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

This topic contains 7 replies, has 5 voices, and was last updated by  pradeepkumar 3 weeks, 4 days ago.

Viewing 8 posts - 1 through 8 (of 8 total)
  • Author
    Posts
  • #10226

    admin
    Keymaster

    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.

    #11474

    admin
    Keymaster

    Hi,

     

    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

     

    #11440

    admin
    Keymaster

    Hi ,

    what u calculated its currect but how taken cable size

    which method u used for calculating the cable size

    #11442

    Karen
    Member

    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.

    #11443

    admin
    Keymaster

    Hello,

    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…..

    #12262

    s
    Participant

    What about power factors of those loads?

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

    #12293

    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 ).

    #18442

    pradeepkumar
    Participant

    Thanks for sharing good information.

Viewing 8 posts - 1 through 8 (of 8 total)

You must be logged or registered in to reply to this topic.

Electrical engineering Community is proudly powered by WordPress
Entries (RSS) and Comments (RSS) and license Creative Commons License