Extension - adding another consumer unit?

[gordonspants]

Hi, hoping there are some electricians here that can help.

We are building an extension to the rear of our house.
Existing consumer unit was replaced roughly 8 years ago, with an MK split unit, RCD for sockets and non-RCD for lights, and new thicker tails from the main fuse. It's pretty much full mcb-wise - not enough capacity for the extension anyway, as we'd like a second cooker down there - just like Ed
Main electric board fuse is still 100A.

The house is an L shaped bungalow with an attached garage, so the extension-to-incoming-mains distance is a good 20 metres-plus through/under/over the house to the extension.

My electrician (who fitted the existing unit) is now retired/AWOL so I'm going to have to 'get quotes'.
I'd like to stick with MK as its a decent known brand.

What I would like to ask for is....

Keeping the existing consumer unit in place.

Teeing off the main fuse with new tails to a small consumer unit with 100A switch and one 50A mcb, probably with RCD capability.

Then feeding from that 50A mcb a single thick cable to the extension, where that 50A supply would go into another consumer unit.

That final consumer unit in the extension would then have enough mcbs to safely supply a small ring main, lights, dishwasher/fridge circuit, and a 13A electric cooker. It would all be RCD protected back at that 50A rated consumer unit near the main fuse.

This way only one (large) cable needs to be run the 20 or so metres from main fuse area to extension, instead of a whole load of lighting/sockets/cooker cables all bunched together.

My questions are
- is this acceptable regs-wise?
- am I asking for the right set-up?
- is there a better way?

Many thanks in advance for any assistance

 

[JohnW2]

Keeping the existing consumer unit in place. ... Teeing off the main fuse with new tails to a small consumer unit with 100A switch and one 50A mcb, probably with RCD capability. .... Then feeding from that 50A mcb a single thick cable to the extension, where that 50A supply would go into another consumer unit.

Conceptually OK, but one would normally just use a 'switch-fuse', not a small CU (it is only there to protect the cables to the new CU in extension). It is usually best not to have RCD protection at that point (see below), unless that cannot be avoided. With some cable routing, it may be necessary to use armoured cable in order to allow omission of an RCD.

That final consumer unit in the extension would then have enough mcbs to safely supply a small ring main, lights, dishwasher/fridge circuit, and a 13A electric cooker. It would all be RCD protected back at that 50A rated consumer unit near the main fuse.

Yep, but if, per above, you did not have RCD protection upstream, this CU would need to have RCD(s) - ideally two, to allow circuits to be split between them (so one fault does not 'take out' all power to extension (and that's also the reason for not having an RCD at the other end of the feed cable).

Kind Regards, John

 

[ericmark]

My mothers house has a second consumer unit in the kitchen supplied from a 50A MCB with SWA cable around outside of house and has around 6 RCBO's fitted. Oven, hob, ring final, boiler etc. Works well and compliant until 2016 when metal consumer units come in.

 

[JohnW2]

Works well and compliant until 2016 when metal consumer units come in.

I don't think you should loose any sleep over that. I'm sure that there will be plenty of plastic CUs (quite probably some of them being compliant) still in service in 25, if not 50, years time!

Kind Regards, John

 

[westie101]

Yep, but if, per above, you did not have RCD protection upstream, this CU would need to have RCD(s) - ideally two, to allow circuits to be split between them (so one fault does not 'take out' all power to extension (and that's also the reason for not having an RCD at the other end of the feed cable).

Sorry but is that not a bit OTT
The house already has a split CU so only one half of it would be lost.
By simply adding an RCD protected CU that is reduced to only one third could be lost.

 

[JohnW2]

Yep, but if, per above, you did not have RCD protection upstream, this CU would need to have RCD(s) - ideally two, to allow circuits to be split between them (so one fault does not 'take out' all power to extension (and that's also the reason for not having an RCD at the other end of the feed cable).

Sorry but is that not a bit OTT ... The house already has a split CU so only one half of it would be lost. By simply adding an RCD protected CU that is reduced to only one third could be lost.

Maybe, but the general interpretation of the concept of 'separation of circuits' is that it exists to avoid a situation in which a single fault can result in loss of all power (lighting, sockets etc.) in one part of a building (hence 'lights up' + 'sockets down' on one RCD, 'lights down' + 'sockets up' on the other). It obviously depends upon the nature and extent of this 'extension', but there certainly could be an argument for having separation of circuits within the extension.

Of course, as you sort-of imply, if some of circuits (e.g. lighting) in the extension were supplied from the house's main CU, there would not be an issue.

Kind Regards, John

 

[gordonspants]

Hi again, many thanks for the replies, much appreciated.

