Underground SWA Cable

skyline2500cc · Jan 13, 2015

A new supply has been installed by SSE and its a single phase TN-C-S in other words neutral and Earth combined at the cutout or point of supply.
Meter tails have been installed to a distribution board which feeds one circuit. From that distribution board we need to run a SWA conductor from an MCB, 600 meters underground to an outbuilding which will have another distribution board to feed additional circuits.
We have looked into the cost of a 3 core SWA copper cable and its between 4-5K. I was wondering as the earth and neutral are combined do the regulations permit you from running a 2 core i.e Live and Neutral and using the SWA or (CPC)
as the earth.
Look forward to any advice
:icon12:
Skyline2500cc

telectrix · Jan 13, 2015

what size conductors in the SWA? generally armour is good for cpc up to 95mm or thereabouts.

skyline2500cc · Jan 13, 2015

Hi Telectric
Thanks for your response we are looking at 70mm2 CSA. The question I would like to know is there any technical, or regulatory reason why the SWA could not be used, saving us the expense of the third copper conductor.
Regards
Skyline2500cc

ImpededLoop · Jan 13, 2015

600 metres is a long run for an LV cable. I'd imagine that volt drop will be your biggest challenge.

With regard to earthing, you can always make the remote end a TT installation so you're not reliany on the TNCS earth.

ImpededLoop · Jan 13, 2015

skyline2500cc said:
Hi Telectric
Thanks for your response we are looking at 70mm2 CSA. The question I would like to know is there any technical, or regulatory reason why the SWA could not be used, saving us the expense of the third copper conductor.
Regards
Skyline2500cc

It is permitted, but you'll need to do all the necessary calculations to ensure it complies with BS7671. If you have any extraneous parts in the outbuilding you'll need to consider bonding them. You may be better to use TT, but the volt drop will still be a big factor to consider.

Leesparkykent · Jan 13, 2015

70mm 2 core SWA has a CSA of armouring of 84mm2 ,with 70mm 2core the required CSA of the armouring would be 79mm2 so using 70mm two core SWA will be fine.

Leesparkykent · Jan 13, 2015

Out of interest, what is the required load of the out building that is 600m away from the supply?

skyline2500cc · Jan 13, 2015

Hi Leesparkykent
We anticipate the load to be around 5000 watts max, the supply is only good for 8000w

skyline2500cc · Jan 13, 2015

Sorry a correction on the cable length its actually 400 meters not 600 as originally mentioned

Bobster · Jan 13, 2015

The armoring will only be suitable for the circuits CPC, not if this building needs any extraneous metalwork bonding.

Just be aware of your Zs for a circuit this long, in the past I have used enhanced conductive SWA, one armor strand in 5 was copper coated to get around it.

richy3333 · Jan 13, 2015

600m Thats from one end of Inverness to the other!

Darkwood · Jan 13, 2015

Our advice is baseless until the OP divulges the calculated load of this shed and also whether it has structural steel and/or other services in metal.... as it stands the advice given has no bearing until we know these points.

Also a discussion with the DNO will be needed to enquire if additional earthing would be required due to the Network set-up if this other building requires any sort of bonding.

So lastly what is the CSA of the SSE supplied N to the service cutout?

At these distances 70mm might give you enough power to boil a kettle and be within acceptable VD of 3% assuming lighting load.

Knobhead · Jan 13, 2015

RoB2 said:
Just be aware of your Zs for a circuit this long, in the past I have used enhanced conductive SWA, one armor strand in 5 was copper coated to get around it.

I didn’t think they still made that.

I was looking for some before I retired to extend a PILC. I gave up in the end and took a separate CPC out of the end of the through joint. It looked like a bizarre breach joint.

Bobster · Jan 13, 2015

Depends how deep your pockets are, I had 300m at an eye watering amount the last time I used it.

Christ must be 10years ago now, doesn't time fly.

telectrix · Jan 13, 2015

i thought christ was 2000 years ago.

Engineer54 · Jan 13, 2015

Leesparkykent said:
70mm 2 core SWA has a CSA of armouring of 84mm2 ,with 70mm 2core the required CSA of the armouring would be 79mm2 so using 70mm two core SWA will be fine.

Only as the cables CPC, If any main bonding is involved then it will not comply!! BTW the CSA of armouring on a 2 core XLPE swa cable (which is now what you would receive from the wholesalers unless you specify PVC insulated) is 80mm...

