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Advice Needed for Home CU to Garage CU

Postby Jbol1985 » Thu May 23, 2019 5:56 pm

Hi all

After a little confirmation with a job please.

I plan to install a consumer unit in my garage. It will eventually be powering a 32amp hot tub (outside garage) ,2 double sockets and two LED lights.
The distance of the garage from the house is 10m. I plan running the cables under my concrete drive once I’ve cut a channel.

1. Is it ok to use un sheathed single cables inside 20mm steel conduit under the concrete drive exiting inside the garage?

2. What size RCD/ RCBO is advisable to use inside the house consumer unit to supply the garage consumer unit?

3. What size single/ T&E cable is advisable to use inside the conduit?

I’m an ex electrician who’s been in the navy for 11 years so confident I can complete this work myself. Any answers to my 3 questions would be much appreciated Thankyou.
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Postby Mr White » Thu May 23, 2019 10:03 pm

As it is an additional circuit in a special location it comes under part P, which means you can not install it your self*

Also, you do not run conduit under concrete, it will corrode, (and plastic conduit will give no mechanical protection) since you can't use conduit your other questions are void.

I would however suggest you use SWA cable.

* You can, but it has to be designed and checked by the person who is going to sign it off, or you can pay the council a large fee and they will sub it out and you get it signed off
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Postby ericmark » Mon May 27, 2019 3:44 am

Twin and earth is not rated for outside use, you would need a cable designed for outside, in the main that would be steel wire armoured cable.

20 mm is too small to really serve as a duct, one would normally use a plastic pipe so large enough to pull a new cable through if ever required.

Size of RBCO or RCD+MCB is dependent on what will fit, some consumer units will take a 45A unit but many will not take over 32A.

Cable size depends on two factors, one is current rating, the second is volt drop, the current rating varies on any cable depending on insulation material be it thermal plastic, thermal setting or mineral insulated so no quick answer likely either 6 or 10 mm but need to look on tables etc once you know what MCB will be used, often MCB in house consumer unit and RCD in garage consumer unit.

You would need to draw up plans, apply to the county council building control and pay fees, with may include paying for a selected by council electrical contractor to inspect the work, although in theory you can DIY in practice it can cost more to DIY than get a scheme member to do the work.

There is often a charge starting at first £2000 worth of work which is around 10% plus vat plus and charges from non council inspectors so looking at paying council around £300 to be allowed to DIY plus you need to show the council inspectors you have the skill.

I wanted to do a job with my son on my dads house, both my son and I hold C&G2391 which shows we have the skill to inspect and test, the LABC would not at first allow us to DIY, after my son insisted that if anyone was to be testing our work they need to have better qualifications than ourselves, he finally allowed us to do work, but I had to inspect and test, as I had the degree in electrical engineering he would not permit my son to test even with his qualifications.
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