Two Cu's in One dwelling.

[Spudnik]

Hi all,

Went to look at a job today that needs a little fettling.

Set up is.

House (very old) with meter situated on the first floor landing.

From here we have 25mm tails to a dp service block, with two 16mm T&E's, one going to a CU in the same cupboard, which feeds various circuits in the loft conversion.

Second drops down stud wall to the under-stairs cupboard to an old wylex which does the rest of the 'existing' house.

Can anyone see any reason why i cannot replace these with a standard CU with RCD main switch for each one, rather than dual RCD for both?

Much obliged.

J

 

[telectrix]

can't see a problem with that. if 1 board is only , say a 4/6 way, you could consider RCBOs for better separation.

 

[Spudnik]

Cheers Tel,

I was thinking that however, its a Proteus CU and they only do 2 mod RCBO's that i could find and so they wont fit.

 

[rich.250]

314.1 and 2 require that every installation should be divided into circuits as necessary to avoid danger and minimise inconvenience in the event of a fault. Designers are required to reduce the possibility of unwanted RCD tripping due to excessive protective conductor currents but not due to an earth fault.

I see this can be achieved in one house with two DBS.
Or as tel says rcbos if the customer wants to pay a little bit more for a far better set up lol

 

[Spudnik]

Thanks fellas i thought as much

2 CU's with RCD mains it is

 

[Guitarist]

Jason, is the 16mm T&E that runs inside the stud wall mechanically protected in any way as I'm assuming it will not be RCD protected? I'm also assuming that the run is less than 3M as it is straight off the splitter block.
Apologies if I have the wrong end of the stick.

 

[Engineer54]

314.1 and 2 require that every installation should be divided into circuits as necessary to avoid danger and minimise inconvenience in the event of a fault. Designers are required to reduce the possibility of unwanted RCD tripping due to excessive protective conductor currents but not due to an earth fault.

I see this can be achieved in one house with two DBS.
Or as tel says rcbos if the customer wants to pay a little bit more for a far better set up lol

When does dividing circuits extend to dividing into sub-mains in a standard size house?? lol!! The Reg your quoting, is basically stating that you shouldn't have too many circuits being covered by a single RCD where accumulated natural leakage can give rise to nuisance tripping... It certainly doesn't mean install another CU/DB!!

All this lazy electrician has done, is supply an un-needed sub-main CU to cover a tiny roof conversion that probably could have been adequately covered by extending existing circuits on the first floor!!

 

[Richard Burns]

Thanks fellas i thought as much

2 CU's with RCD mains it is

I like that as well, best solution all round.
I found one house that had two single RCD boards one in the front hall, one in the living room, thought it was OK.

 

[Spudnik]

Jason, is the 16mm T&E that runs inside the stud wall mechanically protected in any way as I'm assuming it will not be RCD protected? I'm also assuming that the run is less than 3M as it is straight off the splitter block.
Apologies if I have the wrong end of the stick.

Yes, i did miss out the part where i would also install a switch fuse before the DP service block.

When does dividing circuits extend to dividing into sub-mains in a standard size house?? lol!! The Reg your quoting, is basically stating that you shouldn't have too many circuits being covered by a single RCD where accumulated natural leakage can give rise to nuisance tripping... It certainly doesn't mean install another CU/DB!!

All this lazy electrician has done, is supply an un-needed sub-main CU to cover a tiny roof conversion that probably could have been adequately covered by extending existing circuits on the first floor!!

You are correct, however, it does not state how the division of circuits is achieved

I like that as well, best solution all round.
I found one house that had two single RCD boards one in the front hall, one in the living room, thought it was OK.

Indeed, and that is what i proposing to do.

 

[Jay Sparks]

I have just done a EICR on a bungalow that had two boards in, both with rcd as main switch. Basically, they had some work done (bedroom in loft space) and the electrician fitted an new board for the excisting install with rcd main switch and a new board for the loft with lights and ring main on it, this also had rcd main switch. Now the sparkys did one without giving the client an EIC and so when the LABC asked for one, they were knackered. So, they asked me. I said I could only do an EICR (100%) on the install and they would have to ask the LABC if this would be ok....anyway I did the EICR and as far as I know, the LABC didnt have a problem with two boards in one dwelling, both with rcd main switch's.
Hope this made sense, Ive been out for a few games of snooker and Ive had a few...........