With the sort of distances were talking about here, i'd still be calculating if that SW armouring is of a suitable size, even as a CPC!!

skyline2500cc · Jan 14, 2015

Hi Darkwood
We have calculated the load to be approx 5Kw and its actually a service building, so it will have some building services which will need to be earthed.
As I understand it if we cannot obtain the desired Zs value from the DNO, then we would install a separate earth and treat the service building as a TT system.
Is that not how you see it?. I look forward to hearing from you
Skyline2500cc

Darkwood · Jan 14, 2015

Can you show your VD calcs please and is lighting load included - at that distance you may be pushing it with 5KW ? I did a quick rough calc and get nearer 3kw hence my comment on the kettle.
For us to comment we need to know how your arriving at these figures.

Your use of the word Approx also means I would be figuring room for expansion within reason

What is the measured Ze at origin and what calculated Zs do you get + what is the nature of the OCPD and its type.... rather than using terms like 'if we cannot meet Zs' why not estimate it once you have selected your cable of choice.

richy3333 · Jan 14, 2015

OK, maybe a dumb question but wouldn't it possibly be cheaper to get Hydro to put in a separate/new supply into this building (subject to the overhead cables, transformers etc)?

SPARTYKUS · Jan 14, 2015

richy3333 said:
OK, maybe a dumb question but wouldn't it possibly be cheaper to get Hydro to put in a separate/new supply into this building (subject to the overhead cables, transformers etc)?

Must be worth a shot!

skyline2500cc · Jan 14, 2015

The charges for works associated with new supplies are extremely expensive here, and hence why we are looking to putting an underground cable in.

shanky887614 · Jan 14, 2015

have you checked how much if you dig the trench and supply the cable etc.

yeah its expensive in inverness they must have spent a fortune when they upgraded all those overhead lines.

i drove past it last year and they dwarfed the old ones (are they 133kv or higher im not what exactly, we dont touch distribution)

SPARTYKUS · Jan 14, 2015

Good advice above ^^^^^^^^^

shanky887614 · Jan 14, 2015

my great aunt owns a chalet up in avimore a short drive from inverness so i can understand when you say its very expensive.

just so we are on the same page, how much were you quoted for say a 60a supply?

telectrix · Jan 14, 2015

one of my clients needed a 3 phase supply. 400 yards from nearest pole. he dug the trench himself. still cost him 15K.

skyline2500cc · Jan 14, 2015

We already have the supply and it was installed by others with cost unknown. We install new LV plant and historically it can cost thousands to get a supply from an existing transformer to a new location.
I was once quoted £600 to remove the meter and cutout so that we could replace the backboard with new because it was wood and had rotted away.
It took the guy two hrs tops from leaving his premises to returning.

Silly Sausage · Jan 14, 2015

shanky887614 said:
have you checked how much if you dig the trench and supply the cable etc.

yeah its expensive in inverness they must have spent a fortune when they upgraded all those overhead lines.

i drove past it last year and they dwarfed the old ones (are they 133kv or higher im not what exactly, we dont touch distribution)

Do you mean these Muther F£$%ers?
Benvironment ? Scotland, meet your giant new pylons

So they can flog us all their Green power.

shanky887614 · Jan 14, 2015

Archy Styrigg said:
Do you mean these Muther F£$%ers?
Benvironment ? Scotland, meet your giant new pylons

So they can flog us all their Green power.

thats the one.

i past them on a drive from glasgow to avimore.

and its 400kv, i thought it was a big one

Knobhead · Jan 15, 2015

Archy Styrigg said:
Do you mean these Muther F£$%ers?
Benvironment ? Scotland, meet your giant new pylons

So they can flog us all their Green power.

Bloody typical environmentalists. We need more renewable energy but we don’t want the means to transport it.

They’d moan if a damn big trench was dug to bury cables.

telectrix · Jan 15, 2015

al i can say to these whingers is " bin your washing machine/tumble dryer/range cooker. do the washing on a stone by the river and cook over an open fire in the back yard".

SPARTYKUS · Jan 15, 2015

telectrix said:
al i can say to these whingers is " bin your washing machine/tumble dryer/range cooker. do the washing on a stone by the river and cook over an open fire in the back yard".

Youve been to Shropshire haven't you, Tel!