Looks like I am on the right track - I have also had a read of the screwfix catalogue and MK do a small 2 way consumer unit with a 63A rcd (which it says can be used as the incoming isolating switch) and a 50A mcb - something like that would do fine for the incoming end, then they can use standard cable (cheaper than the steel braided stuff) rated at 50 amps to get to the extension and the final consumer unit that will split the supply up to serve lights/sockets etc.

Compliance for the future, other than what will keep BCO happy, is waaaay down my list. Give it a few more years and EU regulation 6543/A/7624587 will require anyone flicking a lightswitch to complete a risk assessment before they even get their insulated rubber gloves on... I remember when fuses were bits of wire you changed with a kitchen knife. All this used to be fields you know...

Ta

 

[JohnW2]

Looks like I am on the right track - I have also had a read of the screwfix catalogue and MK do a small 2 way consumer unit with a 63A rcd (which it says can be used as the incoming isolating switch) and a 50A mcb - something like that would do fine for the incoming end, then they can use standard cable (cheaper than the steel braided stuff) rated at 50 amps to get to the extension and the final consumer unit that will split the supply up to serve lights/sockets etc.

Fair enough, but I would suggest you make sure that your electrician is happy with that approach. Actually, depending upon the routing of the cable, it is possible that you might not need armoured cable, even if you did not have an RCD at the upstream end. It might also possibly be necessary to run a separate earth cable to the new CU in the extension - your electrician will be able to advise.

Kind Regards, John

 

[ban-all-sheds]

We are building an extension to the rear of our house.

What did you/will you say, or allow to be assumed, will be the way that you'll ensure compliance with Part P?

 

[securespark]

We are building an extension to the rear of our house.

What did you/will you say, or allow to be assumed, will be the way that you'll ensure compliance with Part P?

Mist Al-f*cking Chrighty...

 

[Adam_151]

I remember when fuses were bits of wire you changed with a kitchen knife.

Believe it or not BS3036:1956 (semi enclosed re-wireable fuses) is still a current British standard. It is merely 'suggested' that they should not be preferred for a new installation these days.

Back to your extension, It is considered bad form to provide RCD protection at the start of a sub-main, sensible design practice puts this at the local board. One should also try and avoid supplying sub mains from MCBs... they generally do not discriminate with one another at all. It is not uncommon for a fault on a 16A final circuit to take out the 63A C type supplying the board. a 60A switch-fuse makes more sense.

As to avoid RCD protection of the submain, it might be possible to route a twin and earth in such a way that it does not need RCD protection, even if a small sub of galv conduit has to be used to get down to the new board. Alternatively 20m of 16mm 3c armoured is not going to break the bank in comparison to other costs of the extension. Around the £100 for 20m

 

[JohnW2]

Back to your extension, It is considered bad form to provide RCD protection at the start of a sub-main, sensible design practice puts this at the local board. One should also try and avoid supplying sub mains from MCBs... they generally do not discriminate with one another at all. It is not uncommon for a fault on a 16A final circuit to take out the 63A C type supplying the board. a 60A switch-fuse makes more sense.

Indeed.

As to avoid RCD protection of the submain, it might be possible to route a twin and earth in such a way that it does not need RCD protection, even if a small sub of galv conduit has to be used to get down to the new board.

Yes, I have made that point and, anyway, as you go on to say ...

Alternatively 20m of 16mm 3c armoured is not going to break the bank in comparison to other costs of the extension. Around the £100 for 20m

Quite, but, more to the point, 20m of T+E would cost about £93 (TLC price), so the price differential is almost non-existent (even if one adds on the cost of the SWA glands).

Kind Regards, John

 

[Adam_151]

Yes, I have made that point
Kind Regards, John

The OP didn't seem to be hearing you properly...

 

[JohnW2]

Yes, I have made that point

The OP didn't seem to be hearing you properly...

Maybe, but to be fair, the OP hasn't been back since I made that point yesterday evening. I had previously made the point that, without an upstream RCD, it might (depending on installation method) be necessary to use SWA. Yesterday, I re-stated it the other way around, by saying that, with some installation methods, SWA might not be needed!

Whatever, as we have said between us, the marginal price difference between 16mm² SWA and T&E is so small (particularly in the context of the total cost of an extension) that I don't think that cost should enter at all in to the decision melting pot!

Kind Regards, John

 

[gordonspants]

We are building an extension to the rear of our house.

What did you/will you say, or allow to be assumed, will be the way that you'll ensure compliance with Part P?

Mist Al-f*cking Chrighty...

Part P doesn't apply here. Not in my house anyway.