Jay

 

[Engineer54]

I like that as well, best solution all round.
I found one house that had two single RCD boards one in the front hall, one in the living room, thought it was OK.

It's anything BUT the best solution!!

 

[Engineer54]

I have just done a EICR on a bungalow that had two boards in, both with rcd as main switch. Basically, they had some work done (bedroom in loft space) and the electrician fitted an new board for the excisting install with rcd main switch and a new board for the loft with lights and ring main on it, this also had rcd main switch. Now the sparkys did one without giving the client an EIC and so when the LABC asked for one, they were knackered. So, they asked me. I said I could only do an EICR (100%) on the install and they would have to ask the LABC if this would be ok....anyway I did the EICR and as far as I know, the LABC didnt have a problem with two boards in one dwelling, both with rcd main switch's.
Hope this made sense, Ive been out for a few games of snooker and Ive had a few...........
Jay

Just goes to show the standard of electricians these day's, or i should more correctly say, DI trained persons!! lol!!

 

[Deleted member 9648]

When does dividing circuits extend to dividing into sub-mains in a standard size house?? lol!! The Reg your quoting, is basically stating that you shouldn't have too many circuits being covered by a single RCD where accumulated natural leakage can give rise to nuisance tripping... It certainly doesn't mean install another CU/DB!!

All this lazy electrician has done, is supply an un-needed sub-main CU to cover a tiny roof conversion that probably could have been adequately covered by extending existing circuits on the first floor!!

Unless I'm reading jasons OP wrong the loft conversion CU is in the same cupboard as the meter....it is the existing circuits CU which is remote downstairs. There are a multitude of reasons why a separate CU for the loft conversion may have made sense to the original installer....(heavy loads added to the 16mm submain..shower?....existing circuits not suitable for extending?)....because of your illogical hatred of more than one CU in a dwelling you jump in all guns blazing without knowing the circumstances.

 

[Spudnik]

It's anything BUT the best solution!!

Why?

Unless I'm reading jasons OP wrong the loft conversion CU is in the same cupboard as the meter....it is the existing circuits CU which is remote downstairs. There are a multitude of reasons why a separate CU for the loft conversion may have made sense to the original installer....(heavy loads added to the 16mm submain..shower?....existing circuits not suitable for extending?)....because of your illogical hatred of more than one CU in a dwelling you jump in all guns blazing without knowing the circumstances.

Indeed, you are correct wp.

There is no shower upstairs, purely a couple of rooms and a wc.

No, its not ideal, however, the installation will not be left any worse that it was originally.

I am all for sticking rigorously to the regs, however, common sense also has to be applied.



Now, of course with the submain, this will be split concentric and will be run inside a stud wall from top to bottom within 150mm of the corner.

Therefore, of course, as it will be less than 50mm from the surface will need to be RCD protected.

With that in mind, i am considering installing a 80A 30mA RCD in the meter cupboard to protect this, which will mean that the downstairs CU will just be a standard 8 Way with a main switch.

Alternatively, i could always run in down it the corner of the stairwell, external of the stud wall and box it in as they are yet to decorate.

Hmmm.

 

[rich.250]

When does dividing circuits extend to dividing into sub-mains in a standard size house?? lol!! The Reg your quoting, is basically stating that you shouldn't have too many circuits being covered by a single RCD where accumulated natural leakage can give rise to nuisance tripping... It certainly doesn't mean install another CU/DB!!

All this lazy electrician has done, is supply an un-needed sub-main CU to cover a tiny roof conversion that probably could have been adequately covered by extending existing circuits on the first floor!!

Yes E54, it isn't the best solution I agree. But all I said was it could be achieved this way. Although now reading that the submains or concealed in a wall, they will require Rcd if not mechanically protected(unlikely)
So you would have no division would you??
I would say rcbos every time.
And find out exactly how the subs are installed.
Also re reading, there is no OCPD for them currently?

