Wiring Garage

[deckertim]

A friend of mine(part p aware) wants to rewire his garage because it currently doesn't have any RCD protection. I am sure he will get part p building control approval etc. etc.

There is currently a small CU in the garage that is fed by a single

2.5mm t&E cable about 5 metres long. This is connected to the main house CU via a 16A mcb. The garage CU has a 16amp mcb for an electric feed to a double socket, this also feeds a fused spur for an electric garage door. CU also feeds a 6A mcb for the lights.

The new Contactum CU is an RCD protected unit with 32A and 6A MCBs.

Any issues with having a 32A CU fed from a 16A MCB. In the event of overcurrent, it would be inconvenient to have to go in the house, but the main reason for fitting this is to get RCD protection.

Are MCBs from different manufactures interchangeable? If so, I suppose these could be swapped over. Alternatively it won't break the bank to get a contactum 16A MCB for the garage CU.

The intention is also to have a radial circuit with a few double sockets feeding off the CU. The Whitfield book indicates this is ok. However all the diagrams I have seen show

CU-----[socket]------cable--------[socket]---------cable-----[socket]

is it permissable to do a star type arrangement. Where two or more spurs lead off in different directions. I

CU-----[socket]------cable--------[socket]---------cable-----[socket] |----cable--------[socket]---------cable-----[socket]

I couldn't see anything wrong with this because the circuit would still be protected by the 16A mcb. But I would welcome any advice.

Thanks Tim

[Christian McArdle]

You shouldn't really. It might be confusing.

It will be hard to guarantee discrimination. However, you could go some way by replacing the 16A MCB in the house with a cartridge fuse carrier.

Physically, they usually are. However, guarantees and homologation may be affected and it isn't worth doing.

Any arrangement you can think of will be fine. However, I would be tempted to run it as a ring in 2.5mm. Then it can be upgraded to a full 32A ring in the future, simply by replacing the main 2.5mm feed with a 6mm+ cable (SWA armoured if it goes outdoors, T&E indoors).

Christian.

[Andrew Mawson]

electric

electric

suppose

CU-----[socket]------cable--------[socket]---------cable-----[socket]

CU-----[socket]------cable--------[socket]---------cable-----[socket]

If there is no other way into the garage I would be nervous having the electric door opener on an rcd

AWEM

[meow2222]

I fail to see how this would be a constructive priority for his time, attention and money, unless there is some special reason that an rcd is really needed. There is more risk in doing the diy than there is in not having an rcd.

If he still going to do it, replacing the double socket with an RCDed double socket would be far easier. Having your lights go out when an rcd trips is not a safety feature. If he wants more sockets in future, extension lead into that socket is fine.

NT

[John Rumm]

It might be better to use the 16A MCB in the garrage, and get a 20A cartridge fuse holder for the house CU.

In many cases yes - but it does depend on the brands in question.

I would not be as keen on that since the 2.5mm^2 cable would not be adequately protected by the 32A MCB. (it would give adequate short circuit protection, and you could argue that the cumulative total of the downstream circuits is low enough to provide the overcurrent protection, but it would still be a non standard way of doing things. It also relies on the MCBs in the garrage CU not being changed in the future. Another limitation is loss of discrimination - i.e. chances are you will trip the feed to the whole CU and not just a circuit on it)

Yes this is ok, since any segment of the cable should be protected from overload by the CUs MCB.

[deckertim]

I fail to see how this would be a constructive priority for his time, attention and money, unless there is some special reason that an rcd is really needed. There is more risk in doing the diy than there is in not having an rcd.

I suppose it depends on who is doing the diy. My "friend" is a trained electronic engineer with several years experience. So there would actually be much more potential danger in having extension leads lying around in the garage. Equipment plugged in outside, such as fountain, lights etc.

But thanks anyway for your concern for his safety!

[Roger]

The message from "Christian McArdle" contains these words:

No always. My current electrical refurbishment fell foul of that. I was going to use my old CU in the barn (workshop/garage) with RCBOs to protect the ring mains but the MK RCBOs wouldn't fit the Vynckier CU. Not only was the RCBO too deep it wouldn't clip on the rail either. I was fairly sure (wrongly) beforehand that these were all to a standard as I already added a MEM MCB to the old unit which looked out of place but fitted perfectly.

Anyone want a 12 year old Vynckier CU with 8 MCBs plus one spare?

[The Medway Handyman]

Why don't you ask Alan Bryant at the next magic club meeting? :-)

[meow2222]

no-one suggested having extension leads lying around. If hes a tron eng he should see the issues more clearly.

NT