Outbuilding issues

[MRP]

Hi all.

First off what a good newsgroup and for the UK!

Have an outbuilding i.e. somewhat falling down big shed, which I want to put in electrics, couple of sockets and lights - woodworking use.

I know I need a csu with RCD for the building with seperate MCB's for light and sockets.

My concerns are :-

1. My house csu is a cartridge type without RCD or MCB's, should I upgrade this? Why? So the cable from house to building will receive protection.

1. What cable size & fuse for a run of 8 metres? Also, if I upgrade the house CSU what RCD threshold should I use for this cable run?

2. Earthing - House to building earth should not be connected to the building csu I believe, where should it connect (if needed)? I know the building csu must be earthed to an earthing rod, I'm just concerned about the house to building earth.

1. Is it ok to run the cable down an existing socket conduit in mu lounge? I want to do this to avoid having to chase a new conduit in my newly decorated lounge! The idea being is that I'll drill the hole to outside of the house behind this socket.

Any assitance much welcomed.

[Christian McArdle]

You should anyway, but don't actually need to do this to install the shed. You can always run the shed off a separate box. This would involve inserting a Henley block before the current CU. Separate tails then go to a switchfuse unit for the shed run.

However, it is probably better to install a new 100A CU and just run the shed off a 32A MCB or similar.

For such a short run with a 32A capability, TLC calculates 2.5mm XLPE SWA. However, this seems a little small to me. I'd use 4mm or even 6mm, for mechnical protection and reduced voltage drop if nothing else.

I'd install the cable on the non-RCD side at the CU (or on a switchfuse, if applicable). At the shed end, I'd go into an insulated CU with a 30mA DP RCD as the incomer, followed by a 20A MCB for the sockets and 6A MCB for lighting. If the woodworking use involved dangerous machinery, such as table saws, I would use a 100mA time delay DP RCD as the incomer. The socket circuit MCB would be replaced by a 20A/30mA RCBO.

There seems to be some disagreement about this. I suspect it would be fine isolated at the shed end or not, provided a good local earth rod is installed. However, the armour MUST be connected at the house end.

You may need to derate cables to do this. The existing cables may well still be OK after the derating, but some calculations may be required. There is no need to run the SWA internally. Just convert from T&E to SWA at the point you leave the house.

Christian.

[chris French]

In message , MRP writes

See the current thread about shed electric's as well.

[Matt Pearson]

What sort of protective junction boxes would you recommend. Does SWA have glands and require specialist fitting tools?

Any suggestions would be most welcome. The cable is supplying the socket ring. Would the 2 cables running side by side be a cause for concern?

Many thnx

[Christian McArdle]

Do the junctions inside. Avoid external junction boxes if at all possible. SWA does need glands, but internal glands are much easier than ones buried

1m underground.

Probably not. However, they might. We need to know how the cables are installed. We know they are in conduit, but is there any wall insulation? Do you know the thickness of the original cable. Also, is it a ring or radial? If a ring, do both cables go through, or just one?

Christian.

[chris French]

In message , Christian McArdle writes

For the junction box on mine inn the garage I used a metal adaptable box, with choc block connectors inside.

The SWA glands don't require any specialist tools. There was a discussion with some posts giving the method of using the glands here a while back