CPC size help

[pencilpusher]

Hi

I am about to export TNCS from a dwelling to the detached garage. (The garage is originally a TT system with a poor EFLI, over 200 ohms) I am aware that TNCS can be exported and I am going to do this. There are no extraneous conductive parts in the garage either.

My problem is this: 2.5mm T&E leaves the consumer unit and joins 2.5mm SWA at an adaptable box on the outside wall. It travels 3 meters underground to the garage CU. Ze is 0.22 ohms and my calcs are that the short run of SWA will give a ZS at the garage CU of about 0.28 ohms, ish. That will give me a PFC of about 0.8KA at the garage CU. The SWA has a CPC bunched and I can parallel that with the armour. Fine, BUT when the 1.5mm CPC in the T&E meets the swa, I don't think that is adequate to comply with the adiabatic equation. My calcs give me a minimum cpc of about 5mm sq.

Have I got this right and do you agree I will need to upgarde the T&E meeting the SWA?

I have looked at other threads but they don't quite answer my specific issue.

Thanks

PP

 

[Deleted member 26818]

To determine the CSA required for a CPC, you need to furnish the type and rating of the overcurrent protective device.
If for instance you intend using a 32A type B MCB, the time/current graphs in Appendix 3 of BS7671, indicates that the PEFC required to provide disconnection between 0.1 and 5s is only 160A.
Your calculated value of 800A will be more than sufficient to to allow the device to operate, and according to the adiabatic equation, will require a minimum CPC CSA of 0.44mm².

 

[pencilpusher]

To determine the CSA required for a CPC, you need to furnish the type and rating of the overcurrent protective device.
If for instance you intend using a 32A type B MCB, the time/current graphs in Appendix 3 of BS7671, indicates that the PEFC required to provide disconnection between 0.1 and 5s is only 160A.
Your calculated value of 800A will be more than sufficient to to allow the device to operate, and according to the adiabatic equation, will require a minimum CPC CSA of 0.44mm².

Thanks Spin. Forgive me for being a muppet but it totally threw me. I will be using a type B 20A RCBO which only needs 100A to operate within 0.1 secs and that gives me a minimum csa of 0.27mm.

Thanks again.

 

[RodMan50]

Are you really going to export a PME earth ?

 

[pencilpusher]

Are you really going to export a PME earth ?

Yes and there is nothing wrong with doing so.

http://www.electriciansforums.net/a...ted-pme-outbuildings-garage-shed-supplies.pdf

 

[RodMan50]

Yes and there is nothing wrong with doing so.

Well that depends on what side of the fence you sit on

Plenty of export this and export that guides here, there and at the ESC etc

Shame really :ack2:

 

[pencilpusher]

Well that depends on what side of the fence you sit on

Plenty of export this and export that guides here, there and at the ESC etc

Shame really :ack2:

I respect your views but as I say, there is nothing wrong with doing so from my side of the fence!

 

[RodMan50]

Good Luck

How much experience have you with TNCS,PME and TT and exporting earths and the problems associated with it ?

 

[gottahugenoggin]

Are you really going to export a PME earth ?

[h=3]A good guide For 'exported PME' and outbuildings![/h]

 

[pencilpusher]

Good Luck

How much experience have you with TNCS,PME and TT and exporting earths and the problems associated with it ?

Enough to know I can do it ! :rolleyes2:

 

[netblindpaul]

You can't export a PME earth outside the equipotential zone, check the subtleties of ESQCR & 7671.

You CAN extend the equipotential zone though, thus allowing you to utilise a PME earth in an outbuilding!

 

[RodMan50]

Enough to know I can do it ! :rolleyes2:

Like i said good luck, hope you dont make any mistakes :dita:

 

[pencilpusher]

Like i said good luck, hope you dont make any mistakes :dita:

I won't !:santa1:

 

[RodMan50]

Hmmm

Sorry but you already have to be honest :santa1:

 

[pencilpusher]

Hmmm

Sorry but you already have to be honest :santa1:

Thanks for your input :ack2: