Adding a second consumer unit in a extension

[cookie999999]

Hi only recently qualified and gone out on my own just landed my first big job wiring a new extension. I recomended to the client that a second cu for the extension might be a good idea as the existing house wiring is all in need of up dating and did not want to get involved with the old cu unit. The problem I have is that he wants the new cu in the new part and it is 15 meters away from the supply and meter. He has gone out without my advice and bought 20 meters of 25mm tails and earth. Which is not cheap. And I have told him that the tails should not be over 3 meters long. Could I get round this by putting a 100amp isolator after the meter then in to Henley blocks and the reconnect the old cu and then run the tails to the new cu ? Any help would be appreciated

 

[Sintra]

I would be more concerned at how you would run tails for 20 meters and give them adequate protection. IMO a SWA cable would be needed. Why even is the client supplying materials?

 

[Leesparkykent]

exactly what sintra said how would you make this comply?

 

[Guest125]

As above but also if you disconnect the existing board to "chop" into the tails you should be sure the existing installation is safe to re-energise.

 

[Pete999]

Did you tell the client that to do what they wanted would involve running in new tails and what size they would need, without giving them any other options? sounds like you told them that new tails were required and he has gone out and bought them, don't know how you feel but it seems like they don't trust your judgement that much, mind you that's only my opinion, just can;t understand why he has gone and paid for new tails, as has been said how would your method comply IMO SWA would have been the best method.

 

[davesparks]

If the existing house wiring is in need of attention then that needs to be included in the works. What exactly is wrong with it that requires updating?
Whatever you do you will need to ensure the correct bonding is in place so will almost certainly be needing to do work in the existing house to achieve this.

Also the million dollar question is, as always, what have you just qualified as and with what qualifications?

 

[Leesparkykent]

how big is the extension/whats going in there to require 25mm? IMO running a swa over to new consumer unit is a lot of effort and cost when you could just run the new circuits from the extension (same route you would of prob used for swa/tails)and put them on its own consumer next to existing consumer unit. plus you will have to install bonding to gas and water if not already in place.

 

[Guest125]

If the existing house wiring is in need of attention then that needs to be included in the works. What exactly is wrong with it that requires updating?
Whatever you do you will need to ensure the correct bonding is in place so will almost certainly be needing to do work in the existing house to achieve this.

Also the million dollar question is, as always, what have you just qualified as and with what qualifications?

Nooooooooooooo!!! Please not this one again.

 

[telectrix]

Hi only recently qualified and gone out on my own just landed my first big job wiring a new extension. I recomended to the client that a second cu for the extension might be a good idea as the existing house wiring is all in need of up dating and did not want to get involved with the old cu unit. The problem I have is that he wants the new cu in the new part and it is 15 meters away from the supply and meter. He has gone out without my advice and bought 20 meters of 25mm tails and earth. Which is not cheap. And I have told him that the tails should not be over 3 meters long. Could I get round this by putting a 100amp isolator after the meter then in to Henley blocks and the reconnect the old cu and then run the tails to the new cu ? Any help would be appreciated

1. you need a switch/fuse where the fuse has a lower In than the DNO fuse, to protect the DNO fuse from a fault downstream.

2. tails must comply with reg 522.6.101. how you do this is up to you.

 

[Octopus]

Hi only recently qualified and gone out on my own just landed my first big job wiring a new extension. I recomended to the client that a second cu for the extension might be a good idea as the existing house wiring is all in need of up dating and did not want to get involved with the old cu unit. The problem I have is that he wants the new cu in the new part and it is 15 meters away from the supply and meter. He has gone out without my advice and bought 20 meters of 25mm tails and earth. Which is not cheap. And I have told him that the tails should not be over 3 meters long. Could I get round this by putting a 100amp isolator after the meter then in to Henley blocks and the reconnect the old cu and then run the tails to the new cu ? Any help would be appreciated

Matey - if you are going to be the one signing off the work and dealing with the Part P - you and you alone need to design and install and test.

Letting clients supply materials is always going to end in tears.... so tell him he's wasted his money and go and buy the correct materials.

I hope you've got your tin hat on.

 

[D Skelton]

Recently qualified with what qualifications?

 

[HandySparks]

1. you need a switch/fuse where the fuse has a lower In than the DNO fuse, to protect the DNO fuse from a fault downstream.

TBH, providing that the Zs on the distribution circuit is OK, I think you could use the same rating of fuse as the DNO, if that's what you need. It's just there to comply with the DNO's requirements that they're not providing sole protection for long tails. You'd have to go a LOT lower to get reliable discrimination. If the DNO's fuse blows in the event of a fault, it's not dangerous, just inconvenient.

