Installing a sub main to DB2

[DomSparky]

Hello guys I need some help to find out the best way to install a sub main to a new DB let me explain a few details first...

Iam wiring a new extension the circuits required are just lighting and sockets the lighting will be on a 6A rcbo and only 4 sockets so Iam running a 2.5mm radial on a 16A rcbo.

I can't touch the existing board as it isn't to current standards the last electrician had to remove the rcd due to a fault that couldn't be found. The customer isn't interested in paying for a new board and fault find so my plan is to run a swa from a mains switch isolator fed from a new set of meter tails from a henley block then run a swa from here to the new DB in the extension it only needs to be a small 2 way DB 16A and 6A rcbo.

Can you please advise me on the best way of going about this the smallest mains switch isolator is 63A and smallest DB main switch is 40A which would mean the smallest swa cable I could use would be a 10mm which seems a waste when all it needs to power at the other end is a maximum of 22A!

Many thanks for your help guys look forward to hearing your solutions!

 

[Sharpend]

Why do you think that the cable size is determined by the switch? 

 

[Murdoch]

I can't touch the existing board as it isn't to current standards the last electrician had to remove the rcd due to a fault that couldn't be found. The customer isn't interested in paying for a new board

Why can't you add a RCBO for the new circuit? Assuming one is available that's the way I would go

 

[DomSparky]

 I would have thought the 63A fuse would need to protect the cable so the cable would need to be able to handle at least 63A or in a fault the cable would melt but then again the draw should never be that high because the 6A and 16A rcbo would disconnect the supply first.

 

[DomSparky]

Thanks for your reply Murdoch.

There are no spare ways on the current DB otherwise that would be what I would normally do. Iam starting to think my best option would be to install a small garage DB in the meter cupboard and run my T&E cables from there the only problem with that is I will need to fish the cables through the void under the house which will be a nightmare lol

 

[kerching]

would have thought the 63A fuse 

Where's this fuse appeared from...is it the service fuse?

 

[Sharpend]

Where's this fuse appeared from...is it the service fuse?

That is exactly what I was going to ask but been side tracked with 18th edition revision! 

 

[Sidney]

I can't touch the existing board as it isn't to current standards the last electrician had to remove the rcd due to a fault that couldn't be found

So the customer has a potentially dangerous installation but isn't willing to fix it?

 

[Murdoch]

Thanks for your reply Murdoch.

There are no spare ways on the current DB otherwise that would be what I would normally do. Iam starting to think my best option would be to install a small garage DB in the meter cupboard and run my T&E cables from there the only problem with that is I will need to fish the cables through the void under the house which will be a nightmare lol

if you do add a submain etc just make sure your quote / estimate and your EIC description of works makes 100% clear that you haven’t touched the existing installation and that an existing fault is understood to be why the RCD was removed before you worked on it or you could walk into a trap

 

[binky]

 I would have thought the 63A fuse would need to protect the cable so the cable would need to be able to handle at least 63A or in a fault the cable would melt but then again the draw should never be that high because the 6A and 16A rcbo would disconnect the supply first.

put in a mini board with main switch and 32A MCB or MCB of your choice to protect sub main cable.

 

[DomSparky]

Thanks for all your answers guys.

I have managed to get T&E cables through the cavity under the floor to the incoming supply intake so I will be installing a mini DB to provide the power and lighting to the new extension. 

The existing DB and installation is a shambles and I don't want anything to do with it as the customer puts it "but everything works ok, why should I pay to upgrade the board" So I will do what you said Murdoch I will be making it very clear on my EIC as to why I can't touch the existing installation.

Yes Sidney the customer knows he has a dangerous installation but doesn't care he has had the house decorated to a good standard with laminate floors so doesn't want a rewire (why don't people think about these things before making the house look nice?) 

 

[Andy™]

So the customer has a potentially dangerous installation but isn't willing to fix it?

sounds about right

 

[Sidney]

Yes Sidney the customer knows he has a dangerous installation but doesn't care he has had the house decorated to a good standard with laminate floors so doesn't want a rewire (why don't people think about these things before making the house look nice?) 

Do you want me to answer that??? 

 

[kerching]

Thicker than springs to mind!....then the tw@s complain about the invoice! W️S

just saying

 

[binky]

Thanks for all your answers guys.

I have managed to get T&E cables through the cavity under the floor to the incoming supply intake so I will be installing a mini DB to provide the power and lighting to the new extension. 

The existing DB and installation is a shambles and I don't want anything to do with it as the customer puts it "but everything works ok, why should I pay to upgrade the board" So I will do what you said Murdoch I will be making it very clear on my EIC as to why I can't touch the existing installation.

Yes Sidney the customer knows he has a dangerous installation but doesn't care he has had the house decorated to a good standard with laminate floors so doesn't want a rewire (why don't people think about these things before making the house look nice?) 

 Have encoutered this many times and done exactly what you are proposing. It makes it much easier to be BS7671 compliant anyway, plus you know all your work is good.