Yet Another Garage Question

[superfish]

Hi everyone,

Long time lurker on these forums and have always valued the honest advice from you lot.

Our garage wiring is a mess and nothing to do with me. It's obviously a combination of previous owners going back likely 40 years or more but we've a mix of cable colours, brown bakelite junction boxes and in one case, what looks like a length of garden hose being used as conduit.

The project is an attached garage with the house CU on the other side of the wall in the house, so a short run of 2.5mm T&E has already been used coming from a 16A MCB to form a radial to a twin socket. After that, it gets hairy and the brown bakelite box is the next stop before it spiders off to security lights, other sockets and suchlike.

What I'd like to do is rip it all out. I'd like to swap the 2.5mm from the main CU for some 10mm T&E (and swap the MCB for a 40A) and connect to an IP55 "garage" consumer unit with a 40A RCD, 32A and 6A MCBs. From the 32A, I'd like to run 2.5mm T&E around the garage clipped to the wall to connect to 4 metal-clad double sockets in a ring final circuit. From the 6A I'd like to run 2.5mm in a radial to an IP66 outdoor socket.

The main reasons I want to do this myself are because a) I'm interested and b) I want to save some money. Mainly a) though I have access to some of the materials through work so my hardware costs will be low. For peace of mind, I've sorted an NECEIC registered spark through work who's going to do final testing, certification and connection to the main CU as a favour.

My reason for running a ring in 2.5mm instead of 4mm as a radial are that the garage isn't huge and I already have a bit of 2.5mm kicking about. I could get the extra 2.5mm I need for £20 (or even "borrow" from work) rather than spending close to a ton for the required 4mm. I'm going right the way round the garage for sockets anyway so even with 4mm, I'd be finishing not far from where I started the run

Does my plan sound okay?

Assuming it does sound okay, my only question mark is do I have to enclose the cable in conduit? I was just planning on clipping to the wall for ease, cost and current-carrying capacity but not sure if there's a building reg that states otherwise. If so, will the reduction in capacity affect my plans or will the 2.5mm still suffice for 32A in a ring final or should I drop the MCB to 20A?

Thanks for any and all advice

 

[winston1]

Way over the top. What about lights?

You don't need a garage consumer unit. If there is no RCD in the house simply replace the 16A MCB with a 32A RCBO. Run the ring main round the garage in 2.5 twin and earth from this point. If there is an RCD just replace the 16A MCB with a 32A one. The outdoor socket does not need a separate circuit, it can be a spur off the garage ring.

Lights? Well you could probably take them off the downstairs circuit, you could fit a 6A MCB if there is a spare way, or you could fit a switched fused spur unit off the garage ring main.

In a garage I would use conduit especially at low level. No need for steel though. 20mm white plastic is OK and cost effective.

 

[AndyPRK]

10mm feed to the garage is OTT. How about 6mm ?

Not a good idea to run an outdoor socket off a 6A MCB. It would trip out.

Either 16A MCB or spur off the ring.

 

[superfish]

I've always preferred to over-spec than under-provision - habit from my data background!

I really think I just wanted to do the neatest job possible and isolate as much as I could. In all honesty, I've likened the current consumer unit to looking like it's got a giant spider squashed behind it - there are cables literally coming out at all angles - so thought the more I could do away from that unit, the better the job would be.

Good point on using an RCBO - didn't think of that, with the current consumer unit being so close.

Lights - also a good point, deliberately left out of my description because the strip light in there has a power source of unknown origin, the downstairs light ring I think.

I was going to use 10mm between the consumer units because part of the cable will be encased in breeze block so I've used table 4D2A and erred on the side of caution. Otherwise, yeah, I'd much prefer to not use 10mm Over-cautious?

Will probably use a 32A RCBO instead - would certainly simplify things, and will certainly use a spur for the outdoor rather than the 6A MCB - I'd specced it with the idea that the lawnmower would draw 5A tops but in hindsight that was stupid. If I use the garage unit I'll probably leave it for a later date and keep the 6A for lights.

Thanks for the advice guys.

 

[EFLImpudence]

I've always preferred to over-spec than under-provision

Obviously but why not 'what is required'?

I was going to use 10mm between the consumer units

A lot of flats have 10mm² supply cable for everything.

because part of the cable will be encased in breeze block

Method C

Over-cautious?

Yep.

Will probably use a 32A RCBO instead

4mm² then.

 

[winston1]

Will probably use a 32A RCBO instead

4mm² then.[/quote]

2.5mm on a ring then.

 

[EFLImpudence]

Yes, of course, but my point was as opposed to 10.

 

[PrenticeBoyofDerry]

Things to consider do you already have RCD protection at the board protecting the garage way?
If not, does your CU facilitate for fitting an RCBO?
If not, you could still use a 32A MCB on 4mm SWA or 40A on 6mm SWA to the garage and have a small garage CU with a 20A MCB for radial sockets on 2.5mm and 6A MCB lights or 32A 2.5mm RFC or 4.0mm radial and 6A MCB for lights.

But get an electrician on board first and the electrician does not need to be Niceic registered, there are other scheme providers!