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Sub - main supply cable - 2 core or 3 core SWA?

Discuss Sub - main supply cable - 2 core or 3 core SWA? in the Electrical Forum area at ElectriciansForums.net

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elbar

I need to install a sub-main supply cable from the house to a shed approx 100M away. The SWA needs to be 25mm csa for volt drop requirements - I intend to install a 100mA RCD and 40A MCB to protect the cable for overload / fault protection using the armouring as a means of earth. Then with the armouring (which will be PME connected) isolated in an insulated adaptable box, the installation within the shed will have an earth rod ( TT) as a means of earthing.
Is this ok or do I need to run a 3 core sub main?
 

malcolmsanford

Electrician's Arms
I need to install a sub-main supply cable from the house to a shed approx 100M away. The SWA needs to be 25mm csa for volt drop requirements - I intend to install a 100mA RCD and 40A MCB to protect the cable for overload / fault protection Why if it is a TNC-S earthing system

using the armouring as a means of earth. If using a normal 70c cable the SWA should be fine, most likely a XPLE might struggle, I would calc it out to be on the safe side.

Then with the armouring (which will be PME connected) isolated in an insulated adaptable box, the installation within the shed will have an earth rod ( TT) as a means of earthing.Why are you wanting to TT this installation. Is this for cost as you have bonding requirements
Is this ok or do I need to run a 3 core sub main? Depends on above
See post
 
E

elbar

It is my preference to install a 100mA RCD for additional earth leakage protection as the cable will be cleated at low level along a garden wall for 12M. I am aware that I do not need to do this especially as the Zs reading at the shed will be well within the maximum Zs permitted by BS7671.
The reason for wanting to TT the shed installation is cost reduction in using 2 core SWA rather than 3.
 

malcolmsanford

Electrician's Arms
Well that is fair enough about the RCD, would never decry anyone wanting to give additional protection if that is what they want to do. Can I ask why 100mA one.

You can export the earth via the SWA cable, providing the SWA is capable and you should achieve that on 70c SWA, the XPLE kind is a little close, but you can use the adiabatic to check.
 
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Engineer54

It is my preference to install a 100mA RCD for additional earth leakage protection as the cable will be cleated at low level along a garden wall for 12M. I am aware that I do not need to do this especially as the Zs reading at the shed will be well within the maximum Zs permitted by BS7671.
The reason for wanting to TT the shed installation is cost reduction in using 2 core SWA rather than 3.
The only reason to run a 3 core SWA is if you have extraneous metalwork, such as water pipe etc, the 3rd core is then used as a bonding conductor to extend the TNC-S Equipotential zone. Otherwise there isn't any need for that third core. So do you have a water supply at this shed??
 
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elbar

The only way I can forsee any damage occuring to the SWA cable would be if someone penetrating the armouring with a pick or something similar, so a 100mA RCD would be ok. A time delay 30MA would be better I know.

Thanks malcolmsanford.
 
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Engineer54

For a 2 core SWA cable you need a CSA of 36.1 to comply with the steel wire being your CPC. Your XLPE 25mm cable has a CSA of 42. So the use of the armouring complies as a CPC.

If you use a PVC insulated SWA cable the required CSA remains the same at 36.1, but the PVC insulated Cable gives you a CSA of 60. I think Malcolm is a little worried about the length of your run at 100m, and he may have a point. So if you can get hold of PVC insulated SWA cable, so much the better....


 
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elbar

The only reason to run a 3 core SWA is if you have extraneous metalwork, such as water pipe etc, the 3rd core is then used as a bonding conductor to extend the TNC-S Equipotential zone. Otherwise there isn't any need for that third core. So do you have a water supply at this shed??
Hi Engineer54,

No there is no water supply to the shed, but you have raised an interesting point. The main purpose of this shed is to house a solar PV inverter. The PV array is to be ground mounted just outside the shed, and with the frame work for mounting the panels being all metal it will be extraneous and shall need to be bonded. I think that the sub main cable needs to be a 3 core. Correct?
 
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Engineer54

Hi Engineer54,

No there is no water supply to the shed, but you have raised an interesting point. The main purpose of this shed is to house a solar PV inverter. The PV array is to be ground mounted just outside the shed, and with the frame work for mounting the panels being all metal it will be extraneous and shall need to be bonded. I think that the sub main cable needs to be a 3 core. Correct?
If your mounting them on concrete block foundation footings that are partly or fully embedded into the ground, probably!! You would need to test from the MET to the framework and check if the value is 23 Kohms or not. If over the 23k then no need, but you will i think, need to earth the framework, via the CPC.

Having said that, the frame is outside of this shed, so just may need earthing. No time at the moment to check on typical earthing /bonding arrangements for PV systems. So best to ask the solar boys here what earthing/bonding Reg's cover there installations...
 
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