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Topic Title: Buried supply to a shed/garage
Topic Summary: List of requirements
Created On: 11 May 2009 02:57 PM
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 11 May 2009 02:57 PM
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RevMark

Posts: 53
Joined: 15 May 2008

Hi there, I am trying to piece together a specification for a supply to a domestic garage that is 8m from a T-N-C inlet.

1. Mains inlet with second set of tails to a second consumer unit.
Reasons
- No need to consider main consumer unit compliance with regs? (within reason)
- Do I need a RCD here, the forum suggests not, is a fuse/breaker needed ? I cannot see where the 17th Ed talks about this? Obviously RCD and breaker needed somewhere, see below.

2. 4mm2 SW cable buried at 600mm from house to garage.
Reasons:

Current capacity (page 280 17th Ed) table 4D4A in ground = 37A to supply final ring of 32A (as Appendix 15, page 362) plus lighting at nominal 5A.

600mm deep, I see only caravan parks spec 707.521.1.1 to support this, the more vague 522.8.10 (sufficient depth to avoid damage) may be easier, what is usual in domestic use?

3. Cable terminating in consumer unit with 2.5mm2 ring and radial lighting on 1.5mm2. 32A Circuit breaker, 30mA RCD and Type C RCD to allow for use of welder/induction motors (ie Appendix 15 ring set-up).

Does this sound right? I am not asking for checking, what is usual for this kind of set up?

Thanks,

Mark
 11 May 2009 03:03 PM
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OMS

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I assume you mean TN-C-S not TN-C.

Does the garage have any extraneous conductive parts.

Are you planning on exporting the PME earth or Are you intending a TT supply to the garage.

600mm deep sounds fine ( as a minimum I would aim for 450mm and use a marker tape above)

OMS

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Failure is always an option
 11 May 2009 03:06 PM
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AJJewsbury

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is a fuse/breaker needed

You'll certainly need to ensure fault protection for the buried cable - and overload too unless overload is impossible.

37A to supply final ring of 32A (as Appendix 15, page 362) plus lighting at nominal 5A.

Are you really going to dissipate 8.51kW in the shed? (It would get quite warm quite quickly!) A more realistic appraisal of the loads (rather than circuits) might result in a more economic solution. For comparison, it's not unknown for a normal "shed" to be supplied by 13A/16A/20A radial with the lights taken from a fused spur.

Depending on the construction etc. of the shed you might want to dispense with the TN-C-S earth and TT the shed. (Or possibly your DNO might want you to!)

- Andy.
 11 May 2009 03:19 PM
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OMS

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I think the point I was trying to make was the cable "size" might be driven by the requirement to be a bonding conductor rather than by its current carrying capacity

Regards

OMS

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Failure is always an option
 11 May 2009 03:26 PM
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RevMark

Posts: 53
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Thanks Guys,

Yes, I meant TN-C-S.

Extraneous parts - yes, metal clad switches (I assume that the cladding is not considered part of the installation).

By using an armoured cable I asume that the house earth will be extended to the shed, if I use TT then does that not mean an earth electrode will be nedeed.

Perhaps I am over engineering this: By the sounds of it a 1.5mm2 armoured cable from the consumer unit (which has 30ma RCD) to a smaller unit in the garage (16A breaker 30mA RCS) with 5A breaker on same RCD for the lights would do?

My problem is that I cannot see how the regulations treat a garage - is it asumed to be part of the house? or is it a totally seperate set up.

Thanks again
 11 May 2009 03:54 PM
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OMS

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Yes, I meant TN-C-S.


OK

Extraneous parts - yes, metal clad switches (I assume that the cladding is not considered part of the installation).


I suspect they are exposed condcutive parts - is there a water/gas/oil supply in the garage or a reinforced concrete floor grid that would normally need bonding (as opposed to earthing)

By using an armoured cable I asume that the house earth will be extended to the shed, if I use TT then does that not mean an earth electrode will be nedeed.


As I said, if you are exporting the earth from the house, be mindfull of the need for the armour to be both a CPC and a bonding conductor. If you are using TT an electrode would be needed, but perhaps more importantly a potential need to seperate the house earth from the shed earth.

Perhaps I am over engineering this: By the sounds of it a 1.5mm2 armoured cable from the consumer unit (which has 30ma RCD) to a smaller unit in the garage (16A breaker 30mA RCS) with 5A breaker on same RCD for the lights would do?


My concern is you may be under engineering the earthing arrangements.

My problem is that I cannot see how the regulations treat a garage - is it asumed to be part of the house? or is it a totally seperate set up.


They treat a garage just like anything else - perhaps this (Autumn 2005) may be of some use.

Regards

OMS

-------------------------
Failure is always an option

Edited: 11 May 2009 at 03:54 PM by OMS
 11 May 2009 04:02 PM
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spinlondon

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A bit out of date now.
 11 May 2009 04:09 PM
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OMS

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Yes I do know that - all though it still gives a reasonable appraisal of the requirements and the risks with each type of system - age doesn't diminish the laws of physics does it.



OMS

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Failure is always an option
 11 May 2009 04:10 PM
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Parsley

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Metal clad switches are exposed conductive parts not extraneous. incoming water service, steel frame of garage etc may possible be exposed conductive parts and therefore will need to be bonded using a suitablably sized conductor.
Generally in a domestic TNC-S installation 10mm conductor will be adequate, therefore the submain will need to be sized accordingly. Alternatively you may prefer to isolate the submain CPC at the garage consumer unit and install a suitable earth rod/spike and TT the garage installation and connect all exposed and extraneous parts to it. To avoid possible touch voltages to true earth, due to possible loss of combined neutral and earth conductor on the DNO's installation or voltage drop in the submains neutral conductor, unlikely as the garage is only 8m away
 11 May 2009 04:15 PM
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RevMark

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Thanks for the reference to the magazine article. I will read and digest it before going further.

