Garage conversion - installing a secondary Garage CU

Postby zygo » Mon Apr 07, 2008 2:39 pm

My first post - so of course I need help, please :D

My situation is not unlike some previous postings but there are special factors so hence this new topic. I am converting the rear half of a double-length garage into a study/den. The main consumer unit for the house was installed about four years ago. I now want to add a two-way consumer unit dedicated to the study/den. One circuit will be for lighting (8 x low volt halogens on two transformers, plus a bulkhead incandescent light. The other is for a small ring main of about 8 new sockets in there.

I want to do the physical work myself and then have a certificated electrician sign it off after testing. I propose to fit the new unit on the garage side of the new stud partition wall and run the two new circuits into the den from it. I shall link the new CU with the main / existing one (about 5 metres away) with 40 amp cable. The new CU (A Crabtree Starlighter) will have its own63A-30mA RCCB so I will take the power from from a 40amp MCB on the unprotected side of the main CU.

Should this approach present any problem?

I realise that I could just run a 5 amp and a 15 amp supply into the new den straight off the existing CU but I am short on available ways of the right level and think it would be tidier to have a new CU dedicated to the new den.

All comments are most welcome, gentlemen.

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Simply Build It

Postby ericmark » Mon Apr 07, 2008 8:02 pm

Follow links in Projects and down load Part P and read it. It does tell you a lot about what you can do.

The problem is if an electrician is registered under Part P in the main he has to design, install, and inspect and test. If he is not using the self certification then you can have three signatory.

Inspection and test

But to use this you will have to pay the council building control and before you start work lay out exactly what you intend to do.

You must also prove you are skilled enough to do this. You may well be able to do as you hope but have a read first and then say if you still want to go down that route. I will guess it will cost about £60 to use building control and you still are going to get a spark to check the work on top of that.

If you still want to DIY then find a spark first as many have already had problems where house holders have not done as they should have and getting a willing spark may be a problem.

If you are going ahead by all means ask for more advice I am not trying to put you off. But I do want you to start with eyes wide open to problems not getting caught out half way through.

Postby zygo » Mon Apr 07, 2008 8:52 pm

Thanks for this piece of good advice.

I am already dealing with BC as the conversion requires me to make a Building Notice. Heance the cost is not an issue. But I'm glad the general approach I am making is on the right lines.

Any other observations, anyone?

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Joined: Mon Apr 07, 2008 2:24 pm

Postby ericmark » Tue Apr 08, 2008 12:37 am

Now I know your eyes are open I will try to answer the other questions. 40 amp cable I would consider 10mm² as 6242Y but 6mm² 6242X would take current but temperature considerations would not be in its favour. But timing is also a consideration and after 1st July with no RCD protection it would need to be protected or surface or 50mm depth so SWA may be worth while. With timing a consideration then replacing the existing consumer unit may be better than the problems of protecting the interconnecting cable? I assume you are aware of the new regulations coming in on the 1st July where any cable not either surface or buried to 50mm will need either to be protected by an RCD or by being surrounded by a metal sheaf. Steel wire armoured will be OK or the new Flexishield designed as a cheap alternative to SWA with a light aluminium sheave although I am not sure if available in 10mm² it does however have some of the advantages of FP200 cable being far more fire resistance than normal twin and earth. With the new regulations only being published this year many suppliers are struggling to being able to supply new consumer units etc complying with the new regulations and there is a big push to complete works before that date. There also will be a problem finding electricians qualified to the new regulations and able to sign off work. By Wednesday I will know if I will be one of those few qualified electricians. Wish me luck in the exam.

Postby zygo » Tue Apr 08, 2008 7:57 am

First and foremost, ericmark, good luck in the exam! And secondly, thanks for the July 08 warning. If I read this correctly, however, I won't be affected by it. As the existing main CU is in the garage, and the small one I want to install is also in the garage, the cable linking them will be surface mounted so I think this means it will not need to be protected by an RCD. Of course, I could connect it to one of the MCBs on the (RCD) protected side of the main CU. This would mean that anything on the MCBs in the new CU would be doubly RCD protected (first by the RCD in the new CU and then by the RCD protecting the MCB in the main CU). Would that be desirable or cause a problem.

One final thing, would the small CU need to have its own direct earth connection - I was thinking that a 40A T+E cable link would allow me to use the earth of the main CU (which, of course, is itself earthed by a link to a water pipe).

