How to protect existing 16mm T&E feed to CU when converting to sub main at source

[Dartlec]

Currently quoting for a job that needs to be done next week and working out how best to proceed, so thoughts welcome.

Current situation:

-->PME incomer that has just been moved by UKPN within a porch (brick built permanent type) from one wall to another.
-->60A main fuse
-->Meter
-->Large MK metal isolator on same board that is so old it's back in fashion...
-->16mm Twin and Earth which runs up and through the ceiling of the downstairs toilet (about 4-5m length) to the existing CU, which is a metal AMD 3 board, with dual RCD and apparently has a certificate.

Because of the move, the 16mm twin and earth no longer reaches the isolator properly, so currently dangling through air across the porch.

Client wanted power and light in the new larger porch and the 16mm feed extended.

My solution is currently to install a new small CU in the porch, which will contain RCBOs for the 2 local circuits, then connect the 16mm feed into this as a submain on a 63A breaker.

Bonding of Gas and Water has been run to the incomer earth terminal fortunately, so don't need to worry about the submain sizing for that. I haven't done the adiabatic yet, but I think it will comply for fault current - although the Ze was low at 0.1.

I'm planning to install a fusebox CU with SPD included, which will then upgrade the entire installation so seems the best method in the circumstances.

The issue is that the 16mm cable currently runs through the ceiling of a downstairs toilet, with bathroom above. Neither are accessible (bathroom tiled, ceiling plastered), so it is impossible to verify whether the cable is >50mm deep from both. It does not appear to run through any walls at least.

Installing an RCBO as protection for the submain clearly causes discrimination issues.

If I was running it all as a new install, I would likely run the sub main in armoured, or at least ensure that it was >50mm, but it's likely been like this since the property was built (70s I believe)

Any thoughts on my current solution or other options that I'm missing?

 

[Wilko]

Hi - I agree, it’s possible but unlikely the existing 16mm twin and earth cable has sufficient protection and you are making a new circuit... So, I’d try repacking the existing feed, but many of these cables have been clipped which will make it difficult to pull. Doing this sort of job I have converted many ceilings into upside down golf courses, just saying.

 

[bigspark17]

I disagree with wilko and agree with your suggestions, youve given it alot of thought and have a clear vision of the end product your trying to achieve.
good luck.

 

[Dartlec]

Hi - I agree, it’s possible but unlikely the existing 16mm twin and earth cable has sufficient protection and you are making a new circuit... So, I’d try repacking the existing feed, but many of these cables have been clipped which will make it difficult to pull. Doing this sort of job I have converted many ceilings into upside down golf courses, just saying.

I'm certainly used to cutting holes everywhere, but usually when it's for a bathroom fitter and I have a plasterer following me....

I might be able to get my cheap inspection camera in there and have a look - I do have some of the 'nail safe' plates that I bought years ago and have never used, which might be one option to consider. The bathroom above is tiled I believe so definitely no access from there.

Although I'm clearly changing the source of this cable and therefore altering the circuit, I wonder at which point that line blurs between my 'install' and existing installation.

When changing a CU, adding RCD protection on all circuits is an easy way to ensure cables <50mm in walls are covered, but the regs for ceiling are slightly more nuanced.

RCD protection is needed because of the circuit normally, but for cables in a ceiling that are not >50mm from top/bottom of joist, 522.6.204 has a list of options, which do not include RCD protection. (earthed covering, conduit, protection, etc).

If I'm adding to a lighting circuit I will ensure that the cabling I install meets that, but will not physically check the route of any existing cabling that is feeding my part of the installation, or the part that continues to other lighting points in the house - I assume that's how others would also do it?

This is probably much closer to an edge case where the feed cable should be replaced, but is it one where it must be (to comply with BS7671). I don't know the answer to that.
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I disagree with wilko and agree with your suggestions, youve given it alot of thought and have a clear vision of the end product your trying to achieve.
good luck.

Thanks - I'm happy to admit that I don't know all the answers, and find other viewpoints interesting (when it's a discussion not an argument).

In this case I'd be making the install 'safer' by adding MCB protection to the feed and certainly not introducing dangers that aren't there, but that doesn't always mean that it's appropriate if you uncover a 'risk' and don't correct it.

The question is where the tipping point is between the theoretical and the practical.

 

[Wilko]

Although I'm clearly changing the source of this cable and therefore altering the circuit, I wonder at which point that line blurs between my 'install' and existing installation.

Hi - if I understand correctly, you are planning to put in a new CU which will have a 60A mcb connected to the existing 16mm cable. I think this is a new circuit from a new board, but am happy to be corrected.

Above the ceiling it’s when the cable is passing through a joist that 50mm+ is applied in Reg 522.6.201. So my thought is - as long as the existing cable is not notched in you have a chance

 

[Dartlec]

Hi - if I understand correctly, you are planning to put in a new CU which will have a 60A mcb connected to the existing 16mm cable. I think this is a new circuit from a new board, but am happy to be corrected.

Above the ceiling it’s when the cable is passing through a joist that 50mm+ is applied in Reg 522.6.201. So my thought is - as long as the existing cable is not notched in you have a chance

I have no idea when it was installed (house is 70s Ex Council), but as long as it wasn't installed by a plumber it hopefully wasn't notched!

I had a spare hour this afternoon so rang the NICEIC technical helpline.

His view was that it is effectively reconnecting an existing circuit (albeit to a new mcb) and therefore as long as that was noted in the comments of the certificate I would be compliant for my installation, subject to ensuring that the cable was safe to re-energise - which in this case would likely mean Zs or R1+R2, R1+RN and IR for the feed as far as the existing board, though I will definitely give the CU a visual once over and maybe test polarity on a suitable circuit to be sure.

Had quite a useful discussion on the difference between 'maintenance of existing installation' and 'new installation' and the answer made sense to me, and since they're my scheme I'm not going to question it - especially when it makes my job easier!

tbf He was extremely helpful, so may have to make more use of that facility since I'm paying for it!

 

[Midwest]

I have no idea when it was installed (house is 70s Ex Council), but as long as it wasn't installed by a plumber it hopefully wasn't notched!

I had a spare hour this afternoon so rang the NICEIC technical helpline.

His view was that it is effectively reconnecting an existing circuit (albeit to a new mcb) and therefore as long as that was noted in the comments of the certificate I would be compliant for my installation, subject to ensuring that the cable was safe to re-energise - which in this case would likely mean Zs or R1+R2, R1+RN and IR for the feed as far as the existing board, though I will definitely give the CU a visual once over and maybe test polarity on a suitable circuit to be sure.

Had quite a useful discussion on the difference between 'maintenance of existing installation' and 'new installation' and the answer made sense to me, and since they're my scheme I'm not going to question it - especially when it makes my job easier!

tbf He was extremely helpful, so may have to make more use of that facility since I'm paying for it!

I was with Elecsa. Being a sole trader, I found the technical helpline a useful way of discussing BS7671, and getting advice. Much like visiting this forum.

One thing I did find, was I got a more detailed and technical reply, from emailed query, rather than a phone conversation.