Re: sizing load ,cable& breaker

Spir Georges GHALI

Dear ;


I think that you posted another topic for Cables' Section Calculation where I explained the procedure to define the suitable sections of Cables to electrify the  70 AC, the cables are :

– For each Phase : ” 4 x Single Core Cable of  300mm² “

– For Neutral       : ” 2 x Single Core Cable of  300mm² “


About the Circuit Breaker size :

we do as follow to define the specifications of the CB

– The Nominal Current for CB :

          In = 1250A

– The Thermal Protection :

          Ir = 1200A ( as the cables can carry ” 1246A ” with the installation conditions )

– The Magnetic Protection :

          Im : we should calculate : the minimum Short Circuit Current at the end of cable ” Isc1min “, and the Earth Fault at the end of cable, then the value of ” Im ” will be smaller by ” 20% ” than the smallest one.

– The Breaking Capacity :

          BC: we should calculate the maximum Short Circuit Current at the beginning of cable ” Isc3max “, then the value of ” BC ” will be equal or bigger than the calculated value.


But as I don't know : the Power of Transformer, the Earting system, and the Length of cable, I can't calculate the 2 last specifications.