 

[Guest77]

Hi only recently qualified and gone out on my own just landed my first big job wiring a new extension. I recomended to the client that a second cu for the extension might be a good idea as the existing house wiring is all in need of up dating and did not want to get involved with the old cu unit. The problem I have is that he wants the new cu in the new part and it is 15 meters away from the supply and meter. He has gone out without my advice and bought 20 meters of 25mm tails and earth. Which is not cheap. And I have told him that the tails should not be over 3 meters long. Could I get round this by putting a 100amp isolator after the meter then in to Henley blocks and the reconnect the old cu and then run the tails to the new cu ? Any help would be appreciated

What existing earthing arrangement is in the house?
is there any bonding main or supplementry anywhere?
how are you planning to run the cables?
have you told the house owner to take the tails back yet and get some proper cabling for the job?
are you registered with a scheme to sign off the work?
have you got a MFT?
What insurance have you got?
what load is going into the new extension?

 

[Guest77]

Nooooooooooooo!!! Please not this one again.

Why not, I think the question is justified, especially after reading the first post which was pretty worrying.

 

[Guest125]

Why not, I think the question is justified, especially after reading the first post which was pretty worrying.

only because I think I know where it will lead.

 

[Guest77]

Well hopefully the guy will come back and answer some of the questions we asked, we can then re-assess the job, there is a lot more he could tell us which would help us answer his questions, so far we have nothing.

 

[telectrix]

have nothing, i know nothing. i'm from barcelona. que?

 

[Octopus]

This is the classic example of a poor thread where the OP seems to have scarpered.

It should be locked by the mods until the OP comes back with information........... or else it'll just end up in the forum sewer with so many others....

 

[Guest77]

This is the classic example of a poor thread where the OP seems to have scarpered.

It should be locked by the mods until the OP comes back with information........... or else it'll just end up in the forum sewer with so many others....

I have just read his previous 3 posts before this thread, don't build your hopes up.

 

[GMES]

only because I think I know where it will lead.

But if you think about it we have to ask these questions to be able to gauge if the op is up to the job,and also to be able to offer him the best advice based on information we receive.
Think about it, if we just told him how to do the job, we could be putting someone in danger.
If we are not allowed to ask questions and interact with a new member then we may as well all leave the forum and just leave an automated answering system in our place.

 

[trev]

Press 1 if you are a diyer
2 if you're a Electrical Trainee
3 if you need help with an RCD tripping issue
4 if you have left your regs book somewhere/had it eaten by rabid dogs
etc

 

[Tony Barnes]

Hi cookie

My view is definitely don't use metre tales I would use a 16mm twin & earth with a separate 16mm earth along side it providing this can take the load of the extention. Split the existing tales with a Henley block (ensure that you get a DPI fitted by electric board first) then supply the existing consumer unit and also a 60A fused isolator to supply extention sub main. Upgrade the bonding conductors to gas & water services if required.

Hope it helps

 

[Octopus]

Press 1 if you are a diyer
2 if you're a Electrical Trainee
3 if you need help with an RCD tripping issue
4 if you have left your regs book somewhere/had it eaten by rabid dogs
etc

5 if you don't have your tester with you!

 

[GMES]

Press 1 if you are a diyer
2 if you're a Electrical Trainee
3 if you need help with an RCD tripping issue
4 if you have left your regs book somewhere/had it eaten by rabid dogs
etc

I was only joking Trev but it may just be the future!!

 

[Guest77]

This is Electrical Forums.

Press 1 for new shed supplies
press 2 for exporting PME
Press 3 for distribution board changes
Press 4 for shared neutrals
Press 5 for changing lights with no earth wires
Press 6 for faulty RCDs

pmsl haha

 

[HandySparks]

What existing earthing arrangement is in the house?
is there any bonding main or supplementry anywhere?
how are you planning to run the cables?
have you told the house owner to take the tails back yet and get some proper cabling for the job?
are you registered with a scheme to sign off the work?
have you got a MFT?
What insurance have you got?
what load is going into the new extension?

Good point. I may not have asked ALL those questions, but there's a fair amount of info missing.

 

[davesparks]

Nooooooooooooo!!! Please not this one again.

Not what one? Asking what is wrong with the existing installation? Or stating that the bonding will need upgrading as a matter of course? Or asking what he has recently qualified as?

They all seem to be perfectly relevant questions, and the OP even gave me a thanks for that post!

 

[davesparks]

how big is the extension/whats going in there to require 25mm? IMO running a swa over to new consumer unit is a lot of effort and cost when you could just run the new circuits from the extension (same route you would of prob used for swa/tails)and put them on its own consumer next to existing consumer unit. plus you will have to install bonding to gas and water if not already in place.

Or else it is entirely possible that extending the existing circuits may be all that is needed.

But we'll probably never get to find out unless the OP reappears.

