Re: Calculation of cable wire.

Spir Georges GHALI

excalibur said:

I have a project. It consists of a lot motors, my problem is how can i know the right ampacity of cable wires and breaker to be use? can you share me some tables?

Dear ;


First, you should know from the name plate the following specifications for each motor :

     – Powe ” kW  or  HP ” , Single Phase  or  3 phases “, Cos φ , methods of running as we have ” Direct on Line , Star-Delta , Auto-transformer , Soft Starter , Variable Speed Drive ” , the distance between the motor & the supply's panel , and the methode of installation of cables with the ambiant temperature.

By these info, you can calculate the nominal current of the motor ” In ” then accordingly to the methode on running, you can estimate the insush current. Then accordingly to the methods of installation you can define the correction factors and the total ” K “, then you calculate the value ” Iz ” that's :

Iz = In / k

Refering to the tables of ” IEC 364-5-52 ” and the value of ” Iz “, we can know the section of cable, after that we should calculate the percenrages of drop voltage of cable in normal working & in running time, if the values of drop votltage from the source till the motor are equal or less the standard percentages the section of cable is OK,but if not, we should over-size the section. Noting that the standard persentages of drop voltage is :

     – in normal working ( after running ) : 5 %, can be exeeded to 6 or 7 % in some special case

     – in running time : 10-12 % ( not more )

but I prefer to draw your attention that these percentages are from source to the load ( not from the supply's panel to the motor ).