Separate CPC required for 10mm2 cable to attached garage consumer unit?

[4StringBass]

Hello!

We have just signed the contract for our new garage build. This will be attached to the side of the existing house. We are getting the architect round this week to measure up so we are a good few months away from even starting. However...

At the moment I have one of the floors up on the first floor of our house to replace the shower drainage. Our consumer unit is in the middle of the house under the stairs . I have the perfect opportunity to put in the supply cable for garage consumer unit as I can get to the two walls required to get the cable in. To do this later will be a major pain and a very messy job as this floor won't be coming back up again.

I obviously have not spoken to the electrician the builders will be using so I'm just going to run a 10mm2 twin and earth and leave both ends neatly coiled away for his/her future use. It will be more than enough for what we will power in the garage and I will let the electrician choose the appropriate RCBO for the existing board. I will not be doing any wiring up, I'm simply trying to reduce problems later on.

My question is... Is it likely that a separate CPC will be required to bolster the existing earth in the twin and earth? The cable run is only 7m from main board to garage board. I can put a draw cord in for a CPC as it would be possible to pull this through, but no chance I can do this with stiff T&E.

I know it can be common place in large commercial installations to use a separate CPC where the armour may not be sufficient, but that's usually attached to massive sub-main boards/circuits.

Thank you in advance as ever

 

[sparkwright]

Should be fine, though is the garage detached?

If so, and the cable is to go underground, you may want to consider SWA.

 

[RF Lighting]

Depending on how and where it's run, twin and earth cable may not be suitable.

 

[DetlefSchmitz]

I'm a fan of flexible conduit with a pull-string if I don't know what is needed.

 

[sparkwright]

Hang on, it says attached garage in the title...

Didn't see that.

 

[Taylortwocities]

Maybe need some clarification from the OP, but if the garage is attached, and cables are run in roof/ceiling spaces, etc, then T&E cable would be fine.

Having said that, I would be having a chat to the electrician at about this time. He will be the one that certifies that the cable you have run complies with the Regs.
He may want to see it before its covered over, I certainly would!

 

[chivers67]

Yeah, take some picture to show it's been suitable installed.

Getting some adaptorflex/copex with a draw rope certainly may help and future needs as Detlef suggests.

 

[OwainDIYer]

What are you planning powering in the garage? An attached garage would usually have one light and one double socket, both wired to the corresponding house circuit.

10 mm cable is eg pottery kiln territory, especially as at 7 metres volt drop is unlikely to be an issue.

 

[4StringBass]

Thank you all for your replies.

Apologies for not clarifying in my original post. The garage will be attached to the house, with the existing side wall of the house becoming one of the 4 walls of the new garage. The cable will not go outside. It will rise from the cupboard where the consumer unit is, through the ground floor ceiling void and fall within the external wall void prior to going through the existing external wall and connecting to the new CU. The floor void and one of the walls it will run through are uninsulated but one of them is insulated.

Thank you for the Copex suggestion, I'll do that as I've got some somewhere. I will also try and get a hold of the electrician and take photos of what I do.

OwainDIYer, our new garage will hopefully be 42sq metres and will be a bit of a car/motorbike workshop for me so it would be nice to have appropriate breakers for air compressors, motorbike lift and large power tools. One socket and a light isn't going to cut it, wife says so . It will also future proof any need for an electric car charging point, DIY welder or similar. I do not want to lift this floor again hence 10mm2 and my asking if a separate CPC will be required. I've no need for a kiln, but you never know

Just to confirm, I'm doing nothing other than planning on running this one cable for future use. I'm no sparky although I can wire a plug

Thanks again.

 

[Taylortwocities]

Thanks for clarification. A separate cpc is not necessary. The one contained in the T&E will be sufficient.

 

[4StringBass]

Thank you!

 

[securespark]

Cables in cavities and insulation as well?

 

[OwainDIYer]

OwainDIYer, our new garage will hopefully be 42sq metres and will be a bit of a car/motorbike workshop for me so it would be nice to have appropriate breakers for air compressors, motorbike lift and large power tools.

In that case I would put in 16mm now as the extra cost will be not-a-lot over 7 metres. That should be good for up to about 14 kW load assuming no derating due to thermal insulation/grouping etc.

Even that might not be enough for electric vehicle fast charging, for which you might want to research what the requirements are (and they're unlikely to get smaller). More than 16mm you'd probably be better just to put in a straight pipe through which cable can be pulled - SWA doesn't pull nicely round bends.

 

[securespark]

It will...fall within the external wall void prior to going through the existing external wall and connecting to the new CU. The floor void and one of the walls it will run through are uninsulated but one of them is insulated.

You will have to consider factors for thermal insulation and also the type of insulation: if it is polystyrene, it will have a detrimental effect on PVC.

 

[winston1]

You don't need a separate consumer unit in the garage any more than you need a separate CU in any other room of the house. Run a 1.0mm T&E for lights which will probably connect to downstairs lighting circuit. Run two 2.5 T&E for a garage ring which can go to a new MCB in your existing CU. Electric car charging points are somewhat special, some are 3 phase, so difficult to future proof for that. However electric cars can also be charged, slowly, from an existing 13A socket.
Don't forget battery backed emergency lights.