 

[Guest77]

Not what one? Asking what is wrong with the existing installation? Or stating that the bonding will need upgrading as a matter of course? Or asking what he has recently qualified as?

They all seem to be perfectly relevant questions, and the OP even gave me a thanks for that post!

They were, nothing wrong with them at all.

 

[Engineer54]

Why a separate CU for an extension?? If the existing installation needs cleaning up at the existing CU's location then so be it, clear it up and install a suitable sized CU in the process....

As has already been pointed out, some existing circuits could well be extended into the extension saving both money and time... Never understood this mentality that house extensions and the like need a remote separate CU!! It's far more like, it just makes things easier for the installer, and nothing much else matters. It would have to be a bloody big place before i'd contemplate installing remotely located CU's in a domestic house!!!

 

[Guest125]

Why a separate CU for an extension?? If the existing installation needs cleaning up at the existing CU's location then so be it, clear it up and install a suitable sized CU in the process....

As has already been pointed out, some existing circuits could well be extended into the extension saving both money and time... Never understood this mentality that house extensions and the like need a remote separate CU!! It's far more like, it just makes things easier for the installer, and nothing much else matters. It would have to be a bloody big place before i'd contemplate installing remotely located CU's in a domestic house!!!

I think it is a get around for requiring to bring the the existing installation upto current standards with a DB change. Don't touch any of the existing circuits no need to test and give the owner a load of bad news. (Potentially)

 

[Leesparkykent]

most of the time it comes down to cost and some customers are not willing to spend money on the existing installation, even tho they should! seen it plenty of times, they just see it as its worked fine for x amount of years.

 

[andyelectric]

My suspicion is that the original cu is a wylex rewireable, and makes the inexperienced op (possibly) wrongly think some rewiring is required to the original circuits. I am with E54 in that the new seperate CU is probably the wrong way to go. But as often happens the op has disappeared so it is crystal ball time.

 

[jase]

As has already been mentioned, no need to touch the existing installation, just split the tails at the meter, put a new 2w cu in for the extension and run in power and lighting circuits. Job done. Check the bonding and install if missing as this is a requirement of any job regardless of wether you're running in a new lighting circuit or a new sub-main!

 

[davesparks]

As has already been mentioned, no need to touch the existing installation, just split the tails at the meter, put a new 2w cu in for the extension and run in power and lighting circuits. Job done.

No need for bonding or a single point of isolation in your regs book then?

 

[Simlec]

I want to know why Tony would install a 16mm Earth alongside the 16mm T+E ?

 

[TJ Anderson]

I want to know why Tony would install a 16mm Earth alongside the 16mm T+E ?

I thought the same. Maybe he gets Nectar points when he spends on materials?

 

[davesparks]

I want to know why Tony would install a 16mm Earth alongside the 16mm T+E ?

You've had me reading through the whole thread looking for a post by Tony, thinking that doesn't sound like him at all, only to realise that there is someone else of the same name on here.

It does seem like a daft idea doesn't it, and if you did need a 16mm earth it would surely be cheaper to get a cable with a full size earth like NYYJ or similar

 

[ayjay]

I want to know why Tony would install a 16mm Earth alongside the 16mm T+E ?

me too-- overkill i suspect

 

[Simlec]

You've had me reading through the whole thread looking for a post by Tony, thinking that doesn't sound like him at all, only to realise that there is someone else of the same name on here.

Lol yes not that Tony

TJ, I bet the staff at his wholesale always greet him with a bit smile.

 

[ronalsparks]

Hi using 25.0mm tails and a 16.0mm earth gets over any bonding issues and the way you could run the supply to the second consumer unit is to use a small 100amp switch fuse which you could rate at 60 / 80amps which are readily available (assuming a standard domestic installation supply) to provide some discrimination over the supply fuse and any load on the existing area. The only problem is that the cabling needs to be below 50mm to comply with the current regs on buried cables otherwise this requires RCD protection and therefore an SWA would be required to comply with this unless you can meet the conditions of the regs by protection, placing in a route where it is not going to be subject to being nailed through and deep enough. Of course if this is a TT supply it will require RCD protection anyway? I am assuming a TN-C-S / PM-E supply in this instance but you also need to check the earth loop of the incoming supply and disconnection times of the BS fuse used in the switch fuse for disconnection times over the length of the cable run and final circuits fed from there on in. I am further assuming that the final circuits will be twin and earth and less than 50mm buried and will be RCD protected thus using a 30ma RCD given the touch voltage of 50v divided by the trip current allows a maximum earth loop value of 1666.66 ohms. A more realistic value on a TN-C-S /PME installations is likely to be at maximum 0.35 ohms at the origin and final circuits not in excess of the rated values for Mcb's as per Current IET wiring regs.

