Shed Electricity

4 Sep 2008
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United Kingdom
I'm looking in to taking electricity into my new shed. From what I've read in various books and online, this is my plan (not in order):
1) Fit a new 30A MCB in the main consumer unit, RCD protected.
2) Run ordinary 2.5mm^2 3 core cable from the new MCB to a second consumer unit in the shed. The cable will run down the side of the house, into a plastic conduit and then underground for about 8ft under a paved path, at a depth of 2ft. It will then continue, still on conduit, on a garden wall above head height for about 10ft and then into the shed.
3) The shed consumer unit will have it's own RCD protection, and a 30A MCB for the sockets and a 5A MCB for the lighting circuit.

How does this sound? Should it meet the wiring regs (BS 7671)? Also, can I get hold of the regulations online or is this gloing to involve a trip to the library.
Any comments/suggestions welcome.
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1. You have two 30mA RCDs in series. Not nessecary and not to regs.

2. You have two 30A MCBs in series. Not nessecary and not to regs.

3. Plastic conduit offers no mechanical protection. You need steel conduit or armoured cable for the entire outdoor run, no conduit nessecary.

4. 2.5mm² cable of any description IS NOT RATED TO 30A

5. Where exactly will you find 30A MCBs for a modern CU?

Take a non-RCD feed from the house into an RCD CU in the shed (the cable doesnt need RCD). This way, when your drill trips it out, you dont take out half your house, and you're standing next to the RCD to reset it. I also suggest the use of an RCBO in the shed, so your lighting in there is non-RCD. It doesnt need to be.

You got one bit right - the 2ft depth.
I believe that when running cable underground it is neccessary to use SWA. If not then it is certainly a good idea.

There would also be no discrimination between the shed 32A MCB and the 32A MCB in the main consumer unit supplying the cable for the shed. You could look at swapping the shed MCB for a 16A one in this case.

Also from your description you will have a radial circuit so cannot use a 32A MCB to protect 2.5mm PVC cable.

Don't forget to take account of volt drop while desinging the installation.

Also this work is notifiable under Part P.
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