 

[Rockingit]

E54 - sometimes life in domestic isn't perfect! It's not necessarily through laziness or lack of skill / experience, just sometimes you have to find a solution to solve a problem. If one solution is running another CU at x amount of cost, and another solution is to extend existing circuits at x2 amount of cost, then I know which option Mrs Jones is going to ask for. Have you also ever considered that most of the 70's UK housing stock have meter cupboards built around the shoe box size of a Wylex 3036 with 5 or 6 ccts - you try shoehorning a modern split board in when you need an extra couple of circuits.....without destroying most of the adjacent kitchen units.

I don't think that any of us would CHOOSE to be pushed down this route, just sometimes you have to play the cards you've been dealt.

 

[Spudnik]

The division of circuits would be sw fuse, Dp SB, tails to new RCD protected cu in meter cupboard, then to an RCD in the cupboard, then split con internally of the stud wall with 150mm of the corner to new cu under the stairs with a std main switch.

Alternatively i could run the spilt con external to the wall which could then be boxed in, so the RCD could in the CU downstairs.

 

[Spudnik]

Yes E54, it isn't the best solution I agree. But all I said was it could be achieved this way. Although now reading that the submains or concealed in a wall, they will require Rcd if not mechanically protected(unlikely) Correct
So you would have no division would you?? Yes there would be , as the upstairs CU is separate from CU for the rest of the house.
I would say rcbos every time. This isnt the issue
And find out exactly how the subs are installed. Downstairs CU is fed from SB with 16mm TE down a stud wall
Also re reading, there is no OCPD for them currently? For the downstairs sub main, no.

My bold.

 

[rich.250]

What I mean Jason is if you put 30mA on the the supply cable(cos buried) you will then have no segregation between that and the board.

I think rcbos would be the best solution as each cct is then completely segregated.

 

[Spudnik]

Ahh i see rich, however, a std dual rcd CU would give the same result, which we all generally fit now to achieve the division.

 

[rich.250]

Yes it would to the minimum level.
I see your problem. The sub needs 30mA but then no discrimination between the Rcd's in the board and the start of the sub.
Without seeing the job it's difficult to say what I'd do to make the subs comply

 

[Guitarist]

Jason, I would think carefully about boxing non-RCD'd cable in, as it is then technically no longer surface mounted (ie. obviously a cable there). Has to be surface mini-trunking/conduit or else earthed steel conduit inside the stud imo (for what it's worth). I think your idea protecting the cable feeding the downstairs CU (which then has a main switch) is a safer one.

 

[spark 68]

What he could do,
is put a standard split load dual RCD U/S, supply all the U/S circuits from MCB's on one side of the board,

and then supply the sub main and D/S board from a 40A, 45A, 50A (whatever is the largest MCB for that board) from the second side of that (U/S) board.

The downstairs board having no RCD's fitted at all, this will then solve the submain rcd problem, not ideal but workable.

 

[Engineer54]

Unless I'm reading jason's OP wrong the loft conversion CU is in the same cupboard as the meter....it is the existing circuits CU which is remote downstairs. There are a multitude of reasons why a separate CU for the loft conversion may have made sense to the original installer....(heavy loads added to the 16mm submain..shower?....existing circuits not suitable for extending?)....because of your illogical hatred of more than one CU in a dwelling you jump in all guns blazing without knowing the circumstances.

Your right i don't like them, they make no sense at all in a typically sized house, even less so in a bungalow, ...especially when a new CU is being installed!!

Have no idea what your on about here, with the heavy loads?? Are you saying that a typical 80A supply on 16mm tails CU isn't suitable or capable of supplying a typical house?? So when is an existing upstairs outlet circuit or lighting circuit not suitable to be extended?? They are probably going to be the most under loaded and easily got at circuits in the house!!

There are very few circumstances where a second CU would be the correct path to take, ....and there is Nothing illogical about that!! ...lol!!

