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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. Ifran
    Sep 07, 2022

    Short circuit current at TC secondary = 1203 / 5.75 = 20919 Amp.
    Short circuit current at TC secondary = 12038×100 / 5.75 = 20919 Amp.

    which is correct?

  2. Nagendra Mathur
    Jun 24, 2022

    Should we add Motor Starting KVA in Denominator to the transformer Short Circuit KVA ie the % drop = Motor Starting KVA / ( Transformer Short circuit KVA _ Motor Starting KVA

  3. Gousemydeen
    Jan 29, 2022

    In our industry, We have 4 nos of 260 KW motor, which are connected with 1600 KVA transformer. Is this correct design? Please clarify me. We are facing voltage drop upto 48 V

  4. sonam T
    Apr 13, 2021

    hello, if sir could help me out to find the size of transformer for a given load.

  5. Sougata Mukherjee
    Apr 09, 2019

    kindly mention IS or IEC standard from which voltage drop formula used for transformer sizing has been furnished.

    Feb 15, 2019

    Hello sir,

    We have to purchase new transfirmer 33 kv/ 440 volt,i know the LOAD IN KW,how to calculate how much kva transfomer we need …what is formula

    • Ahmed Sheikh
      May 27, 2019

      You need to use an application.

  7. Md. Arifuzzaman
    Feb 11, 2019

    if we start 1000 kva generator( prime power-800 kw, output voltage-440 v) and we want to produce 11 KV voltage with a tranformer(step up) to the load of generator.
    what rating s of x-er to be connected.
    if 1000 kva rating of x-er , the generator tripped?
    or if uses 1500/2000 kva x-er is it safe?

  8. 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.

  9. 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 (%)

  10. ramesh kumar
    May 10, 2018

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

  11. Lilantha Neelawala
    Dec 04, 2017

    Awaiting reply for all above comments.

  12. 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.

  13. 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.

  14. 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.

  15. 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

  16. umair aziz
    Sep 28, 2016

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

  17. 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

  19. 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


  20. 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.

  21. 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

  22. 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

  23. Minu
    Apr 20, 2015

    There is a error in the attached excel calculator. It calculated kVA by multiplying kW and power factor. Actual equation is kVA=kW/power factor.

  24. bhavin mistry
    Jan 28, 2015

    Dear sir, i want know about maximum secondry connectable load as per transfor mer rating

  25. Djarot Prasetyo
    Nov 15, 2014

    Hi! I’d like to know, what standard did you use for the locked rotor current?

  26. Benn Richey
    Sep 02, 2014

    There’s an error in the calculations above: “Motor inrush Kva at Starting (Irsm) = 460 x 2118 x 471 x 1.732 / 1000 = 1688 kVA”
    It should be: Motor inrush Kva at Starting (Irsm) = 460 x 4.5 x 471 x 1.732 / 1000 = 1688 kVA

  27. Mujeeb Raza
    Aug 28, 2014

    Hi All,
    I want to get this into detail, if the output of this transformer is connected to load of small industries (assume same data), What factors are to look into while selecting cable for the secondary of transformer up to the LV panel.

  28. Zulfiqar Ali
    Aug 20, 2014

    Dear sir i hope u r doing fine..your website is very helpfull for engineering articles.many thanks for creating a such type of website.
    but in above article there is a slight mistake in calculations.i hope u ll remove and correct it immediately to avoid more confusions between engineers
    Motor inrush Kva at Starting (Irsm) = 460 x 2118 x 471 x 1.732 / 1000 = 1688 kVA
    in above equation 471 is repeating .which should not b repeated
    this equation should be
    Motor inrush Kva at Starting (Irsm) = 460 x 2118 x 1.732 / 1000 = 1688 kVA

    hope u got my point.

    Thanks And Regards,

    Engr Zulfikar Ali

  29. Mayank P Shah
    Aug 13, 2014

    We are going to set up a new project.Biggest motors are of 260 KW x 2 and 245 KW x 4.
    Total running load is expected of 3100 KVA.Please let us know the size of Dist.Transformer of 11 / 0.433 KV when we starts these heavy motors.All motors are proposed by VFD.We are working around 3500 KVA Transformer.Please guide us.

  30. haytham151
    Jun 21, 2014

    It’s v.g refreshable exp , but l have to wander if i missed something here or you make amistake by the formula :
    Motor inrush Kva at Starting (Irsm) = Volt x locked rotor current x Full load current x 1.732 / 1000
    I think you point to :

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

    • haytham151
      Jul 12, 2014

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

  31. vinod
    Jun 08, 2014

    Please furnish relevant standard for the portion” Motor full load current ≤ 65% of Transformer full load current”.
    Efficiency not considered while calculating full load current of motor.

  32. Mohammed zougti
    May 30, 2014

    •Motor inrush Kva at Starting (Irsm) = 460 x 2118 x 471 x 1.732 / 1000 = 1688 kVA

    Should be •Motor inrush Kva at Starting (Irsm) = 460 x 2118 x 1.732 / 1000 = 1688 kVA

  33. P.Paulpandy.
    May 21, 2014

    I have found formula of motor inrush kva at starting time wrong because when i use your formula the motor kva must be 795353 kva ( motor capacity is 300 kw). but, i see your electrical article 1688 kva.

    Please check and clarify.

  34. Maher M. Al Qadhi
    May 18, 2014

    Dear Sir

    I hope you are doing well.
    And thank you for this useful example
    I have one question which is if I want to calculate the transformer size & volt drop due to building of 11.89 Km square.
    Can I use the important above tips to calculate it ?

    thank you again.

  35. Amr Ahmed
    May 18, 2014

    This calculation for DOL motor starting only or for any starting method , also kindly give me information about the following standard in the point ”Motor full load current ≤ 65% of Transformer full load current ”.

    Thanks & Regards

  36. subrata deb
    May 16, 2014

    Following points to be considered even assuming that prior to starting of this motor

    there is no other load which is fed by the transformer:

    1) 300KW is the output rating of motor.Therefore efficiency of motor should be considered
    to find F.L current 2) +10% tolerance on percentage impedance of transformer should be considered

    As per IS 6600, all the parts of transformer are designed to carry maximum overload

    of +50%. Therfore it should be checked that motor starting kva must be within 1.5 times of
    transformer capacity.

    The logic that motor F.L current shall be less or equal to 65% of transformer rated current is not clear. Please furnish the relevant standard.

  37. Lalit Kumar
    May 13, 2014

    The above calculations does not consider additional loads which might be fed from the same transformer feeding the large motor.

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