CU Extension

I've had an extension built and have installed a ring main and lighting circuit within. I have brought the circuit tails back to the existing CU, but have since been informed that the CU MCB's are now obsolete.
I now plan to add another 2 way CU alongside rather than replace the complete CU, but do not know how to feed this new CU?

If the mcb's are obsolete, it seems a waste to spend money on a 2 way board for your extension. Supposing an mcb fails in the near future on your current cu - you would then have to change the cu anyway. Why not just get a new cu with plenty of spare ways , and a good branded one, and then you'll never have any problems.

At the minute , tlc are doing 17 way Wylex units with RCD split for about £40 , and the mcbs are £3 each, they also do versions with 9 and 12 ways also, but get what suits you! If you dont want do fill all the mcb slots, they also have the blanking plates for them. And you have room for more expansion.

If you work out what it would have cost for a seperate 2 way cu, the henley blocks, 16mm tails & earth etc , I think you'd find it to be a much better (and neater) approach!

If you want to get it up and running for the time being I suppose you could have always add the new ring to your current one (and this could now easily be done at the consumer end by removing one of your current cu ring cables , joining it to the new ring circuit with an accessible junction box , and put the other end of your new circuit back onto your cu where you removed the other cable) ! Again this approach is house size dependent and depends how much loading you already have on your sockets.

You could also add your new lighting circuit to the consumer, providing you dont have too much load on your current lighting circuit, by just adding it to the 6a mcb thats already there!

I'm very surprised that building control has'nt forced you to get somebody part p in (or even came out to inspect the work!!) I know they do like the extensions to be on seperate circuits (and seperate consumers) , but seen as they seem to be happy letting you carry on with your own thing, and obviously as they are'nt inspecting it, then its down to your choice , although they will probably want some paprework and test certificates etc.

These are just general views, I'm not saying ' This is what you HAVE to do!!' Seriously though - Consider the new consumer option.

jondiy said:

If you want to get it up and running for the time being I suppose you could have always add the new ring to your current one (and this could now easily be done at the consumer end by removing one of your current cu ring cables , joining it to the new ring circuit with an accessible junction box , and put the other end of your new circuit back onto your cu where you removed the other cable) ! Again this approach is house size dependent and depends how much loading you already have on your sockets.

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you'll get better specs (lower volt drop, lower earth fault loop impedance etc) if you just put all four wires in the breaker terminal.

Thanks jondiy.

Regarding the Building Control, the officer had made several building checks, and explained that he would require test certificates for the electrics. No mention of a Part P electrician to perform the work, but we did explained that I was once a JIB Approved Electrician(expired), but have not been on my tools for almost 10 years. I Didn't mention that I have never done house bashing, all experience on ships and factories!!!.

If I was to install a complete consumer, would all circuits then require testing or only the new circuits. (But I would get a Part P to do this work though).

Your help is appreciated, as i've lost touch with all my sparky contacts.

I have now thought of another option but not sure if it is a goer.
I do have a spare 15A MCB within the CU. The boiler has now been removed.
Can I use this MCB for the downstairs lighting circuit?.
Current downstairs wattage = a potential maximum of 1100W. My new extension has a maximum potential of 640W.
So a total current 7.5A, and I believe the maximum current capacity of 1mm T&E is 16A.
I would then extend the ring as you had detailed previously.
So leaving me no need to buy any additional CU's, or am I just wishfull thinking??.

<i>I do have a spare 15A MCB within the CU. The boiler has now been removed.
Can I use this MCB for the downstairs lighting circuit?. </i>
its not really low enough to use for a lighting circuit directly but what you CAN do is connect it to a FCU fitted with a 5A fuse and then supply your lighting circuit from that.

Or depending on the length of the run, I suppose theres no harm in terminating one end of your new ring , and running the circuit as a 15 amp radial! (Although it should be 20 amp). Or possibly you could run a 15 amp ring, bearing in mind you won't be able to overload it due to the low rating as you will be forever resetting trips!

Again, its load dependent, as long as your not boiling kettles/tumble drying/electric fire in the extension and you are only running small loads (eg tv/stereo ) then that could be another option!

It will certainly take the load away from your other 32 amp mcb.

plugwash said:

put all four wires in the breaker terminal.

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Yikes!

Then it would not be a conventional ring circuit...

As described previously, it would be better in the short term to wire it temporarily as a radial on the 15A.

securespark said:

plugwash said:

put all four wires in the breaker terminal.

Click to expand...

Yikes!

Then it would not be a conventional ring circuit...

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so what? just because its not a conventional cirucit its not in any way unsafe, just slightly more vulnerable to nusense tripping than two seperate circuits.

Doh!....I've sent you down the wrong path alittle.

I was going to extend the new ring to the existing ring, as per jondiy comments.
And use the spare 15A MCB for the lighting circuit.

But I do like the idea of using the spare 15A MCB as a feeder to another FCU.
I assume it would be okay to use a 2 way MCB enclosure alongside the old CU with the new 6A mcb within for the new lighting circuit, with the new ring main extended into the existing ring main at the CU. I know it's not an elegant solution, but as long it's acceptable and Part P compliant, it'll do for me.

After discussions with the part P sparky, I replaced the complete CU.
TLC had a good offer going with a 12 way CU, c/w 9 MCB's for £73...
I also had to upgrade my incomer tails and all bonding to current regs!!