 

[Engineer54]

E54 - sometimes life in domestic isn't perfect! It's not necessarily through laziness or lack of skill / experience, just sometimes you have to find a solution to solve a problem. If one solution is running another CU at x amount of cost, and another solution is to extend existing circuits at x2 amount of cost, then I know which option Mrs Jones is going to ask for. Have you also ever considered that most of the 70's UK housing stock have meter cupboards built around the shoe box size of a Wylex 3036 with 5 or 6 ccts - you try shoehorning a modern split board in when you need an extra couple of circuits.....without destroying most of the adjacent kitchen units.

I don't think that any of us would CHOOSE to be pushed down this route, just sometimes you have to play the cards you've been dealt.

I think your costings are going to be completely the other way round!!

I have nothing against adding an additional CU, as and when needed, but not via a sub-main to another distant location, not in your standard sized house anyway!!

I think i have stated on here before, i ended up with 3 CU's in the house i had built in Cyprus. I went ballistic when i saw a separate 3 phase CU on the upstairs landing, especially when i had supplied the builders electrician with a comprehensive electrical layout drawing!! ..lol!! The third CU was located in an external service room and covered all the external circuits, swimming pool, pressurised water system and future boiler provision... Which was fine, as i hadn't included a layout drawing, only notes on for ''provision to be made''!! Just a shame he didn't provide any cable ducts/conduits etc, to the outside!!

 

[Deleted member 9648]

Your right i don't like them, they make no sense at all in a typically sized house, even less so in a bungalow, ...especially when a new CU is being installed!!

Have no idea what your on about here, with the heavy loads?? Are you saying that a typical 80A supply on 16mm tails CU isn't suitable or capable of supplying a typical house?? So when is an existing upstairs outlet circuit or lighting circuit not suitable to be extended?? They are probably going to be the most under loaded and easily got at circuits in the house!!
The existing circuits CU is supplied by a 16mm sub from a first floor meter cupboard to a ground floor CU serving the rest of the house....I used the shower as an example...adding a 40-50a shower to a 16mm submain already serving the entire house may not be a good idea. An existing lighting circuit may not be suitable for extending when it does not include a CPC...a socket outlet circuit may not be suitable for extending when it is not a ring or has an excessive number of spurs. All examples of when it may be better to leave the existing well alone (assuming the client does not want addituional work)and install a new CU specifically for the work to be carried out.
There are very few circumstances where a second CU would be the correct path to take,see above ....and there is Nothing illogical about that!! ...lol!! It is illogical to take such a blinkered view as you do ....lol!!!

...........

 

[Engineer54]

I think it's far easier to make-up excuses as you go along... lol!!

My points on multiple CU's in standard sized houses are general statements/observations Not particularly based around Jason's work place, which already has an existing additional CU....

Going back to your last post, ... If both restrictions exist as in your example, then it's a simple job to run a radial socket circuit to cover the bedroom(s) and a lighting circuit. So just 2 regular cables, to be run in (or 3 in the unlikely event of a shower being required...)

A 80A/16mm supply will be well able to supply all these circuits, this standard set up seems to work everywhere else for gods sake!!

 

[Guest55]

Now, of course with the submain, this will be split concentric and will be run inside a stud wall from top to bottom within 150mm of the corner.


With that in mind, i am considering installing a 80A 30mA RCD in the meter cupboard to protect this, which will mean that the downstairs CU will just be a standard 8 Way with a main switch.


Hmmm.

I dont see the point of using split con , why not spend an extra few quid on SWA ? It will mean the sub-mains wont have to be rcd protected , and the rcd can be fitted to the CU that it supplies , where it should be.

Having a rcd remote from the CU when its providing protection for that CU's final circuits is poor design to be honest as you have look in 2 different places to check after loss of supply.

Alternatively , use a time delay s type for submains , and standard rcd in CU.

 

[Risteard]

Alternatively , use a time delay s type for submains , and standard rcd in CU.

An S-type RCD cannot be used for the provision of additional protection.

 

[Guest55]

^^ good point and well done for spotting my intentional error lol
glad someones paying attention ;-)