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Home / Technical Articles / An example of calculating transformer size and voltage drop due to starting of large motor

Calculate voltage drop

Let’s calculate voltage drop in transformer 1000KVA, 11/0.480 kV, impedance 5.75% due to starting of  300 kW, 460V, 0.8 power factor, motor code D (kva/hp). Motor starts 2 times per hour and the allowable voltage drop at transformer secondary terminal is 10%.

Medium-voltage motor starting transformer
Medium-voltage motor starting transformer (man. J. Schneider Elektrotechnik; photo credit: DirectIndustry)

Calculation can be checked by using this MS Excel Spreadsheet dedicated especially to this kind of problem.

Ok, let’s get into the calculations…

Motor current / Torque

Motor full load current = (Kw x 1000) / (1.732 x Volt (L-L) x P.F

  • Motor full load current = 300 × 1000 / 1.732 x 460 x 0.8 = 471 Amp.
  • Motor locked rotor current = Multiplier x Motor full load current

Locked rotor current (Kva/Hp)

Motor CodeMinMax
  • Min. motor locked rotor current (L1) = 4.10 × 471 = 1930 Amp
  • Max. motor locked rotor current (L2) = 4.50 × 471 = 2118 Amp
  • Motor inrush Kva at Starting (Irsm) = Volt x locked rotor current x Full load current x 1.732 / 1000
  • Motor inrush Kva at Starting (Irsm) = 460 x 2118 x 471 x 1.732 / 1000 = 1688 kVA


  • Transformer full load current = kVA / (1.732 x Volt)
  • Transformer full load current = 1000 / (1.73 2× 480) = 1203 Amp.
  • Short circuit current at TC secondary (Isc) = Transformer full load current / Impedance
  • Short circuit current at TC secondary = 1203 / 5.75 = 20919 Amp
  • Maximum kVA of TC at rated Short circuit current (Q1) = (Volt x Isc x 1.732) / 1000
  • Maximum kVA of TC at rated Short circuit current (Q1) = 480 x 20919 x 1.732 / 1000 = 17391 kVA
  • Voltage drop at transformer secondary due to Motor Inrush (Vd) = (Irsm) / Q1
  • Voltage drop at transformer secondary due to Motor inrush (Vd) = 1688 / 17391 = 10%
  • Voltage drop at Transformer secondary is 10% which is within permissible limit.
  • Motor full load current ≤ 65% of Transformer full load current
  • 471 Amp ≤ 65% x 1203 Amp = 471 Amp ≤ 781 Amp
Here voltage drop is within limit and Motor full load current ≤ TC full load current.

Size of Transformer is Adequate.

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More Information

Jignesh Parmar

Jignesh Parmar has completed M.Tech (Power System Control), B.E (Electrical). He is member of Institution of Engineers (MIE), India. He has more than 20 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.


  1. Jeyapandian Nagaraj
    Jun 13, 2018

    Motor inrush Kva at Starting (Irsm) = Volt x locked rotor current x Full load current x 1.732 / 1000

    it is supposed to be Motor inrush Kva at Starting (Irsm) = Volt x locked rotor multiplier x Full load current x 1.732 / 1000

    Typo error.

  2. Sirajuddin Hamdard
    May 28, 2018

    Please explain me Sir the average of actual load in(Amp)
    rated capacity (Amp)
    percetage of Transformer loading
    and over loading (%)

  3. ramesh kumar
    May 10, 2018

    We have 1000kw three motors with VFD How can i select Inverter transformer size

  4. Lilantha Neelawala
    Dec 04, 2017

    Awaiting reply for all above comments.

  5. Giorgi
    Jul 03, 2017

    When you calculated the full load current of the motor, why did you not take into the account the efficiency of the motor, as i know the full formula is as follows: (KW*1000)/(1.732*v*pf*eff)

      Jun 07, 2018

      kW rating given for motors are normally output mechanical power. it must be converted to input electrical that is why efficiency come in to play.

  6. owen ezeagwula
    Jan 30, 2017

    (1) Motor inrush Kva at Starting (Irsm) = Volt x locked rotor current x Full load current x 1.732 / 1000.
    (2) Motor inrush Kva at Starting (Irsm) = Volt x locked rotor current x 1.732/1000

    (2) above is the correct relationship.

  7. shantilal parmar
    Jan 06, 2017

    Dear Jignesh sir,
    Please can you tell me.
    how to find out powe HT TX correct number of turns for primary& secondary winding, size of the winging conductor, core size as we know primary voltage/secondary voltage & secondary load.

  8. rama rao
    Sep 30, 2016

    Good morning,
    we have the loads like 48X2kw(single phase energy meters) + 20X5Kw(3phase energy meters) + 13kw (separate building purpose) = 209KW . what is the capacity of Transformer. Please give the detailed formula to know the actual rating

  9. umair aziz
    Sep 28, 2016

    It correct. But if load increase in future
    Load plus 30% to 50% future load add. Then after calculate.

  10. Vishwajeet
    Jun 29, 2016

    A single phase transformer rating
    33 kva 2400/240 V, 60Hz….here I want to know that the secondary voltage V2=240V is at No Load or Full load…if it is at No load or at full then please explain how..

    Jun 10, 2016

    I want to know design of transformer,
    having data only 1000 VA i/p : 415 V , o/p : 230 V , single phase
    Hoe to decide primary and secondary turns and also cross section area
    please give all details with formulas if possible

  12. Oladipo
    Apr 25, 2016

    Thanks for the calculation its so helpful, however I have a small doubt.
    As regards the motor starting 2 times per hour, no 20% was added to the motor’s minimum kVA rating to compensate for heat losses within the transformer as said in many references.

    Please clarify , if starting the motor 2 times per hour has little or no impact as nothing was mention in your calculation.

    Thanks for the good job once more


  13. Manish
    Jan 16, 2016

    I have one compressor with 20HP motor and our transformer is 200KVA, due to strarting current of motor a voltage drop is there which affects other machines. How can i minimise the same.

  14. siddharth awasthi
    Nov 17, 2015

    sir i m interested in opening a saw.mill . for.that i m installing a 15 hourse.power electricity moter. department.sying thae.above said.moter will 100 killowatt.
    i m very surprise. to tha load calculated by them
    will u please help calculate.the load.
    1.) 15 hourse.power.moter
    2.) 3 normal. 100 watt bulb

  15. Rustico Taguiam
    Apr 22, 2015

    Which better % impedance of distribution transformer, a higher % or lower %?
    5.75% or 4.5%?

      Dec 11, 2017

      Based on my understanding, selection of Z% of the transformer depends on the distribution system. If you have another transformer to be paralleled, where the KVA are similar, we will definitely select the same Z% for both transformers. This will ensure equal load sharing and avoid circulating current in the transformers windings.

      If your installation solely relying on only 1 transformer, now, you can select any size of your Z%. This Z% will be useful especially to limit the fault current. The higher the Z% the lower the fault current can be reduced. However, in the future, if there is a new installation, you would consider the branches being connected to your distribution system. Introducing a load / source in parallel to your system will increase the fault current in your system.

      So depending on your application, you decide which Z% to be applied by considering the philosophy of your system operation, your transformer configuration/connection (parallel?) and considering the fault level in your system. This will also lead to your sizing of cables and busbars as well.

      I hope this help. Appreciate for your feedback / comment.

    • Sudheer ogeti
      Jun 03, 2019

      4.5% impudence is best, because isc will increase

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