Bridge from old meter to new CU. 25mm SWA.

f0rmula · 5 Jul 2013

I’ve completed my new consumer unit, which is about 4m from the old CU and meter. I’ll get the meter moved to the new CU location, as soon as building control are able to test and certify the new CU and installation. 4m is longer than you’re allowed meter tails, so I need to put an isolator on the wall near the old meter. I called the BCO electrical inspector to ask what he’d want to see and he said “100A isolator on the wall near the old meter and run 25mm^2 SWA (3core) over to the new CU”.

I bought a few bits, listed below. First thought: Wow. 25mm^2 SWA is massive.

100A isolator (fused):
http://www.tlc-direct.co.uk/Products/CGFS100.html
Box to abut the CU and terminate the SWA into, as there’s nowhere to put the glad on the CU chassis (TLC didn’t have any metal boxes with a 25mm knockout):
http://www.tlc-direct.co.uk/Products/WK607B.html
25mm piranha nuts and glands, etc.

So, onto the questions: The earth terminal on the consumer unit won’t take the 25mm^2 earth core. There’s only one terminal left, and it’s 16mm^2. Should I:
1) Terminate the 25mm^2 earth core with a lug and attach it to the piranha nut earth terminal:
http://www.tlc-direct.co.uk/Products/BHLUG25.html
Then run a 16mm single with a similar lug from the piranha nut to the earth terminal in the CU. This is the cheapest, easiest, and most simple option, I suspect, but requires me to crimp onto a 25mm^2 core. I’m not sure how well my crimps will do that.

2) Terminate the earth core into an earth block.
http://www.screwfix.com/p/4-way-earth-block/12386
Similar to above, but terminate the earth core an earth block inside the box I’m terminating the SWA into, then run one 16mm^2 up to the CU earth terminal, and one 16mm^2 (or 25mm^2?) to the piranha nut terminal. No crimping of 25mm^2 required, but certainly more faff.

3) Something else entirely.

As an aside, I wonder if I could have used a 63A fuse at the isolator, and used 16mm^2 SWA (or even 10mm^2 which is rated 73 Amps). It would have cost a shedload less, been easier to work with, and 63A is totally sufficient for our usage. At least until we get the meter moved. (In fact, we’ve made do with 26A from two RCD’d sockets for ages while I’ve been re-wiring

)

Anyhow, here are some nearly finished CU pics, for anyone who's interested. The SWA at the top supplies outside sockets, and the 25mm SWA will enter from a box mounted below the CU:

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f0rmula
5 Jul 2013
1

Untitled

f0rmula
5 Jul 2013
1

Thanks for any advice.

riveralt · 5 Jul 2013

Looks neat and tidy.

It is always difficult to tell from photographs but it doesn't look as if you have provided a CPC to the SWA itself.
The same SWA line cable looks like it is 1mm ² which would be a problem if you are running this to an outside socket radial.
Similarly if this is the SWA line cable then it is connected to an MCB without, it appears, any RCD protection.

I don't have the tables to hand but I would expect 16mm ² SWA to be sufficient to cover the tails. You might struggle to fit 25mm ² into that CU (not sure that the CPC is 25mm ² as well).

If he insists it might be better to get 2 core SWA to a box below the CU and run standard 25mm ² tails to the CU - and run a 16mm ² direct from the MET to the CU.

I take it you have conducted dead circuit tests - what were you results like?

JohnW2 · 5 Jul 2013

f0rmula said:
...4m is longer than you’re allowed meter tails, so I need to put an isolator on the wall near the old meter. I called the BCO electrical inspector to ask what he’d want to see and he said “100A isolator on the wall near the old meter and run 25mm^2 SWA (3core) over to the new CU”.

Hmmm. For what it’s worth, IMO that’s a bit OTT. Unless it is particularly vulnerable to damage and/or concealed (which I very much doubt) I don’t really see that SWA is needed for such a short run. Whatever, even if you are using SWA, the earth certainly does not have to travel in the SWA, so you could have used 2-core SWA and a separate (probably 16mm²) earth wire, which would then have avoided the problems you go on to describe. However, the BCO has spoken, and I presume you don’t/didn’t want to argue!

f0rmula said:
So, onto the questions: The earth terminal on the consumer unit won’t take the 25mm^2 earth core. There’s only one terminal left, and it’s 16mm^2. Should I:...
2) Terminate the earth core into an earth block.
Similar to above, but terminate the earth core an earth block inside the box I’m terminating the SWA into, then run one 16mm^2 up to the CU earth terminal, and one 16mm^2 (or 25mm^2?) to the piranha nut terminal. No crimping of 25mm^2 required, but certainly more faff.

