Extension

[youngen1]

Hi doing an extension and I am trying to find the best way to wire this. The old consumer unit is not up to 17th and I am not sure has seperate earthing to gas and water so to try and get around this I was thinking of taking a newset of tails into a 60a incomer rcd proteted and then run a 16mm t+e to a sub-main.

My two questions are will this get over the earthing if not in place (will try and get to earthing correctly at some point) as the primary characteristics will be the 60a incomer? The new sub-main is near the services easy to earth

Will I need to get a time delayed rcd so not to upset the new sub-main consumer unit taking out the incomer rcd and can you get 30ma time delayed rcd if needed

Thank you for your help

 

[sparkwright]

Why don't you just just run new circuits back to the old board, earth gas and water, and replace the board with a new consumer unit?

There will be cases, I expect, where you will have to alter existing circuits, like hall and landing lighting, so you may need these to be RCD protected.

You will probably need to add mains smoke detectors in both the old and new parts of the house, so find out first what is required.

 

[youngen1]

would like to replace the original c/u but customer doesn't want the expense tried explaining so was trying to cover myself by having the sub-main only 3 circuits up to 17th and for the test certificate I can satisfy the criteria by the main supply being a new small enclosure incoporating a rcd and mcb, which should comply.

I wanted to know if the main incomer rcd could be a 30ma time delayed rcd ?

 

[sparkwright]

So you want the new board in the extension, I take it?

If so, why not fit the new board next to the old one? ie not in the extension.

The earthing for gas and water should go back to the MET.

 

[youngen1]

that would be simple but the old board will not be accesable 24hrs as the door is locked, don't let me go into it so thats why the people using the extension have to have access to the c/u controlling there circuits.

Main issue is the trying not to get the rcd to trip by the mains before the one goes in the extension if a fault occurs

 

[sparkwright]

deleted

 

[flameport]

that would be simple but the old board will not be accesable 24hrs as the door is locked

In other words this extension is really another property, which should have its own supply provided by the DNO.

If there is no protective bonding in place, there is no way 'round it' - it needs to be installed. Not doing it means it will never be installed, it certainly won't be done at a later date.
30mA time delay RCDs do exist, but they cost a fortune and are the wrong solution here. A single 30mA incomer for a board supplying this extension/extra property won't comply either, since one fault will disconnect the whole installation.
T+E is a poor choice for a submain for several reasons.

If the customer isn't prepared to pay for the work to be done properly, then leave.

 

[sparkwright]

Would using SWA solve the problem of having an RCD at the origin?

 

[Chri5]

Am I missing something here?

It sounds like a partitioned building, with the new bit being holiday let.

Why not treat the new area as a separate entity- tails direct from the main via henley block.
MET complies on new small CU (since you have stated there's no issue with bonding water / gas in new area).

You then end up with a 17th inst via new board and correct bonding in the new area, the old area being no issue regarding cert since it's not in the works scope.

OK you will need a switch fuse since the tails (via SWA ?) will extend more than 3m from the main head, but at least you end up with a installation that is legal and to current standard.

Obviously the main building service should be updated at some point, but by separating the work that's not your problem.

In my area I've seen dozens of jobs (loft conversions and major rear extensions) where rather than suffer the cost of the existing CU being updated (and loads of work on the existing circuits) the new build area has it's own direct sub main (not via an existing CU).

You wll need a >50mm route (such as under floor) to avoid rcd requirements.

 

[youngen1]

Thank you for that reply it is what I am trying to achieve, I was not sure if from the tails into the fused switch the swa coming out to the new c/u being mechanicaly protected would get over the rcd problem? As you said I would still have to run within of floor/ceiling space 50mm