How to calculate transformer size from total load

Discussion in 'General Electronics Chat' started by elec.engnr, Feb 3, 2014.

  1. elec.engnr

    Thread Starter New Member

    Feb 3, 2014

    Recently when I've visited an electrical panel room, I've noticed that the current readings in the Digi Ammeter were around 160A in each phase. Now for a voltage of 415V and current as mentioned above i.e. Ir = 160A, Iy = 158A, Ib = 145A what would be the transformer capacity (11 / 0.415kV) in kVA and what is the calculation procedure?
    Last edited: Feb 3, 2014
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    Not sure what the 11 divided by .415 is about. 415 volts (V) times 160 amps (A) is 66,400 VA. This is the power per leg of the AC circuit you speak of. Leading or lagging current caused by the power factor would modify this power level giving you the required min. Power rating.
    elec.engnr likes this.
  3. inwo

    Well-Known Member

    Nov 7, 2013
    I come up with 115 KVA

    Using 160 amps.

    I'll let someone else give you the formula when phase currents differ.
    I'd use the average current. Then closer to 110 KVA

    Power factor should not enter the equation as all measurements are VA not watts.
    Last edited: Feb 4, 2014
  4. crutschow


    Mar 14, 2008
    Sounds like a cleverly worded homework problem. :rolleyes:
  5. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    I'd calculate based on the highest current, so 115KVA. If however you are supplying motors, I'd provide extra for load dynamics.