That would be the conventional method. The Main Earthing Terminal (MET) is often an exposed earth block (rather than just using the CU’s earth bar for this purpose) – no need to be within the box if you don’t want.

f0rmula said:
As an aside, I wonder if I could have used a 63A fuse at the isolator, and used 16mm^2 SWA (or even 10mm^2 which is rated 73 Amps). It would have cost a shedload less, been easier to work with, and 63A is totally sufficient for our usage. At least until we get the meter moved. (In fact, we’ve made do with 26A from two RCD’d sockets for ages while I’ve been re-wiring )

I see nothing wrong with that electrically, if 63A is enough for your purposes. An 80A fuse with 16mm² would also seem to have been an option.

f0rmula said:
Anyhow, here are some nearly finished CU pics, for anyone who's interested.

Looks very neat.

Kind Regards, John

JohnD · 5 Jul 2013

RCBOs are great!

JohnW2 · 5 Jul 2013

JohnD said:
RCBOs are great!

...particularly when there is as much space for them as there is in that CU.

Kind Regards, John

f0rmula · 5 Jul 2013

riveralt said:
Looks neat and tidy.

Cheers. I've mostly done this for fun and a learning exercise.

It is always difficult to tell from photographs but it doesn't look as if you have provided a CPC to the SWA itself.

It's 3 core, and the earth core runs straight to the CU earthing block. There's a second run from the same terminal in the CU back to the piranha nut terminal. I think you can see it if you squint a bit

The same SWA line cable looks like it is 1mm ² which would be a problem if you are running this to an outside socket radial.

It's 1.5mm SWA on a 20A MCB, so it should be good to 23A.

Similarly if this is the SWA line cable then it is connected to an MCB without, it appears, any RCD protection.

It supplies a pair of RCD sockets, so that should be OK, by my understanding. :-/

I don't have the tables to hand but I would expect 16mm ² SWA to be sufficient to cover the tails. You might struggle to fit 25mm ² into that CU (not sure that the CPC is 25mm ² as well).

If he insists it might be better to get 2 core SWA to a box below the CU and run standard 25mm ² tails to the CU - and run a 16mm ² direct from the MET to the CU.

Hmm. I can fit 25mm SWA into the terminals of the isolator. It was just the earth that won't fit into the earth block in the CU. Hence the post. I've gone and bought 4m of 3 core 25mm SWA already, so that's become a bit of a moot point, but I really just wanted to know if my gut feeling that this was all a little over the top was justified. Seems like it is.

I take it you have conducted dead circuit tests - what were you results like?

Insulation tests and various R's I took all seemed sensible and remarkably consistent but I'm at work at the moment. I only have a DIY multimeter, but have a friendly electrician coming to inspect and test before the BCO come, as I wasn't sure exactly what level of testing they will actually do.

JohnD · 5 Jul 2013

I am trying to remember if there is a problem having that horrible unsheathed bell-wire inside the CU enclosure which also contains busbars. i thought there was and would not have used it. Has anyone else got a better memory?

f0rmula · 5 Jul 2013

JohnW2 said:
f0rmula said:

...4m is longer than you’re allowed meter tails...

Click to expand...

Hmmm. For what it’s worth, IMO that’s a bit OTT. Unless it is particularly vulnerable to damage and/or concealed (which I very much doubt) I don’t really see that SWA is needed for such a short run. Whatever, even if you are using SWA, the earth certainly does not have to travel in the SWA, so you could have used 2-core SWA and a separate (probably 16mm²) earth wire, which would then have avoided the problems you go on to describe.

Completely visible. Clipped to the ceiling joists of the cellar, in a well lit room, not running behind anything.

However, the BCO has spoken, and I presume you don’t/didn’t want to argue!

