Re: size of cable

Spir Georges GHALI

ghulam mujtaba said:

HI i am mujtaba want to know that how can we calculate size of cable as per requirement of  load

Dear ;


First, by knowing the load's power ” P “, the kind of load ” Single Phase  or  3 Phases “, and the Cos φ, we can claculate the nominal current ” Ib ” of load by applying the formula :

– For single phase : Ib = P / V x Cos φ

– For 3 phases : Ib = P / 1.73 x U x Cos φ

where :

     – V : voltage between phase & neutral

     – U : voltage between 2 phases

Then you should decide or know the methode of installation of cables as we have many methode ( better to refer to IEC 345-5-52 ), and the ambiant temperature, so, we can find the correction factors and the totat ” k “, so, we calculate the nominal current for cable in standard conditions ” Iz = Ib / k “, and by this value and the table of ” IEC 364-5-52 ” we can do the choice of cable accordingly to the kind of cable ” Single  or Multi “, ” Cu  or  Alu “, ” PVC  or  XLPE “.

After knowing the section, we should calculate the percentage of drop voltage of the cable, if the percentage is equal or less the standard values, the cable is OK, if not, we should ove-size the section of cable. But be carful that the standard values of Drop Voltage percentages are from Source to the load, that means all cables from the source ( transformer or generator ) to the load.