Help on upgrading garage consumer unit for conservatory


Postby murras22 » Mon Apr 11, 2011 6:53 am

Hi,

My main house 10 way split consumer unit, 100A main with with an 80A RCD which supplies power and lighting to my garage. Our house is a large 4 bedroom detatched.

The garage consumer unit has which has a 100A main with a 16A and 6A MCB for lighting and 3 double 3 amp sockets with an outside 13amp weatherproof socket.

My main household CU is full to capacity and due to it's location cannot be upgraded unless our new 2yr kitchen is dismantled and this is a disruption we want to avoid.

My query is can the garage CU be upgraded to supply another consumer unit for a conservatory that we intend to have built which wil supply power for the following below. I've been told by a few local electricians that this could be done but I want to make sure as I'm a bit sceptical os any advice will be greatly appreciated.

The conservatory will have no more than 4 double 13amp sockets, 1 outside weatherproof socket, 1 fused spur for an electrical roof vent opening and 3 lights.

Also we will be seeking power for underfloor heating which for the size of our conservatory would require a 16A or 32A power supply

In the future and if finances allow we would also hope to have a hot tub which I'm told will require a 16A or 20A power supply.
murras22
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Postby ericmark » Mon Apr 11, 2011 3:56 pm

Not really enough information. The garage I would assume is feed from the house. This cable will be protected by some overload device and you need to work out what size this is or can be. Since you say it has a 16A and a 6A in garage one would assume the supply would be bigger that largest MCB it feeds so 20A or more.

So if it was 20A and we will assume just a 2.5mm sq feed then next question has to be.
1. Is 20A enough? Likely it will be enough.
2. Will the impedance be low enough.

The second question should be answered by looking at the Installation Certificate issued for the garage or the Inspection report. However that's in an ideal world and often the paperwork is lost. So job one is to measure the impedance.

OK you are asking why. Well the MCB or fuse must blow if there is a fault. Now if you had a mile of cable then the resistance of the cable would be that high that even if you shorted the ends it would still not draw enough current to trip the MCB or blow the fuse. OK I know it's not a mile long but there is a point when it is considered too long and that is when either the fuse or MCB will take over 0.1 seconds to blow or the volts will drop more that 5%.

Depending on the loop impedance if supplied with 20A is if you can take full 20A to next building or if it will be reduced to 6A and only work lighting.

A MCB will have a marking like B20 and the B means it will need 5 times 20A to blow in 0.1 seconds. C means 10 times and D means 20 times. So a B20 MCB will need 100A to blow it in 0.1 seconds. At 230v simple ohms law 230/100 = 2.3 ohms is the maximum impedance.

The meters are expensive to measure this but easy to use they just plug in then press the button. (Although there are some safety measures required) So any electrician can quickly work out what is the maximum supply that can be taken to new building. Also what size cable will be required.

But unless you have these reading then no one on here can tell you if you can or can't do what you want.
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Postby murras22 » Mon Apr 11, 2011 8:08 pm

Thanks ericmark, obvisously not as straightforward s I'd like it to be.

So I may have to trust in the electricans say so if they say it can be done and it is just to safeguard disrupting my kitchen etc.

No paperwork for the existing garage CU exists so far as Im aware but I have been informed that the work will be covered by a certificate / Part P so I suppose I'll draw comfort from that at least.
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Postby ericmark » Wed Apr 13, 2011 8:45 am

Under Part P the installation certificate is sent to the scheme provider who in turn issue the completion certificate and then pass it on to building control so likely it will be scrutinised by at least two other than electrician doing work so unlikely there will be a problem.
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