Keep on studying! (unless you took the exam a while ago and are simply awaiting the result, in which case good luck with the result!)
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Postby ericmark » Tue Apr 08, 2008 9:42 am

You are not allowed to get an earth from the public supply water pipes. But you should earth the water pipes to your earth. As to your earth there are two main types in the UK. TN-C-S and TT the first brings the earth in from the supplier and the second requires you to fit an earth rod. On the second all items will go through a 100ma earth leakage trip.
I had assumed you already had power in your garage and was just changing a consumer unit but to take new power to the garage needs a lot of consideration.
Since you did no mention having two RCD’s I will assume you have TN-C-S in which case type of ground and distance come into play. Also size and type of feed cable. Since you say surface mounted cable I would assume an attached garage and then a TN-S feed from the house would be in order.
Running T&E outside although done is not really a good idea as for one, the PVC is attacked by the sun. All outdoor PVC cables are black to resist the UV from the sun. And also you will need to consider volt drop and Earth loop impedance. The meters to measure the latter are expensive. 40A for a cable does not mean much cables are rated according to size and material and route. As current goes through cable it gets hot and different cables will be allowed to run from 60 degs to 90 degs so a 10mm cable could be rated 40 to 120 amp the latter being special mineral insulated cable.
All electrical work needs testing, some need the test results sending to the council, these include new circuits which from what you say it will be. I would follow the links to Part P and down load the free on line document as well as do’s and don’t it also gives sample forms and some advice and it would be better to do the work with your eyes open to what is required. Maybe you have already done this?
If you have had any electrical work done in the past they will have issued at least a minor works certificate and this will show type of earthing and the earth loop impedance reading.

Postby zygo » Tue Apr 08, 2008 10:16 am

I fear that I might have misled you - my apologies.

The garage is attached to the house; it is a double length - tandem - garage. There once was a four way fuseboard inside the garage (on the wall that divides the garage from the house), earthed to a 22mm copper pipe (I have just re-checked and it's a GAS pipe not a water pipe). This is the way the house/garage was built 30 years ago.

About five years ago the fuse board was replaced by a Wylex 12 way Consumer Unit, which is in the same place that the old fuse board was in before it. The earthing is the same (fastened to the same 22mm Gas pipe with what looks like the original earth cable).

I am converting the far end half of the garage into a study, leaving me with a single length garage when I've finished. I will run a cable from the Wylex Consumer Unit to a new small two-way unit (a Crabtree Starbreaker) to supply the newly converted part. That part always had power, for a single light and a single socket but I want to run more sockets from it and more lights.

The Wylex is a split board with the protected side having an 80A RCD 80 and I was planning to connect the Crabtree CU to it using a 40MCB on the unprotected side (but it could just as easily be on the protected side, if there's any benefot in that). The two-way Crabtree has a 63A RCCB protecting both circuits (a 6A MCB for lighting and a 32A MCB for the ring).

The Wylex and the Crabtree will both be INSIDE the single size garage (even after the conversion) The T+E cable supplying power (and earth) from the Wylex to the Crabtree will also be entirely inside the garage.

Does this make it clearer? My apologies if the first post was incomplete or misleading.
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Postby ericmark » Tue Apr 08, 2008 2:04 pm

In the main all you say is OK. No point in coming off protected side if there is a RCD in small unit under the new regulations sockets and lights for the same area are not taken through same RCD i.e. Upstairs lights with downstairs sockets is thought to be OK but not upstairs lights with upstairs sockets. Unless emergency lights are installed. Not in yet but you might consider if you give yourself a belt do you really what to be plunged into darkness as well?
As to earth on gas it is normal to earth gas to main house earth but no permitted to use gas as main house earth. I would hope you just have not noticed where the main house earth comes in. Under projects Electrical consumer unit it does show main earth wire. But not very well. Near your meter and incoming fuse you may also have and earth connection the cable would normally follow same route as Line and Neutral cables. As an alternative to that you may have an earth rod. The earthing system is decided by the electric supply company as they have to use the same system for all houses in the area. If you think its missing ring the supply company and ask them what sort of earth you should have. In my area if we phone up and ask for an earth they will say they don’t have to supply one which is correct but if you ask them what earth you should have especially if you drop in conversation next doors got TN-C-S then they will normally come out and provide you with an earth block. That’s assuming your supply is underground. Often overhead supplies are TT and you need an earth rod and RCD protection on all circuits. Old regs a 100ma trip new regs everything has 30ma trips.
I have come across too many houses without any earths to say most likely you have missed it. It does need confirming you have an earth. When you complete your work one of the tests is earth loop impedance and that test will most likely show up any faults.

Postby zygo » Wed Apr 09, 2008 10:57 am

By the time you read this, ericmark, I expect you will have your results. I hope you passed (I can be your first customer!).

The small CU is now in place, connected by exposed internal T+E cable to a 40A MCB on the unprotected side of the main CU. All is well.

The earth to gas pipe is a bit of a worry (to a layman, earthing to gas seems a bit reckless but I presume it's not inherently unsafe). The worrying bit is I cannot see an earth rod or any other connection to the gas pipe. It runs from underground to the meter.

I need to look up more about TN-C-S (whatever that is) but the services enter the house underground.
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Joined: Mon Apr 07, 2008 2:24 pm

Postby ericmark » Wed Apr 09, 2008 12:26 pm

TN-C-S is also called PME if that helps.

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