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How to calculate cable and cb/fuse sizes for motors

Farmboy
I haven't had to calculate cable and cb/fuse size(s) for single and 3 phase electric motors for some time now, but would like to do so as a matter of interest, so is there a contemporary reputable resource I can use that sets out how to do so (I could google/youtube it but don't know about their repute/accuracy)?


Thanks


F
  • 0
    Ask the manufacturer, either of the motor, or the machine.


    Typically, the startup current is 6 x the running current, but you don't need to size the cable for that.
  • 0
    On an inverter type  VSD you can ramp up with almost no inrush at all if you don't have a load with a lot of initial "sticktion" to overcome, and don't mind a slow spin-up compared to DOL,  apart from needing a bit of thought to avoid EMC and earth leakage related problems, the VSD is better for the motor life, better for average power consumption, and better from the lights dimming inrush perspective as well.

  • Former Community Member
    0 Former Community Member
    A rule of Thumb is to size the cable for 1.4 x the running current. This allows for the increased volt drop during starting.
  • Former Community Member
    0 Former Community Member

    Johnboy6083:

    A rule of Thumb is to size the cable for 1.4 x the running current. This allows for the increased volt drop during starting.




     

    and it gives you a useful "thermal reserve" in a cable subjected to multiple or frequent starting actions


    Regards


    OMS
  • 0

    OMS:




    Johnboy6083:

    A rule of Thumb is to size the cable for 1.4 x the running current. This allows for the increased volt drop during starting.




     

    and it gives you a useful "thermal reserve" in a cable subjected to multiple or frequent starting actions


    Regards


    OMS 

     




    And for the edification of most; oft ignored is the permissible starts per hour, see here for guidance. On a a pulp and paper mill during commissioning, they had problems matching the Kw rating of G.E.C. pump motors. With frequent starting, the windings on some were displayed at the ends until eventual failure. What was the condition of others, that were successfully commissioned? You're guaranteed till it expires! 


    Jaymack

    https://www.efficientplantmag.com/2015/05/safe-starting-of-motors-check-for-temperature-increase/

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