3 phase and extending consumer unit

[c0wb0y]

Hi folks,

Hoping you good people could help me out with an idea I've had:

I'm on a 3 Phase Supply (formerly a commercial site turned into small apartment). My terminology is way out but it seems only 1 of these 3 'phases' are metered (@100A). The other 2 'phases' (100A and 80A) are simply security sealed.

The meter extends into a CU which has plenty of blanks. This CU as 3 MCBs rated at 6A (Lights), 32A (Sockets) and 45A (Electric Shower) - they are all RCD protected.

I'm looking to extend the supply to the adjacent (vacant) room which I want as a mini workshop for hobbies & crafts. Money is no object here & I have enough cable lying about. Also, I like it looking all neat & tidy. So here is the plan:

Step 1...Add 1 x 45A RCBO to existing CU
Step 2...Run 5 meters of 10mm 2 Core&E to new 2 Way CU which will have RCD
Step 3...Insert (to new CU) a 32A MCB for sockets and 6A MCB for Lights
Step 4...Begin circuits (3 sockets + 2 lights)

This is perhaps overkill but as mentioned above, I have enough bits lying about. My questions are:

1....whether the above would work fine? and is the way to go
2....are meter tails from CU to CU necessary or would the 10mm twin Core&E suffice? Cable would be surface mounted in trunking with no bends or twists - straight and clean run of 5 meters
3....If I wanted the hobbies room metered (and if I went overboard again), could I 'engage' another phase from the supply and meter it with a separate CU? or would you recommend just Henley Blocking the existing phase and installing a submeter?

4...Do you recommend separate lighting circuits (2 tube lights) or could I simply use the socket circuit for lights too?

Appreciated

 

[DaftPunk]

Just run your new sockets and lights back to the existing cu. adding a new cu is completely pointless.

 

[riveralt]

The work you are planning will require you contacting your Local Authority Building Control before you start work. Completing the appropriate application form and paying their fee - circa £500.
I know money is no object but it would be much cheaper to employ an electrician.

1. The work is fine though there are alternative ways of doing this - like extending the existing socket and lighting circuits - depending on the loads. No need to contact the LABC.

2. 10mm² T&E is fine for the circumstances you describe - remember the cable current carrying capacity after environmental factors have been taken into account must exceed the size of the MCB.
Personally if I was going down the additional CU route then I would run 32A MCB (4 or 6mm ² T&E) from the old CU to the new one and then run 1 (20A RCBO) in 2.5mm² T&E for a radial socket circuit and 1mm² T&E (6A RCBO) for the lights.

3.The three phase kit belongs to your DNO and they are the ones who will need to be informed if you intend to bring either of the two remaining phases into operation - they will connect up to the second meter and then to an isolation switch for you(r) (electrician) to connect to the CU.

4. See 2 above.

 

[ricicle]

Feeding a CU from an RCBO would be unwise due to discrimination issues. This would be a concern as you would lose lights in the workshop as well if there was a single fault.

 

[ban-all-sheds]

Step 1...Add 1 x 45A RCBO to existing CU

Why an RCBO? Even if there's a non-protected side in your CU, you've said the cable will not be concealed in a wall or partition.

Step 2...Run 5 meters of 10mm 2 Core&E to new 2 Way CU which will have RCD

Why another CU?

And why an RCD if the submain has one?

Step 3...Insert (to new CU) a 32A MCB for sockets

Why 32A for just 3 sockets?

Even if you might later add more, are you really going to be burning over 7kW in there?

1....whether the above would work fine? and is the way to go
2....are meter tails from CU to CU necessary or would the 10mm twin Core&E suffice? Cable would be surface mounted in trunking with no bends or twists - straight and clean run of 5 meters
3....If I wanted the hobbies room metered (and if I went overboard again), could I 'engage' another phase from the supply and meter it with a separate CU? or would you recommend just Henley Blocking the existing phase and installing a submeter?

4...Do you recommend separate lighting circuits (2 tube lights) or could I simply use the socket circuit for lights too?

I being the person responsible for the Design of the electrical installation (as indicated by my signature below), particulars of which are described above, having exercised reasonable skill and care when carrying out the Design, hereby CERTIFY that the said work for which I have been responsible is to the best of my knowledge and belief in accordance with BS 7671:2008, amended to .............(date) except for the departures, if any, detailed as follows.

Given the questions you are asking, I really don't think that your competence is sufficient for you to genuinely make that declaration.

The thing is 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 become competent enough to do a job like this just by asking whatever random questions happen to occur to you. You've already shown that you have some dodgy misconceptions - 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?

And then there's all the testing - 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 it, 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?

As for metering your hobby room - why?

 

[c0wb0y]

Given the questions you are asking, I really don't think that your competence is sufficient

The work will be carried out by a qualified sparky so the usual disclaimers don't apply here. As a keen DIYer, I like to know whats happening and why.

