Submain Cable to house extension

[tew1]

Hi, I'm just planning ahead whilst I'm redecorating and installing hardwood floors e.t.c. and want to run a cable 10 metres from my existing house's meter and CU between the upstairs floor joists to the other side of the house (where there is currently a garage which I'm planning on turning into an extension with bedroom, bathroom and bigger garage at a later date. This will allow an electrician to install a separate localised CU for the extension in the future. The new CU will definitely NOT need to supply any heavy loads (electric showers, heaters e.t.c.) just a few socket radials, lighting e.t.c. maybe a garage door opener. The house's supply is TN-S with 25mm tails. Will it be suitable to run 10mm T&E clipped to the joists to supply this CU, or is a larger earth CPC required? (The plumbing is already bonded with 16mm cable to the house CU, and there will be no extraneous metal parts to the timber frame structure). Will this be able to be connected to a 60a switch fuse by the meter, or will it require RCD protection because T&E isn't mechanically protected? It would be virtually impossible to physically run SWA as I need a bit of flexibility to get round the corners, through holes in joists e.t.c. so T&E is preferred.
Much appreciated.[/img]

 

[yorkspark]

I take it the garage is connected to the house from your description?
why do you need another cu? can you not just use existing cabling altered to suit your requirements ie extend ring final around bedroom and garage and extend lighting to suit?
This would be the easiest solution and of course less upheaval of existing house decoration/flooring etc

 

[riveralt]

This will allow an electrician to install a separate localised CU for the extension in the future.

This is the person who you need to be speaking to - since he will be designing the additional circuit - completing the installation - testing and completing the appropriate paperwork and registering the work with your LABC.
But you know this already.

 

[tew1]

I take it the garage is connected to the house from your description?
why do you need another cu? can you not just use existing cabling altered to suit your requirements ie extend ring final around bedroom and garage and extend lighting to suit?
This would be the easiest solution and of course less upheaval of existing house decoration/flooring etc

The existing house CU and wiring is to its limit as far as I'm concerned, I already have a 24-way crabtree board which is full, and I don't want to add or extend any more to what's there now. The supply is adequate, there's just a lot of small radial circuits to various parts of the house, so the easiest, neatest solution is a separate 10-way CU

 

[tew1]

It's fairly simple, all I need to know, is the earth conductor in 10mm T&E suitable to supply a CU only (it will not be bonded to any water pipes, extraneous components e.t.c.), and can a 10 metre T&E sub main be protected by a 60a switch fuse on a TN-S supply or is mechanical protection (conduit/SWA) required?

 

[ban-all-sheds]

Your electrician has to be the one to answer that - he is the one who will be signing certificates to say that his work (which includes design, i.e. things like this) complied with the Wiring Regulations and with the Building Regulations.

And you'll struggle to find one prepared to take responsibility for any cables you install, so you need to get your electrician involved now.

 

[flameport]

It's fairly simple, all I need to know, is the earth conductor in 10mm T&E suitable to supply a CU only (it will not be bonded to any water pipes, extraneous components e.t.c.), and can a 10 metre T&E sub main be protected by a 60a switch fuse on a TN-S supply or is mechanical protection (conduit/SWA) required?

The simple answer is no - the circuit you have proposed is NOT suitable.

 

[tew1]

It's fairly simple, all I need to know, is the earth conductor in 10mm T&E suitable to supply a CU only (it will not be bonded to any water pipes, extraneous components e.t.c.), and can a 10 metre T&E sub main be protected by a 60a switch fuse on a TN-S supply or is mechanical protection (conduit/SWA) required?

The simple answer is no - the circuit you have proposed is NOT suitable.

why?

 

[flameport]

The required disconnection time cannot be met using the items you have suggested.

 

[tew1]

The required disconnection time cannot be met using the items you have suggested.

can you elaborate? 10 metres of 10mm cable clipped on its own with no insulation is rated at 65 amps. A 45/50 amp isolator would be more than sufficient for a Cu with lighting and a dozen sockets, or do you mean that it needs to be RCD protected? It makes no difference to me, I'm not installing anything, I just want to make reasonable provision for cabling from one side of the house to another for the future, along with TV, phone, alarm e.t.c. to avoid ripping out what I'm currently finishing.

 

[JohnW2]

The required disconnection time cannot be met using the items you have suggested.

Would that be the 5 second disconnection time required of a distribution circuit in a TN installation?

Kind Regards, John

 

[ban-all-sheds]

Well, yeah, but..

The corollary, if you like, of that is what does that cable do to the Ze seen by the devices in the extension CU which may need to provide a 0.4s disconnection time?

 

[JohnW2]

Well, yeah, but.. The corollary, if you like, of that is what does that cable do to the Ze seen by the devices in the extension CU which may need to provide a 0.4s disconnection time?

... if I've done my sums right, 10m of 10mm² T&E (with 4mm² CPC) ought to contribute about 0.077Ω to the Zs of circuits it feeds - unlikely, I would have thought, to prevent satisfaction of required disconnection times.

Kind Regards, John

 

[tew1]

right ok, I'm going to go ahead with 10mm. There's no way I'm running a 16mm cable or SWA through the inside of the house just for a few extension circuits, that's totally overkill. I could always run a separate 10mm earth CPC alongside it if anyone thinks that would be necessary??

 

[flameport]

The cable size or type is irrelevant in this instance.
The problem is the BS1361 60A fuse on a TN-S supply.
http://niceic.com/Uploads/File3757.pdf

The real answer is not to install this useless submain at all, but to simply extend a couple of the existing circuits into the new extension, as suggested in the first reply.