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Calculate size of transformer and voltage due to starting of large size motor (MS Excel Spreadsheet)
Calculate size of transformer and voltage due to starting of large size motor (MS Excel Spreadsheet)

Easy way to calculate transformer and motor details

Transformer details:

  1. Transformer size
  2. Transformer secondary voltage (v2)
  3. % Impedance (z)
  4. Allowable voltage drop in system

Motor details for one and more motors:

  1. Motor full load current (ILM)
  2. Locked rotor current multiplier (Min)
  3. Locked rotor current multiplier (Max)
  4. Locked rotor current (L1) (Min)
  5. Locked rotor current (L2) (Max)
  6. Motor inrush KVA at starting (IrsM)
  7. How many times motor starts per hour

and Calculations:

  1. Full load current of transformer (ILX)
  2. Short circuit current at transformer secondary (Isc)
  3. Maximum KVA of transformer at Isc (Q1)
  4. Motor inrush KVA At starting (IrsM)
  5. Voltage drop at transformer secondary due to motor inrush (Vd)
  6. Transformer secondary voltage

Results you get:

  • Motor Full Load Current
  • Voltage Drop at Transformer Secondary Due to Motor inRush
  • Transformer Size:(20% Added due to motor starts more than once/Hour)

Calculation formulas used:

  • ILX = KVA / (1.732 x v2)
  • ILM = (Kw x 1000) / (1.732 x Vm x P.F)
  • Isc = ILx / z
  • Q1 = (v2 x ISC x ILM x 1.732) / 1000
  • IrsM = (Vm x L2 x 1.732) / 1000
  • Vd = IrsM / Q1
Software:Calculate size of transformer and voltage due to starting of large size motor
Developer:Jignesh Parmar
Size:40 KB
Download:Right here | Get Download Updates | Get Technical articles
Code Letter (KVA per HP) -Locked Rotor Current
Code Letter (KVA per HP) -Locked Rotor Current

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Page edited by E.C. (Google).


  1. Khaled Ellithy
    Jan 25, 2018

    Excellent example. Could you please modify it to include 75 HP motor with pf of 0.8 lag and starting pf of 0.3. and constant load 20 KVA, pf = 0.8 lag


  2. Ahmed Madian
    Feb 01, 2017

    What is the rating of transformer which can feed motor 1500 HP , 4160 V and 60 HZ ?

  3. Dr Khaled Ellithy
    Apr 02, 2016

    Ganesh, I agree with you, It should include the KVA of the loads connected to the transformer before starting the motor. Including the loads KVA to the starting KVA of the motor may end up with the transformer size is incorrect (VD% is above the limit. Comment of Jignesh Parmar is highly appreciated.

  4. mohsin raza
    Apr 14, 2015

    Please note that in thi volatge drop calculation existing load on transformer is not considered. only motor starting KVA is considered for voltage drop calculation. Kindly tell me how to consider other conneted load in this calcullation.

  5. Bhagaban Barik
    Sep 22, 2014

    To calculate sizing of Transformer 30% future expansion should be considered.

  6. Ruon
    Nov 21, 2013

    motor kVA from kW and cos phi will be rather E11/E13 than E11/E13
    isn’t it ?

  7. R.Ganesh
    Jul 13, 2013

    This calculation has two mistakes.
    1. You have not considered the existing load in the transformer during motor starting, as if we are starting the motor alone without any other load. Normally transformers will be loaded 50% of its rating. So atleast you have to consider 30 to 40 % load in the transfomer and add that to the motor starting KVA. Here it is around 300 to 400 KVA. IF you add that the transformer selection will be wrong.
    2. Motor starting KVA can be calculated directly from starting KVA/HP, converting KW to HP, which will be 400HP*4.5 = 1800KVA, which is correct way of doing. In that case your figure is wrong. In both cases the transformer will not be suitable.

    Usually in LV MCC, motor KW is restricted to 160KW the maximum due to the above reasons.

  8. vins231
    Jul 11, 2013

    Yup they are right it is not downlodable. i am trying since two months and for your information when we registered ourself then only we are able to give comments here. pls do the needful

  9. Eduardo Vega
    Jul 10, 2013

    Muchas gracias.

  10. Jatin Joshi
    Jul 08, 2013

    Calculation is good but i understand that it does not include starting method of motor??? or does it include this effect in form of motor code. This is one gray area I am not clear about. Can you please confirm the same please….

    Thank you

  11. nirbhik.modi
    Jul 08, 2013

    Not downloadable !

  12. celebrationcometh
    Jul 08, 2013

    the excel spreadsheet is not in downloadable format. so i have not been able to download the table even though i can see it in pictorial form.

    • Edvard
      Jul 08, 2013

      Spreadsheet is downloadable and link works. Did you register at EEP?

      • Paddy Keys
        Jan 28, 2017

        Please check your calculations otherwise give password so we can correct your mistakes (E13 is incorrect as kVA increases with a lower PF not decreases.

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