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Using Existing Cables to supply extension.

Postby DorsetBilly » Tue Sep 16, 2008 6:57 pm

I would appreciate any information regarding my extension which is to be a kitchen/diner. I originally intended getting an electrician to do this work but was let down by the one who promised to provide me with a quote. I have now started the electrics myself with the intention of getting a sparky in to update the consumer unit and carry out the Part P stuff.
At present I have several ( 2.5mm, I think ) cables running from the consumer unit to various areas next to my extension which used to be used for storage heaters. These heaters were removed by myself several years ago when I installed gas central heating.
I have used 2 of these cables to provide a new ring main around the extension without any problems.
My dilemma is that I was hoping to use another of these cables to provide the supply for my lighting, which consists of 8No dimmers, 2No pendants, 4No external lights and the usual under cupbd lighting. The intention was to connect onto this 2.5mm cable with 1.5mm and to loop in from there.
A chap at work who knows a bit about electrics has told me that this is bad practice and would cause problems when trying to obtain the correct readings required for Part P. He says I need to run a new 1.5mm cable from the consumer unit to the extension, which would cause huge disruption as the two are at opposite ends of the house.
I cant see why I cant use these existing cables and any advice would be much appreciated.
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Postby TOPSPARK » Tue Sep 16, 2008 9:56 pm

First off your friend is right what you are proposing is bad practice and could be dangerous. The second thing is the cables you talk about which i assume were for storage heaters should all be separate circuits and therefore rated at 16 amp so you could not make them into a ring main to feed the kitchen easily. My advice on all this would be to call in another electrician and get his advice and go from there as safety is of paramount importance. although awkward your best bet would be to run in two new circuits to the extension for the lighting and power and would be much safer than what you are proposing
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Postby ericmark » Tue Sep 16, 2008 11:48 pm

The “Chap at work” is right in a way in that you should not change cables sizes without some sort of protective device but a simple fused spur unit can do this at the point where the cable size changes.
Reading will be improved using 2.5 feeding into 1.5 than using 1.5 all the way.
The main problem is:-
Providing earth leakage protection for buried cable at less than 50mm.
Getting a completion cert where the work has no installation cert.
The Local council building control are a law unto them selves and can bend the rules as they see fit is issuing completion cert. But the reg electrician is not so lucky and has to sign that he did the work and tested it and may have any of his work checked so has to be very careful as to what he signs for.
Since it is an extension the LABC will be aware and you do need to be careful not to break the rules.

Postby sparx » Thu Sep 18, 2008 9:19 pm

Hi, if taken alone I will deal solely with the question of using an existing 2.5 to connect onto a 1.5 for lighting.
This is perfectly ok, providing the circuit is fused for the smaller 1.5mm cable ie 6A if fact the only thing that will be affected is there will be less volt drop on the 2.5mm section , a good thing!
As the protective device would be less than the rating of the smallest conductor in the circuit there is no point in putting any other device at the point of cable size change, otherwise we would have to fuse down at every light flex for 0.5mm which is why we use only 6A when 1.5 can carry 13A
As for readings for test sheets , it won't matter at all, r1+r2 will be whatever they are as will Ze, so can't see any problem there, a simple note on test sheet to explain 2.5 leaving board is most required.
As for Part-p ... well it's been written about enough for people to make their minds up as to how to comply, i'm not going there again,
regards SPARX
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Postby ericmark » Fri Sep 19, 2008 8:23 am

What sparx says is true. But I was considering 434.2 and 433.2.1 Except where Regulation 433.2.2 or 433.3 applies, a device for protection against overload shall be installed at the point where a reduction occurs in the value of the current-carrying capacity of the conductors of the installation. 433.2.2 does apply in your case as you have neither branch circuits nor outlets for connection of current-using equipment and the prospective fault current shall he determined at every relevant point of the installation.
I had not read the whole section sorry.

Postby DorsetBilly » Fri Sep 19, 2008 3:53 pm

Guys, I really appreciate the time you have spent explaining this issue. I have gone from thinking this was a really simple situation, to it being an impossible one and then back again. I spoke to the electrician in the local electrical shop and he also suggested the same things as you have mentioned. I am going to proceed using the "fused spur" and once the wiring is complete I will get an electrician in just to check that all is correct and at the same time get him to replace the old consumer unit with a bigger and more up to date one.
All the best and thanks again, Billy.
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