On the earth protection I need to meet Zs of 0.92 ohms (Type C, 25A, Table 41.3 on page 49). Presumably I can calculate the 8m of supply cable armouring and what this will come to. Is this a figure (per m) that suppiers give, I assume so). If it is too low than an additional earth will be needed.

Thanks again.
 11 May 2009 04:36 PM
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OMS

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If it is too low than an additional earth will be needed.


Be wary of adding supplementary conductors with armoured cable - the reactance conspires to make the extra conductor a damn sight less effective than you think.

Better off using (in this case) a 3 core SWA in my opinion

Regards

OMS

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Failure is always an option
 11 May 2009 04:48 PM
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spinlondon

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If you are going to export the Earth, then you will need 10mm² SWA. However, if you TT the Garage, you could probably get away with 2.5mm². Do the cable calcs. for volt drop etc.
Bond the SWA at the house, but don't bond at the Garage.
Run a bond from the Earth bar in the Garage CU to a MET along with a bond from the cladding. Then from the MET to an Earth rod.
The Garage CU should have a main switch, and either two RCBOs or one RCBO and one MCB for the lights.
 11 May 2009 05:05 PM
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OMS

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If you are going to export the Earth, then you will need 10mm² SWA


That assumes a 3 core and not relying the armour ?

Regards

OMS

-------------------------
Failure is always an option

Edited: 11 May 2009 at 05:05 PM by OMS
 11 May 2009 05:12 PM
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spinlondon

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Originally posted by: OMS
If you are going to export the Earth, then you will need 10mm² SWA


That assumes a 3 core and not relying the armour ?

Not not at all. Minmum SWA CSA for 70ºC required is 22.6mm², actual CSA 41mm².

Regards



OMS




Edited: 11 May 2009 at 05:14 PM by spinlondon
 11 May 2009 05:16 PM
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AJJewsbury

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Run a bond from the Earth bar in the Garage CU to a MET along with a bond from the cladding. Then from the MET to an Earth rod.
The Garage CU should have a main switch, and either two RCBOs or one RCBO and one MCB for the lights.

If TT then the lights will have to have RCD protection of some sort (given that the supplier's rod can be up to 20 ohms there's no way you can have a Zs good enough for even a 6A MCB). Could be MCB with a RCCB incommer, or RCBO, but not MCB alone.

- Andy.
 11 May 2009 05:24 PM
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OMS

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Not not at all. Minmum SWA CSA for 70ºC required is 22.6mm², actual CSA 41mm².


Really - if you assume that a minimum conductor to meet 54.8 is 10mm then the copper equivalent of 10mm would be around 90mm - you would need a 95mm2 two core XLPE/SWA/LSF to comply as that has an armour CSA of about 110mm

Regards

OMS

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Failure is always an option

Edited: 11 May 2009 at 05:26 PM by OMS
 11 May 2009 06:49 PM
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spinlondon

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Originally posted by: OMS

Not not at all. Minmum SWA CSA for 70ºC required is 22.6mm², actual CSA 41mm².


Really - if you assume that a minimum conductor to meet 54.8 is 10mm then the copper equivalent of 10mm would be around 90mm - you would need a 95mm2 two core XLPE/SWA/LSF to comply as that has an armour CSA of about 110mm

S­­<16
k1/k2 x S = Minimum CSA of CPC in relation to CSA of phase conductor.
70/52 x 10 = 13.46
The minimum CSA of a buried earthing conductor protected against corrosion is 16mm², and as I said, 10mm² complies.


Regards



OMS




Edited: 11 May 2009 at 06:50 PM by spinlondon
 11 May 2009 06:51 PM
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spinlondon

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Originally posted by: AJJewsbury
Run a bond from the Earth bar in the Garage CU to a MET along with a bond from the cladding. Then from the MET to an Earth rod.
The Garage CU should have a main switch, and either two RCBOs or one RCBO and one MCB for the lights.


If TT then the lights will have to have RCD protection of some sort (given that the supplier's rod can be up to 20 ohms there's no way you can have a Zs good enough for even a 6A MCB). Could be MCB with a RCCB incommer, or RCBO, but not MCB alone.

- Andy.


Yes quite right, two RCBOs then.

 11 May 2009 06:56 PM
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OMS

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No it doesn't - you are not trying to verify its thermal capacity as a protective conductor - it would be acting as a main equipotential bonding conductor - hence 54.8 applies - which is quite clear when it mentions the minimum copper equivalent.

If we assume a conductance ratio between steel and copper of about 9:1 then 10mm of copper equates to about 90mm of steel - something like the 95mm 2 core I mentioned above which has about 110mm2 of steel around it.

You may also want to check where you got your values of K1 from

Regards

OMS

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Failure is always an option

Edited: 11 May 2009 at 07:03 PM by OMS
 11 May 2009 07:22 PM
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spinlondon

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Originally posted by: OMS
No it doesn't - you are not trying to verify its thermal capacity as a protective conductor - it would be acting as a main equipotential bonding conductor - hence 54.8 applies - which is quite clear when it mentions the minimum copper equivalent.

If we assume a conductance ratio between steel and copper of about 9:1 then 10mm of copper equates to about 90mm of steel - something like the 95mm 2 core I mentioned above which has about 110mm2 of steel around it.

Why should we assume a ratio of 9:1?
Are you now stating that using the SWA as a CPC does not comply with BS7671?


You may also want to check where you got your values of K1 from

Sorry, should have been 115, which gives a csa for the SWA of Aprox 22mm².

Regards



OMS


IET » Wiring and the regulations » Buried supply to a shed/garage

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