Home › Electrical Engineering Forum › General Discussion › calculations of 3-phase motors and sizes of wires to be used, including respective breakers in every motor › Re: calculations of 3-phase motors and sizes of wires to be used, including respective breakers in every motor
Adding to what said before, we should be careful of the following points :
– It's possible that the Power factors ” Cos φ ” aren't for all ” 0.80 “, so, the nominal current will be changed.
– When we know the nominal current of a motor, we can select the cable's section either from the tables of ” IEC 364-5-52 ” ( best ) or from the tables of the manufactures, but these values are in the standard conditions, so, we should integrate the Correction factors accordingly to : the kind of cable, Method of installation, Ambient temperature, and the no. of cables in the same conduit.
– After the correct slection of cable's section, and as the cable will electrify a motor, we should calculate the ” Drop Voltage ” in 2 cases :
– During the nominal working ( after running ) where the current is the ” nominal current ” for the motor.
– During the running where the current is the ” Inrush current “.
if these calculated values are suitable, we can install & use the selected cable, but if not, we should over-size the section of cable.
– Then we will select the CB, noting that this CB is responsible to protect only the cable ( not the motor ).