Precisely. I'm not confident enough about any of this to push back.

f0rmula said:
f0rmula said:

2) Terminate the earth core into an earth block.
...

Click to expand...

That would be the conventional method. The Main Earthing Terminal (MET) is often an exposed earth block (rather than just using the CU’s earth bar for this purpose) – no need to be within the box if you don’t want.

Hmm. That's interesting. Thanks. I suspect that's what I'll do, considering my crimps aren't going to handle 25mm^2 very well.

I see nothing wrong with that electrically, if 63A is enough for your purposes. An 80A fuse with 16mm² would also seem to have been an option.

Could have saved myself a hell of a lot of hassle here, at the expense of potential hassle of BCO arguments that you mentioned above.

f0rmula · 5 Jul 2013

JohnW2 said:
JohnD said:

RCBOs are great!

Click to expand...

...particularly when there is as much space for them as there is in that CU.

Kind Regards, John

It's an EAD11R, specifically for RCBO installs. Took a bit of effort to source, but definitely worthwhile finding one

Also, note that there's an RCBO protecting the bell transformer :-O

I was planning to put the bell transformer with a lighting circuit, but I bought an extra 6A RCBO by mistake, so used it rather than bother sending it back

ban-all-sheds · 5 Jul 2013

f0rmula said:
3) Something else entirely.

What have you done at the source end re earthing the armour?

As an aside, I wonder if I could have used a 63A fuse at the isolator, and used 16mm^2 SWA

Yes.

f0rmula · 5 Jul 2013

JohnD said:
I am trying to remember if there is a problem having that horrible unsheathed bell-wire inside the CU enclosure which also contains busbars. i thought there was and would not have used it. Has anyone else got a better memory?

I was waiting for someone to mention that

I don't think it was allowed, so I'm going to have to address that somehow. I guess it's a junction box right outside the CU, with bell wire running into it, and 1mm T&E running into the CU. How do people normally get from a bell transformer out of the CU?

ricicle · 5 Jul 2013

JohnD said:
I am trying to remember if there is a problem having that horrible unsheathed bell-wire inside the CU enclosure which also contains busbars. i thought there was and would not have used it. Has anyone else got a better memory?

Is the bell wire insulated to the maximum voltage present ?

f0rmula · 5 Jul 2013

ban-all-sheds said:
What have you done at the source end re earthing the armour?

I haven't done that just yet. The whole bridge is this weekend’s job. I was going to do something similar with an earthing block or lug at the isolator end as I did at the CU. Incidentally, the isolation box earth pole has a similar M8(?) thread to the one on the piranha nut.

JohnW2 · 5 Jul 2013

mutley11 said:
he visible. Clipped to the ceiling joists of the cellar, in a well lit room, not running behind anything.

As I suspected. I really don't see how a requirement for SWA can be justified - I would have thought that 25mm² DI singles (with a 16mm² earth) would have been fine (with 100A fuse), or 16mm² singles with an 80A or lower fuse.

mutley11 said:
However, the BCO has spoken, and I presume you don’t/didn’t want to argue!

Click to expand...

Precisely. I'm not confident enough about any of this to push back.

It really is a bit naughty. Individual BCOs should really not be allowed to 'require' things over and above what can be justified electrically, particularly when advising (for a good price!) someone who they know probably isn't confident enough to argue.

mutley11 said:
Hmm. That's interesting. Thanks. I suspect that's what I'll do, considering my crimps aren't going to handle 25mm^2 very well.

You're welcome. It's probably what I would do.

mutley11 said:
Could have saved myself a hell of a lot of hassle here, at the expense of potential hassle of BCO arguments that you mentioned above.

Again, one might have hoped/expected that the BCO advising you would have mentioned such options - perhaps not 63A, but certainly 80A (and it's quite likely that you've only got an '80A supply', anyway).

Kind Regards, John

ban-all-sheds · 5 Jul 2013

JohnW2 said:
It really is a bit naughty. Individual BCOs should really not be allowed to 'require' things over and above what can be justified electrically, particularly when advising (for a good price!) someone who they know probably isn't confident enough to argue.

Are you assuming that the BCO was himself knowledgeable?

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Bridge from old meter to new CU. 25mm SWA.

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