Power to second CU

[GCarnegi]

Just finished extension including wiring all tied back to new CU inside garage. Wiring has all been inspected by registered electrician and he is happy. (okay I'm only at first fix stage so far but getting there and he is happy so far).

Electrician told me to use 10mm2 t&e to supply the new CU.

I spoke to another electrician and was told to tee-off the existing meter tails (using Henley blocks) to supply the new CU.

Cable length required between meter and new CU is about 5m - albeit they are pretty much back-to-back through a granite wall.

My question is...

Can I put the recommended 10mm T&E into the Henley block without it first going through an RCD, as obviously te bullet fuse is rated at 100A (far more than the 10mm T&E), so what protection does my 10mm cable have?

If I have to take it from the existing CU instead of from the Henley, then I will likely have to rewire something as I don't have enough free space in the existing CU and I don't really want to disturb the existing house wiring if I can help it at all.

PS I have an isolator switch fitted to the output of the meter.

thanks

 

[ricicle]

No offence but by having to ask these questions doesn't it mean that you aren't competant to do this work and should leave it to the electrician ?

 

[ban-all-sheds]

J(okay I'm only at first fix stage so far but getting there and he is happy so far).

Then to keep him happy I suggest you carry on doing what he tells you, rather than asking around on the internet and doing something different.

 

[riveralt]

Tails from the meter to the current new CU should 25mm.
put these into Henley Block - then run two sets of 25mm tails from the HB to the respective CU's.

Can't be that much of an electrician - sounds like this has been a badly designed installation from the beginning. If you have a new CU fitted why are you fitting a second one - unless you live in a mansion you should have been able accommodate all the circuits in one CU would save all this messing about.

Also why 10mm2 T&E? is this second CU for an electric shower circuit?
Seems a very strange todo.

 

[JohnW2]

Can I put the recommended 10mm T&E into the Henley block without it first going through an RCD, as obviously te bullet fuse is rated at 100A (far more than the 10mm T&E), so what protection does my 10mm cable have?

[
You've answered that question yourself. A 100A fuse (or even an 80A one, which is what it may be) is not acceptable as the protection for a 10mm² cable - I think you should ask the electrician who suggested it what (s)he had in mind for protection of the cable - and, indeed, how (s)he arrived at the decision to recommend 10mm².

Kind Regards, John.

 

[GCarnegi]

No offence but by having to ask these questions doesn't it mean that you aren't competant to do this work and should leave it to the electrician ?

Yeah, I know. I was expecting at least one of these....
My thinking is that if I ask the questions, and my electrician is proved wrong, then I'm probably more competent to do this particular work than he is. Of course I could always NOT ask the question and let the house burn down.
No offence taken, but it does question the relationship between registration / certification and competency, doesn't it?

J(okay I'm only at first fix stage so far but getting there and he is happy so far).

Then to keep him happy I suggest you carry on doing what he tells you, rather than asking around on the internet and doing something different.

Maybe, you're right. I should not have asked the question then and let the 10mm cable sink the short to earth. Glad we're all in agreement there then... not.

Tails from the meter to the current new CU should 25mm.
put these into Henley Block - then run two sets of 25mm tails from the HB to the respective CU's.

Can't be that much of an electrician - sounds like this has been a badly designed installation from the beginning. If you have a new CU fitted why are you fitting a second one - unless you live in a mansion you should have been able accommodate all the circuits in one CU would save all this messing about.

Also why 10mm2 T&E? is this second CU for an electric shower circuit?
Seems a very strange todo.

Exactly what I was thinking, but of course he is the experienced one who has done all the courses and has all the certificates so who am I to argue? So if the certification counts for squat, how can I be sure of a professional installation?

The new 17th reg CU is for the extension only. That way the only inspection that is required is of the NEW circuits, and the old ones (on the other CU) don't need to be considered. The old one is wired fuses and has no space for additional circuits. Got to admit it's a lot easier to install a completely new CU and circuits than replacing an old one - at least in my opinion, maybe you disagree.

I think the 10mm2 T&E was just selected due to the load requirements expected in the extension - no shower, just sockets and lights.

Strange todo.... I agree... Hence the question.

Can I put the recommended 10mm T&E into the Henley block without it first going through an RCD, as obviously te bullet fuse is rated at 100A (far more than the 10mm T&E), so what protection does my 10mm cable have?

[
You've answered that question yourself. A 100A fuse (or even an 80A one, which is what it may be) is not acceptable as the protection for a 10mm² cable - I think you should ask the electrician who suggested it what (s)he had in mind for protection of the cable - and, indeed, how (s)he arrived at the decision to recommend 10mm².

Kind Regards, John.

Precisely... I just hope he hasn't got me spending money on things I don't need. 10mm2 cable is not cheap. I think people who do that are rude, careless, disrespectful and in this case downright dangerous and generally taint the image of the profession, which is not what anyone needs or wants. It seems that the requirement for certification has done little to get rid of the cowboys, and diligent DIYers are no less competent than they used to be - though it appears they are often treated as such.

Remember that I'm in Scotland, and here there is no requirement for the installation to be carried out by anyone in particular - my gran could legally do it if she wasn't dead [edit: actually not sure if being alive is actually a legal requirement, can't find it in the regs] - as long as it was passed afterwards by the registered electrician (as required in this instance due to the nature of the build).