Radial circuit 2,5mm and CPC 1mm

[columbus]

Hi

I just found calculation examaple :
"A 20A radial circuit is wired using 2.5mm sigle core PVC installed in conduit.A separate protective conductor of a 1mm cable is use. ......"

My question is :
Can I still use CPC 1mm and live wires as 2.5mm ? Shouldn't they be the same size 2,5mm ??

 

[Spark123]

A 1mm CPC may be used with 2.5mm live conductors providing the adiabatic equation is can be complied with i.e. regulation 543.1.3
For this you need to know the fault current and the time that the protective device will disconnect in for the fault current. You also need to know the K value for the CPC which can be found in BS7671:2008 table 54.3 for your application.
In most instances a 1mm CPC with 2.5mm lives will be fine however can be a problem where there is a larger energy let through by the protective device such as with rewirable fuses.
Regulation 543.1.1 allows you to select the CPC size using regulation 543.1.3 or 543.1.4
Where regulation 543.1.4 is used, this would require you to use the same size CPC as the live conductors for this particular size cable.