17m x 15m Car Enthusiast Domestic Garage

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Dec 7, 2022
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Aberdeen, UK
The garage is 17 mtrs x 15 mtrs in the UK

The house has a 100 amp supply to the house consumer unit.

The garage consumer unit is fed from a 63amp RCD to a 40 amp B rated mcb in the house consumer unit.

The feed cable to the garage is 10mmsq SWA 3 core with the swa bonded to the earth conductor at both ends via the earth bar on the consumer units.

The garage consumer unit has a 100 amp main switch with two 63 amp RCD's which in turn feed 4 mcb ways each.

All the lighting is by LED with a load around 2 amps.

The garage is heated via an oil burner using less than 1 amp of electricity to run the burner controls.

There is a 3kw air compressor fed via a 20 amp B rated mcb in the garage consumer unit via 2 mtrs of 2.5 twin and earth with no issues.

There is a car lift fed via a 32amp b type b mcb via 15 mtrs 2.5 twin and earth.

The question I have is about sockets.

The garage has an internal area of approx 255 sq mtrs

The guidance in the UK tends to be to use a ring circuit for sockets with a max floor area of 100 sqmtrs per ring.

Therefore going by that guidance, it suggest that we have three ring circuits 5.7 mtrs wide by 15 mtrs deep via 32 amp mcbs but I am thinking of reducing the mcb rating to say 20 amps. Run with 2.5 twin and earth.

The main reason I am doing this is to prevent the owner who thinks he knows better and has no interest in keeping to any electrical regulations.

Does anyone have any issues with what is proposed.
Thanks for the reply, the guidance on radial circuits seems to be for a a maximum of 50sqmtrs floor area per for 2.5 twin and earth radial circuit with a 20 amp mcb or 75sqmtrs with with 4mmsq twin and earth radial with a 32amp mcb or as you suggested rcbo's to stop the rcd switching off all the attached mcbs on that part of the board in a fault condition on one mcb.

The client wants the 17mtr front face of the garage to have a say 12 single metal clad sockets. That is 15mts from the far end to the consumer units then the next 2 mtrs to the corner, continuing down the 15mt front to back wall and again another 12 sockets and a further 12 stretching along the back wall. He has no requirement for electrical sockets on the final 15mtr return wall.

How many radial circuits would you suggest in all reasonableness?
The guidance in the UK tends to be to use a ring circuit for sockets with a max floor area of 100 sqmtrs per ring.
1/ that's really aimed at houses.
2/ it's guidance,ergo, drivel!

Your only actually limitation is the Max Zs for the cct, so calculate the voltage drop you are likely to encounter on the cable runs and work with that. As per @Murdoch I would run radial ccts on RCBOs, probably one each side of the garage, and no more than that.
That sound more reasonable than the boilerplate guidance, do a final check of what max load could be expected to be put on each of the two sides of the garage, then run each side of the garage sockets with a 20 amp breaker via 2.5mm twin and earth.

Thank you both!
I'd be looking at that circuit for the car lift.
The car lift has been working for two years without an issue.

I put a clamp meter on it the other day as someone brought one along with them, it recorded 20 amps at startup and went down to 12 amps after say 10 seconds. I 'm well aware that inexpensive clamp meters have a start up lag before they register a reading.

My own car lift used to work of an C20 mcb but now after I had the garage certified by an approved electrician for my local council completion certificate. He changed he changed the mcb to a B32. As I got an electrical certificate from him that the local council accepted and gave me a completion certificate for my garage. I did not question the sparky.

What would you look at in the car lift electrical installation mentioned in the original post?
It worked on a C20 but they changed it to a B32, that is a bizarre thing to do, the cable is not rated for a 32A protective device.
It worked on a C20 but they changed it to a B32, that is a bizarre thing to do, the cable is not rated for a 32A protective device.
Sorry I did not mention my home lift is wired with 6mm twin and earth possibly a bit on the heavy duty side.

On the original garage we were talking about, I would have used 4mm twin and earth with a C16 breaker but it was wired by someone else two years ago.
I would put 4mm radials in probably 2 off. Then you could fuse at 32a. Up to you if you choose to fuse them at 20a to start with.

I would use rcbos rather than a split load board more as a safety issue so a group of circuits did not go off which could cause an injury.

If the sub main is wired in swa then it's fine on an mcb but you could go belt and braces and put a 30ma time delayed rcd at the mains to cover that circuit as additional protection.