calculations of 3-phase motors and sizes of wires to be used, including respective breakers in every motor

Home Electrical Engineering Forum General Discussion calculations of 3-phase motors and sizes of wires to be used, including respective breakers in every motor

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    hi, I have 5 electric motors ( 25hp, 20hp, 15hp, 10hp and 7.5hp), how many Amp.will be my main ckt breaker, and corresponding breakers for each motors. try also to give the sizes of thw wires to be used.




    P(kW) =  HP x 0.746


     I = P *10^3 / (1,73*400*0,8)


    P=5,6 kW  =>  I=10, A  ;  wire 2,5 mm2, Circuit breaker In=10A

    P=7,5 kW =>   I=13,5 A ;  wire 2,5 mm2, Circuit breaker In=16A

    P=11 kW =>    I=19,9 A ;  wire 4 mm2, Circuit breaker In=20A

    P=15 kW =>    I=27 A ;  wire 6 mm2, Circuit breaker In=32A

    P=18,6 kW => I=33,6; ;  wire 6 mm2, Circuit breaker In=40A

    P=57,7 kW => I=104,2 A ;   wire 25 mm2, Circuit breaker In=125A



    Hi ,

    what u calculated its currect but how taken cable size

    which method u used for calculating the cable size


    If you do a google search you will find many “cable size calculator’s” which are accurate in working out the size of the cable you will need to use.



    The process is :


    1º Calculate de current in the circuit

    2º Get a cable manufactures Current rating tables for the type of cable that you will  use

    3º Choose a cable with a current rating bigger than the circuit current

                 Iz (Imax from the cable) > Is (circuit)

       you must be aware that the value in the tables are usually for 20ºC or 30ºC, so youi may have to correct the value, be also aware

       that te value taken form the table is for a single cable alone, so if you have more cables together in the same conduit you must

       correct the value.

    4º Choose a protecion for the circuit

              In > Is and  In*1,45 < Iz*1,45

    5º Verify that the length of the cable is not a problem for the Voltage level in the end of the circuit.


    This is only some guide lines…..


    What about power factors of those loads?

    it's vital to know them, P= 1.73 x V x I x cos(phy)

    Spir Georges GHALI

    Dears ;


    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 ).


    Thanks for sharing good information.

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