Adding a second consumer unit in a extension

[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!

 

[urbanmonkey]

Perhaps if the run is 15m, the OP is terrified of touching the original CU and a 2 way one is going in, why not have it next to the original board and run the circuits out to where they need to go, assuming its a small extension that is! Although its probably a new kitchen and dining room, 3 en-suite bedrooms upstairs and a hot tub on the patio!

 

[D Skelton]

I don't know why the OP can't extend existing circuits to meet the demand of the new extension?! If they are that bad, they need rewiring don't they? Major building work taking place would be the perfect time to bring said circuits up to standard.

If one if my lads suggested a new DB on a domestic extension, supplying just the extension, he would have to work very hard to justify his position and avoid getting the boot!

 

[Slowhand]

I don't know why the OP can't extend existing circuits to meet the demand of the new extension?! If they are that bad, they need rewiring don't they? Major building work taking place would be the perfect time to bring said circuits up to standard.

If one if my lads suggested a new DB on a domestic extension, supplying just the extension, he would have to work very hard to justify his position and avoid getting the boot!

If he did it twice would he get a pair of boots?
Only asking cos mine are split ;-)

 

[D Skelton]

If he did it twice would he get a pair of boots?
Only asking cos mine are split ;-)

If he did it twice you'd have to join him on the dole queue to find out!

 

[urbanmonkey]

I suppose we shouldn't be so harsh on him, we all have to start somewhere......

College usually!

Totally agree it makes no sense to not extend the existing circuits, wouldn't take much testing to find out what (if any) remedial works need doing in order to bring them up to scratch. Job i'm on this week is a kitchen and conservatory fit, old board is coming out, old circuits needed nothing doing to them to take the conservatory, new circuits for the kitchen go in new board. Everybody's happy. Its not hard really is it.

 

[Octopus]

I was working in a house this morning and there was a "handyman" working in the loft - so he claims to be a 30 years + spark, not up to 17th nor a scheme member - he gets his "mate" to sign off work.

The scope of his work today was to put boards down in the loft - and to "avoid" the cables he was cutting notches out of the top of the joists.............

then......he sticks a screw through the lighting cable.....lord only knows how he's repaired the cable

and no sign of any testing on the "new" lights in the loft!

And who says all the newbies are responsible for the lax installations....