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Transformer Losses

Transformers have two major components that drive losses: the core and the coils. The typical core is an assembly of laminated steel, and core losses are mostly related to magnetizing (energizing) the core.

Spreadsheet for Transformer Losses Calculation
Spreadsheet for Transformer Losses Calculation

These losses, also known as no-load losses, are present all the time the transformer is powered on – regardless of whether there is any load or not.

Core losses are roughly constant from no-load to full-load when feeding linear loads. They represent a continuous cost, 24 hours/day, for the 25-year or more life of the transformer.

Formulas in spreadsheet

  • HV Full load current = VA / (1.732 · Volt)
  • LV Full load current = VA / (1.732 · Volt)
  • HV Side I2R losses = I²R · 1.5
  • LV Side I²R losses = I²R · 0.5 · 3
  • Total I² R lossses at Amb. temp = Hv losses + Lv losses
  • Total Stray losses at Amb. temp = Measured losses – I²R losses
  • I²R lossses at 75° C temp = ((225 + 75) · losses) / (225 + Amb. temp) .
  • Stray losses at 75° C temp=((225 + Amb. temp)(Stray losses at Amb. temp)) / 300
  • Total Full load losses at 75° C = I²R losses at 75° C + Stray losses at 75° C
  • Total Impedance at amb. temp = (Imp. voltage · 1.732) / Full load current
  • Total Resistance at amb. temp = I²R losses / I²
  • Total Reactance (X) = SQRT (Impedance² – Resistance²)
  • Resistance at 75° C = (300 · resistance at amb ) / (225 + Amb. temp)
  • Impedance at 75° C = SQRT (R² at 75° C + X²)
  • Percentage Impedance = (Z at 75° C · I · 100)/V1
  • Percentage Resistance = ( R 75° C · I · 100)/V1
  • Percentage Reactance = (X · I · 100) / V
  • Regulation at Unity P.F. = (%R cosø + %Xsinø)
  • Regulation at 0.8 P.F. = (%R cosø + %Xsinø) + 1/200(%R sinø – %Xcosø)2

Efficiency at Unity P.F

  • At 125 % of Transformer Loading = (kVA · 1.25 · 100)/((kVA · 1.25)+(I²R losses · 1.25²)+(No Load Losses))

Efficiency at 0.8 P.F

  • At 125 % of Transformer Loading = (kVA · 1.25 · P.F. · 100)/((kVA · P.F. · 1.25)+(I²R losses · 1.25²)+(No Load Losses))

Contribution to transformer losses

Load level varies widely, with some installations running very heavily loaded and others more lightly loaded.

This difference substantially impacts actual losses incurred. Unfortunately, there is a small body of field data available, driven by the factors such as a lack of awareness of the cost of the losses, and the cost of gathering detailed data from a reasonable number of individual transformers.

Several variables contribute to transformer losses, the most important of which include load level, load profile, and core and coil construction.

Since there are a wide variety of transformers on the market serving different purposes, and available from different manufacturers, actual losses incurred in the field will vary substantially from installation to installation.

Software:Spreadsheet for Transformer Losses Calculation
Version:10.06.2013 New
Developer:Jignesh Parmar
Size:36 Kb
Download:Right here | Video Courses | Membership | Download Updates
Spreadsheet for Transformer Losses Calculation
Spreadsheet for Transformer Losses Calculation

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More Information


  1. Vladimir
    Sep 28, 2023

    Need to calculate transformer looses.
    Need software to download

  2. Bashar A.A
    Jul 11, 2023

    Thanks for everything

  3. Prashant Jagtap
    Jun 14, 2023

    We are using 415 V ac, 50 Hz, 600Amp, dimmerstate (variable voltage transformer).
    Need help in calculating losses. CORE loss and COIL (Copper) Loss.

  4. Prashant Jagtap
    Jun 14, 2023

    We are using 415 V AC, 50Hz, 600 Amp, Variac.
    We want to calculate I2R Losses (Cu-Losses)

  5. Rishi
    May 12, 2023

    In Copper loss, why it is multiplied by 1.5 and 0.5 for HV & LV side?

    Mar 07, 2023

    How do I get the Impedance when having,
    315KVA transformer

  7. Serajahmad
    Dec 27, 2022

    Dear sir
    Thanks in advance
    could you tell me and solve the problem of how to calculate the x/r of Transformer?
    I hope you will learn to me !!!

    Sep 17, 2022


  9. Elaiyaraja
    Sep 14, 2022

    How to calculate the no load loss, copper loss and stray loss for 2500KVA Transformer(33KV/0.433KV). Do share the formulas for all the 3 types of losses in transformer.

