Underground SWA Cable

[skyline2500cc]

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]

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

 

[skyline2500cc]

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]

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]

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]

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]

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

 

[skyline2500cc]

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

 

[skyline2500cc]

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

 

[Bobster]

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]

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

 

[Darkwood]

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]

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]

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]

i thought christ was 2000 years ago.