Probably a a bit over the top but hope this gives a good guide and this is how I would approach it.

Don't you just hate it when customers go off and have half the information and because we drive a van they instantly know more than we do with only part of the information GOOD LUCK!

 

[davesparks]

How does using 25mm tails and 16 mm earth get over any bonding issues?
There will still need to be main bonding installed to extraneous parts whatever size cable is used, and the incoming services are almost certainly going to be in the existing building unless they are going to be moved into the extension.

 

[D Skelton]

Hi using 25.0mm tails and a 16.0mm earth gets over any bonding issues

How?????

and the way you could run the supply to the second consumer unit is to use a small 100amp switch fuse which you could rate at 60 / 80amps which are readily available

A 100A supply for a couple of lights and sockets?????

(assuming a standard domestic installation supply) to provide some discrimination over the supply fuse and any load on the existing area.

Why do you think you need to provide discrimination with the supply fuse? That isn't part of the consumer's installation is it?!

The only problem is that the cabling needs to be below 50mm to comply with the current regs on buried cables otherwise this requires RCD protection and therefore an SWA would be required to comply with this unless you can meet the conditions of the regs by protection, placing in a route where it is not going to be subject to being nailed through and deep enough.

I don't know of a single decent electrician that would run a buried sub main in T+E. Anyone worth their salt would be running sub mains, whatever their reference method, in SWA.

Of course if this is a TT supply it will require RCD protection anyway?

Not necessarily.

I am assuming a TN-C-S / PM-E supply in this instance but you also need to check the earth loop of the incoming supply and disconnection times of the BS fuse used in the switch fuse for disconnection times over the length of the cable run and final circuits fed from there on in.

Huh? What bearing on individual final circuits does the disconnection time of the sub main supply fuse have?????

I am further assuming that the final circuits will be twin and earth and less than 50mm buried and will be RCD protected thus using a 30ma RCD given the touch voltage of 50v divided by the trip current allows a maximum earth loop value of 1666.66 ohms.

Nope, on a TN system the max loop value allowed on a circuit protected by a 30mA RCD is 7667ohms. You're not using touch voltage, instead you're using Uo.

A more realistic value on a TN-C-S /PME installations is likely to be at maximum 0.35 ohms at the origin and final circuits not in excess of the rated values for Mcb's as per Current IET wiring regs.

Rated values of what???

Probably a a bit over the top but hope this gives a good guide and this is how I would approach it.

I wouldn't say over the top, more like you've just swallowed a regs book and got it all muddled up when you've pooped out the words?!

Don't you just hate it when customers go off and have half the information and because we drive a van they instantly know more than we do with only part of the information GOOD LUCK!

So your van is to blame for this gobbledegook?!

I'm totally lost right now!

 

[Simlec]

It will take him all week to respond to that lot, given his lack of understanding.

 

[richy3333]

I want to know why Tony would install a 16mm Earth alongside the 16mm T+E ?

To keep it company?

 

[richy3333]

Press 1 if you are a diyer
2 if you're a Electrical Trainee
3 if you need help with an RCD tripping issue
4 if you have left your regs book somewhere/had it eaten by rabid dogs
etc

Hang on Trev, this smacks of plagiarism in trying to adapt my suggestion of ---- word bingo based upon the usual forum responses! I'll collect my royalties in beer please

 

[trev]

I'm going to plead ignorance here mate.
Meaning I'm completely ignoring your beer money demand. See you in the car park for a discussion and pagga

 

[Guest77]

How?????



A 100A supply for a couple of lights and sockets?????



Why do you think you need to provide discrimination with the supply fuse? That isn't part of the consumer's installation is it?!



I don't know of a single decent electrician that would run a buried sub main in T+E. Anyone worth their salt would be running sub mains, whatever their reference method, in SWA.



Not necessarily.



Huh? What bearing on individual final circuits does the disconnection time of the sub main supply fuse have?????



Nope, on a TN system the max loop value allowed on a circuit protected by a 30mA RCD is 7667ohms. You're not using touch voltage, instead you're using Uo.



Rated values of what???



I wouldn't say over the top, more like you've just swallowed a regs book and got it all muddled up when you've pooped out the words?!



So your van is to blame for this gobbledegook?!

I'm totally lost right now!

Welcome to EF ronalsparks :smilielol5:

 

[urbanmonkey]

Its like the blind leading the blind here, kellogs must have brought out 'cut out and keep' JIB cards on the back of cornflake boxes or something.

 

[D Skelton]

Its like the blind leading the blind here, kellogs must have brought out 'cut out and keep' JIB cards on the back of cornflake boxes or something.

It's only gonna get worse mate!

One day, not too far from now, this place will be the new Screwfix forum! People like myself will be LONG gone by then!