Running supply to detached garage

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Hi all

Am looking for some advice on running a supply to a detached garage.

Some pertinent info:
- I believe the house supply is TNCS
- I have a spare way in the house CU on the non-RCD side
- 15m of 10mm2 T&E (with 4mm2 CPC) runs from the house CU, through the house to the exterior of the house
- ...this connects, via an adaptable box on the outside of the house to...
- 18m of 10mm2 3-core SWA
- ...this runs to the detached garage via a duct, and comes up the inside wall of the garage to where the garage CU will go.
- the garage is a detached double, with stone walls, steel doors and concrete floor. There is no water supply in there at present (maybe a possibility in future, but not now).
- the CPC of the T&E connects to one of the 3 x 10mm cores of the SWA (not the armour) in the adaptable box.

I've a few questions:
1) In the absence of a water supply in the garage, am I OK to export the earth from the house supply, or would I still be better off fitting an earth rod and using a TT system in the garage ?

2) Am I right in thinking that if the equipotential zone had to be exported to the garage (for example if it did receive a water supply), then the 4mm2 CPC of the 10mm2 T&E wouldn't be of sufficient CSA, and therefore I'd have to switch the garage to TT ? (As it would be impossible for me to run a supplementary 10mm earth cable from the house CU area all the way to the garage)

3) I have a spare 50A MCB. From the calcs I've done, I think I should be OK in terms of volt drop and earth impedance. I was planning to use a 50A MCB on the non-RCD side of the house CU, and in the garage, use a MK Sentry "Super Garage Kit" CU with 63A RCD and 4 MCBs. Probably one MCB for external lights, one for internal, one for an internal ring, and one spare way. Does this all sound OK, or is there anything else I need to think about ?

4) If I did export the house earth to the garage, can I do this via the 3rd core of the armoured cable, and run this directly into the garage CU, leaving the armour disconnected at both ends (IE it's just there for mechanical protection) ?


Thanks for any thoughts !

Cheers
 
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1) exporting the earth is fine, and preferable to TT IMO.

2) correct.

3) I haven't done the calls but off the top of my head it sounds ok.

4) the armour MUST be earthed in all circumstances regardless of what earthing type or arrangement you use.
 
1) exporting the earth is fine, and preferable to TT IMO.

2) correct.

3) I haven't done the calls but off the top of my head it sounds ok.

4) the armour MUST be earthed in all circumstances regardless of what earthing type or arrangement you use.

Great, thanks for the reply.

Regarding (4). If I export the earth, does it matter which end of the SWA cable the armour gets earthed at ?
 
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2) Am I right in thinking that if the equipotential zone had to be exported to the garage (for example if it did receive a water supply), then the 4mm2 CPC of the 10mm2 T&E wouldn't be of sufficient CSA,
That's correct.

and therefore I'd have to switch the garage to TT ?
No.

(As it would be impossible for me to run a supplementary 10mm earth cable from the house CU area all the way to the garage)
Bond, not earth - but you may use the CPC(earth) as a bond. :)
You would only need to run a 10mm² form the end of the swa to the relevant points.


4) If I did export the house earth to the garage, can I do this via the 3rd core of the armoured cable, and run this directly into the garage CU, leaving the armour disconnected at both ends (IE it's just there for mechanical protection) ?
It must be connected at the house end.

As above re: both ends.
 
Thanks :)

and therefore I'd have to switch the garage to TT ?
No.

(As it would be impossible for me to run a supplementary 10mm earth cable from the house CU area all the way to the garage)
Bond, not earth - but you may use the CPC(earth) as a bond. :)
You would only need to run a 10mm² form the end of the swa to the relevant points.

I assume I'd have to run 10mm2 from the bonding point in the house, to where the 10mm2 T&E joins the SWA ?
It's not practical for me to do that, now the house is all finished inside - so I guess the practical option is TT in the garage, if I ever run a water supply out there.

Incidentally, is it the mere presence of a water supply that would require extension of the equipotential zone, or does it depend on the type of pipework run to the garage (ie whether it's plastic or copper) ?
 
Incidentally, is it the mere presence of a water supply that would require extension of the equipotential zone, or does it depend on the type of pipework run to the garage (ie whether it's plastic or copper) ?
The issue is whether or not there's extraneous metalwork, i.e. metalwork that could itself be earthed and could therefore be at a different potential to the house and other items bonded to the house earth main terminal. Metal water pipes would be one example, but metal framing might be another. How sure are you that the supply is TNCS?

Regarding the cable run, is the 10mm T&E sufficiently protected that you don't need RCD at the house end? At the garage I'd probably put all lights on one circuit unless there's some specific reason for you to want to split them.
 
Am looking for some advice on running a supply to a detached garage.
Do you intend to apply for Building Regulations approval?

Do you know what tests you would carry out on the installation - what sequence you'd do them in and at what point you would energise the installation, and for each test do you know what is being measured, why it is important, how you would carry out the test, and with what equipment, and what sort of results you would expect to get if everything was OK?
 
Am looking for some advice on running a supply to a detached garage.
Do you intend to apply for Building Regulations approval?

Do you know what tests you would carry out on the installation - what sequence you'd do them in and at what point you would energise the installation, and for each test do you know what is being measured, why it is important, how you would carry out the test, and with what equipment, and what sort of results you would expect to get if everything was OK?

It will all be tested by an electrician; I have an arrangement with someone who is happy to let me do the design & installation, and he does my testing.
It's an arrangement that's worked well on the 5-bed selfbuild that I've designed & wired myself so far...
 
The issue is whether or not there's extraneous metalwork, i.e. metalwork that could itself be earthed and could therefore be at a different potential to the house and other items bonded to the house earth main terminal. Metal water pipes would be one example, but metal framing might be another.
Do you think garage doors would fall into the "extraneous" category ?
The doors themselves are made of a steel frame with timber inlays. The actual frames are steel, and bolted directly to the blockwork. The bottom ends of the frames are in contact with the garage floor, but also partly in (damp) contact with the driveway paving at the bottom.
How do you define whether metalwork could actually be earthed ?
 
Do you think garage doors would fall into the "extraneous" category ? ... The doors themselves are made of a steel frame with timber inlays. The actual frames are steel, and bolted directly to the blockwork. The bottom ends of the frames are in contact with the garage floor, but also partly in (damp) contact with the driveway paving at the bottom. How do you define whether metalwork could actually be earthed ?
The official answer is that one should measure the resistance from the doors to earth - if it is sufficiently high, it doesn't count as an extraneous-conductive part. However, the problem there is that the answer one will get will obviously depend upon the state (dampness etc.) of the floor and paving at the time of the measurement.

Certainly if it were only the floor that the doors could touch, I would say that the pragmatic answer would probably be that doors would not need to be treated as an extraneous-c-p (which hence needed bonding). I say that because, if the only route to earth were through the damp floor then, even if one bonded the doors for that reason, one would not be able to bond the floor itself - so that bonding just the doors would be a fairly ineffectual gesture!

Kind Regards, John
 
It will all be tested by an electrician; I have an arrangement with someone who is happy to let me do the design & installation, and he does my testing.
It's an arrangement that's worked well on the 5-bed selfbuild that I've designed & wired myself so far...
And so clearly it's an arrangement that your LABC are happy with. Hope the supply to the garage was all part of the original house approval - it would be annoying to have to pay a separate fee.
 

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