Admittedly, I wouldn't know where to begin with testing. In terms of loads, its basic math to find peak loads (and without knowing code/regs, I usually minus 30% and that covers all the usual blurb). So 32A x 230 is approx 7kW - but I would always say (to myself) max permissible is 5kW. Where I'd need 2.5mm cable, I use 4mm. Where 6mm is needed, I use 10mm.

Just run your new sockets and lights back to the existing cu. adding a new cu is completely pointless.

Can do - but its a room which may in the future be sub-let to artists so want it separated now.

And why an RCD if the submain has one?

The 2-way CU I purchased had an RCD fitted and 2 blanks.

Feeding a CU from an RCBO would be unwise due to discrimination issues. This would be a concern as you would lose lights in the workshop as well if there was a single fault.

Yep - never thought of that. Do you recommend feeding the 2nd CU from a MCB on the 1st CU? And avoid RCBOs altogether?

Why 32A for just 3 sockets. Even if you might later add more, are you really going to be burning over 7kW in there?

I have a spare box of 32A new Wylex MCBs so will bung em in. And no, there will never be 7kW but if I was to sublet, pigs will start flying with tenants using 3kW heaters, Irons etc. etc.

 

[c0wb0y]

if CU#1 has all circuits RCD protected and a 2nd CU was 'extended' off a MCB on this 1st CU, can or will the 2 RCDs clash?

Doing it this way, does the entire installation become more 'sensitive' to differences in current discrepancies?

So, for example, if RCD#1 tripped on a 30mA discrepancy, will adding a 2nd RCD protected CU now reduce the 'tripping' threshold to 15mA ?

Not sure if the Q makes sense but thought I'd ask.

 

[ban-all-sheds]

The work will be carried out by a qualified sparky

In which case she has to do all of the design work. She too cannot honestly sign the declaration above to say that she did it if she did not.

Nor can she certify to Building Control that her work complied with the Building Regulations if it was not her work.

And also don't be surprised if she is not prepared to install materials which you happen to have lying about, because she's got to take responsibility for those too.

In short, you need to stop asking questions like these here, stop trying to second guess the electrician you don't have, and get an electrician involved now.

Can do - but its a room which may in the future be sub-let to artists so want it separated now.

You need to take legal advice on metering and charging tenants for electricity.

The 2-way CU I purchased had an RCD fitted and 2 blanks.

So why purchase that type?

Anyway - it could well be irrelevant what it is.

I have a spare box of 32A new Wylex MCBs so will bung em in.

1) You can't - that's your electrician's job - just like she has to do the design if she's to say she did it, she has to do the construction as well.

2) You've checked that they aren't the ones subject to the recall?

 

[ban-all-sheds]

if CU#1 has all circuits RCD protected and a 2nd CU was 'extended' off a MCB on this 1st CU, can or will the 2 RCDs clash?

Doing it this way, does the entire installation become more 'sensitive' to differences in current discrepancies?

So, for example, if RCD#1 tripped on a 30mA discrepancy, will adding a 2nd RCD protected CU now reduce the 'tripping' threshold to 15mA ?

Not sure if the Q makes sense but thought I'd ask.

I'm afraid that all you are doing is re-enforcing the impression that you need to leave it alone and get a competent electrician involved now.

 

[c0wb0y]

And there goes the thread - off on a tangent like so many before. Cut n Paste disclaimers about buildings regs etc. are being used to deny access to knowledge - and that's all it is Im asking - just info/knowledge. For every Q, the same 'ask the sparky' reply makes me wonder why this forum was set up in the first place. Oh well - onwards and upwards.

 

[ban-all-sheds]

Cut n Paste disclaimers about buildings regs etc. are being used to deny access to knowledge.

No they are not. You only have to look at the articles in the Wiki, particularly the ones containing links to outside sources of information and recommended reading to see that there is a great deal of access to knowledge provided here.

You are clearly doing more than asking for knowledge, you are seeking conformation of, and assistance with, electrical design work. You are talking about actually doing installation work.

What we are trying to explain to you is that you MAY NOT do any of those things, YOUR ELECTRICIAN HAS TO DO THEM.

Your electrician cannot legitimately sign an EIC if she didn't actually do the things she's signing to say she did. She cannot legitimately certify to Building Control that her work complied with the Building Regulations if it's not her work.

You can't ask an electrician to turn up and do a bit of testing on what you've done and "sign it off" - it doesn't work like that.

If you DIY it you're required to apply for Building Regulations approval before you start, and as part of that you'll have to explain how you will ensure that your work will comply with P1, which will involve convincing your LABC that you are competent to design and install in compliance with BS 7671.

If you think you'll want a separate DNO supply for the workshop, be prepared for disappointment if you don't have the necessary certificates. If you want to get approval from the council for an HMO, ditto.

Please accept the reality that you need an electrician, and that whatever your current plan is it won't fly.