Secondary (expansion) Consumer Unit

Hi all,

first post here. Hopefuly I wont go down in flames for asking an electrical question

I am rennovating my kitchen at the moment and I would like to install a new ring main for the sockets I will add. My current CU has run out of space and so I would like to add a small 4 or 6 way unit as an addition rather than replace the whole unit.
My question is this:
As the existing CU is protected by an RCD can I in theory remove one of my ring mains from the existing unit and add an higher rated MCB to connect to the existing unit to the 100A isolator in the secondary unit?
I would then have 2 ring mains running in the secondary unit.
What rating of MCB would I requir in the primary unit?
Would the secondary CU be protected by the RCD in the primary?
Would 10mm T&E be sufficient to connect the primary to the secondary?


Hope that makes sense!


Look forward to hearing from you.


Tim

Easier connection the sub board direct to the mains, bypass the existing unit and include all the 17th ed gubbins in the new board.

Reason being that if the existing board isn't 17th ed, all new work must be 17th and by keeping away from the original board you don't end up having to update the old and supply the new.

You just supply the new.

It also means that if something trips on the old board, by not being connected the kitchen board doesn't lose power (think freezer / fridge- rotten food).

Look and see if between the existing board and the main head / meter there's a black box, a Henley unit. If so that has additional terminals to take a fresh pair of tails (or 16 / 25 mm sq cable if the new CU is being located away from the area) to connect to a new board.

Even if there isn't, any good pro can provide one.

tlawrence127 said:

What rating of MCB would I requir in the primary unit?

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How come you think you are competent to design and install circuits and CUs when you don't know something as basic as that?

Would the secondary CU be protected by the RCD in the primary?

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How come you think you're competent to install CUs when you don't know how they work?

Would 10mm T&E be sufficient to connect the primary to the secondary?

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Ib ≤ In ≤ Iz

• For a circuit to supply a load (doesn't matter what), how would you go about deciding what cable and protective device to use?
  
  
• How do you calculate maximum demand and how can diversity be used?
  
  
• Do you understand how the way in which you install cables affects how much current they can carry?
  
  
• What are the rules concerning cables concealed in walls, partitions and under floors?
  
  
• Where cables need to be joined, how should this be done / not be done and in what circumstances are different methods acceptable?
  
  
• Can you identify extraneous conductive parts, and do you know the requirements for main and supplementary bonding of them?
  
  
• Which circuits should be RCD protected?
  
  
• If you supply the new CU straight from the tails how will you isolate the supply so that you can connect up your new CU?
  
  
• Do you know what tests you would carry out on the installation - what sequence you'd do them in and at what point you would energise the installation, and for each test do you know what is being measured, why it is important, how you would carry out the test, and with what equipment, and what sort of results you would expect to get if everything was OK?

The thing is, designing and installing new circuits, installing new CUs, submains etc is not a trivial job, and I can assure you that it involves knowing far more than you think it does.

Asking questions here can be a useful part of a learning process, but they are not a substitute for proper structured studying. The key term there is "learning process" - you cannot learn all the things you need to know just by asking questions here. It isn't structured enough - it won't provide you with a way to progress where each step builds on what you learned before.

You can't carry out a job of this magnitude by asking whatever random questions happen to occur to you. What if you get something wrong because you have no idea your knowledge is wrong? What if you miss something because you simply have no idea it even exists, and just don't realise you don't know it?

Plus, of course, all the work you propose to do is notifiable.

I cannot urge you too strongly to get an electrician.

hmmm not sure how helpful the last reply was but thanks very much to Chri5 for some very useful input. Having re-investigated the CU this evening I am now wondering if it is possible to replace the RCD to free up 2 ways. Currently the feed from the meter is via a 100A DP Switch in the CU fed into a 2 pole 80A RCD which is using up 4 "ways " in the CU. Can I fit one of these?


This is the 5780s and my RCD is the 5780 I think they are the same model but different sizes.

If your existing RCD is a 4 module one, then that 2 module RCD is no use.
It won't fit, as the terminals are in different places. Even if it and the board was hacked around and made to fit in somehow, you won't gain any extra ways, as the busbar will still be the same length.
A replacement busbar would be required, and as this is an old MK board, finding any parts which fit into it will be one or more of difficult / expensive / time consuming / pointless.

The sensible solution here is to buy a new CU for the whole lot, and dispose of the existing one completely.
The cost difference between a 6 way and 16 way unit is minimal, and will be far easier and quicker than trying to connect the new into the old, moving circuits about and so on.

tlawrence127 said:

hmmm not sure how helpful the last reply was.

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It was being very helpful in trying to alert you to the fact that everything that you are contemplating is outside your level of electrical competence and knowledge.

You won't get that on an Internet forum. All of the work (including even just fitting sockets in the kitchen) is notifiable work. This is a legal requirement.

Either get a recommendation for a reliable registered electrician or find one at www.competentperson.co.uk

tlawrence127 said:

hmmm not sure how helpful the last reply was

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Not helpful?

I highlighted gaps in your knowledge which you should take as a serious warning that you don't know enough to be doing this.

Installing CUs and new circuits is not just like wiring a plug but with a few more, longer cables.

You asked if 10mm² T&E would be OK, and I gave you the formula for working it out. Was that not helpful?

I also asked you several questions, all of which are related to things you must know and must be able to do if you are to competently tackle the task you have set yourself. Did they not help you to decide what you should do?

Seriously - do you know how to design circuits, work out what derating factors will apply to your cables, work out what voltage drop will be, predict what the EFLI will be given the Ze of your supply?

Do you know what tests you'd need to carry out, and in what sequence? Do you have the appropriate test equipment, and the knowledge to use it?

I pointed out that the law requires you to apply for Building Regulations approval for this work, in advance, and as part of that application you will have to say what you will do to ensure that the work complies with P1. How will you do that?

I'm not trying to blind you with science, or to make myself look superior to you - I an genuinely concerned that you have absolutely no idea of the complexity of the work you plan to do.

Can I fit one of these?


This is the 5780s and my RCD is the 5780 I think they are the same model but different sizes.

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This is the old 5780:



Don't you see that this is one more indication of your complete lack of familiarity with CUs? How on earth will the 2-module one you show connect to a busbar which used to go into one of those?

And even if it did, how would that magically create 2 more spaces? Where do you think those 2 extra busbar prongs might be at the moment?

There's nothing wrong with you not knowing about electrics, just like there's nothing wrong with me knowing about sailing and navigation etc. But given my ignorance it would be incredibly foolish of me to get into a boat and push off from the shore, thinking it will be OK.

And given your ignorance it would be incredibly foolish of you to decide to start fiddling with or installing CUs, to try and design circuits, to dispense with testing...

Please get an electrician.