

25mm and 35mm cables are quite difficult to terminate until you have done a few big cables, when you learn how to bend the cores and get used to working with bigger stuff. Personally I have no trouble terminating 35mm SWA. Like everyting, practice and experience make a huge difference.

250mm 4core SWA is a bit more difficult though (but you do get bigger switchgear and spreader boxes for that stuff)

 

[Lectrician]

Ive done the lot luckily. 400mm single core alu-armour is my largest. Commonly run submains in 70 - 185mm 4c - all good fun.

35mm would be tight in a standard CU, and some will not be able to accomodate a 35mm erth in the earth bar.

You need to make sure the cable is formed well - not stressing the terminals they are in, or trying to force something from the dinrail.

When getting metal clad boards, don't get the ****e ones. MEM ones are not great unless you go for the type 'a' board which is built to the same spec as the type 'b' 3 phase boards. The 'standard' metal clad are very thin.

 

[jeepmadmike]

630mm singles is my biggest to date its a 300 4c is fun , 35mm is easy if its an electricain who is used to it, some will make a pigs ear of the job! Sounds to me like it needs to be designed correctly and then you can be advised approperatly, 16mm might be big enough even....what size is the supply you are getting?

 

[JohnD]

...When getting metal clad boards, don't get the s***te ones. MEM ones are not great unless you go for the type 'a' board which is built to the same spec as the type 'b' 3 phase boards. The 'standard' metal clad are very thin.

I think you mean the grey "Memshield 2" ones which are industrial rather than the cream "Memera" domestic/light commercial range? Funny thing is, the internal devices are the same size and are interchangable (though the domestic ones have a lower fault-breaking capacity).

 

[Lectrician]

The domestic breakers will NOT fit the memshield 2 boards - the terminals are narrower. The memshield 2 breakers WILL fit the domestic board

I would always go for the memshield 2 boards.

 

[JohnD]

The domestic breakers will NOT fit the memshield 2 boards - the terminals are narrower.

So they are! never tried that way though.

I prefer the memshield or equivalent rebrands, as you can put the RCBO pods on them, and they are very widely available, one way or another.