  10. ksaravanan
    Jul 29, 2022

    it is very useful for me

  11. Denis
    Jun 16, 2022

    Dear Jignesh,
    I don’t find Lv resistance nor Hv resistance (cells C16 and C17 in your Excel file) on my transformer user manual. I though at first that it was primary side resistance and secondary side resistance. But in that case, I don’t understand why you multiply by 0.5 in the I²R losses formula (cells H7 and H8).
    I thought it might be total resistance seen from primary and secondary, thus I computed I²R losses / I1² and the same with I2², but figures are different.

    Please, can you explain me how to find these values (Lv and Hv resistance) ?

    With kind regards

  12. Pankaj Patel
    Dec 30, 2021

    Dear sir,
    How to calculate 2500KVA (11kv/415V) Transformer no load loss and copper loss at 50% rated load.
    Transformer details:
    Make- Crompton
    YOM- 2007
    KVA- 2500
    Impedance volts- 7.10%
    HV Amp- 131
    LV Amp- 3333
    Volts HV- 11000
    Volts LV- 433
    Type of cooling- ONAN

    Please reply me as soon as possible.
    Pankaj Patel
    Gujarat, India

    • Wasiullah
      Apr 21, 2022

      Dear Mr. Pankaj patel,

      This is very simple to measure no load losses.
      First of all Switch off the transformer output breaker, which is installed on LT side called ACB. Then take Clamp meter and measure the Amps from transformer input (HT side) and put the amps in power formula, (P=HT Volt*HT Amps*pf*1.732/1000) P=11000*0.5*0.8*1.732/1000


  13. Andrew Smith
    Oct 05, 2021

    Dear Sir,

    Thank you for posting this calculator. I used it to check my own which was almost correct.

    Aug 06, 2021

    Sir, please note that we have old 400 kva tranformer,year of manufactured 2007 and MAKE: CROMPTON GREAVES. OIL COOLED TRANFORMER Hence kindly suggest us for replacement of the TRANSFORMER , since it is very old.we are assuming that lot of energy getting wasted due to the above conditions.

    • Esennar Transformers
      Sep 14, 2021

      You can go with Esennar Transformers.. where you can save upto 50 thousands rupees per year..

  15. Mohd Zulhizam
    Mar 11, 2021

    Assuming the losses and impedance of the transformer is negligible, calculate the HV and LV currents if the transformer was loaded at 120%

    What formula use ?

  16. Shafique
    Jun 26, 2020

    Sir, coul you explain temperature gradient calculation of HV and LV winding , if temperature rise 40/50

  17. Rohit Saini
    Dec 12, 2019

    Please tell how to calculate no load losses of transformer.

  18. Nandini Basavarajappa Doddaveerappla
    Oct 15, 2019

    Tell me how to get transformer copper loss
    I have no load losses n total power loss

    • vivek kumar
      Jan 08, 2021

      Total Power loss = No load loss + Copper loss + Stray loss

      If assume Stray loss negligible then

      Copper loss = Total power loss – No load loss

    Aug 28, 2019

    Dear sir,
    Dry type and oil type transformer losses how to calculate.

  20. FLOYD
    Jan 19, 2019

    specs of hi pot tester from 10 to 35kv?

  21. S.M.Tayyab
    Nov 12, 2018

    How can I calculate no load losses and load losses of power trafo when I have total losses of trafo available only?

  22. Duaa Alqaruty
    Sep 20, 2018

    Great work, keep it up ^_^

  23. Carcky
    Jul 25, 2018

    On this formula: Total Stray losses at Amb. temp = Measured losses – I²R losses, if I measured V= 384, I= 20 and W= 1015, for a 24MVA, 231200V (on tap 1), how do I relate the measured power into the equivalent input power which I can use in the formula?

  24. sam oketch
    Apr 18, 2018

    The spreadsheet for transformer losses calculations is a good work

  25. mani
    Feb 04, 2018

    how do find out the value of resistance in hv and lv side

  26. Genaro Lucas Jr
    Nov 25, 2017

    please review cell H33, i think efficiency should not be highest at 125%

  27. Fernando Pereira Andrade
    Oct 01, 2016

    Dear all,

    How can I get the value 1.5 in ” HV Side I2R losses = I²R · 1.5″ and 0.5*3 in
    ” LV Side I²R losses = I²R · 0.5 · 3″ .

    Thank you in advanced.

  28. Fernando Pereira Andrade
    Oct 01, 2016

    Dear all,

    What are the values 1.5 in HV and 0.5*0.3 and LV ?

    HV Side I2R losses = I²R · 1.5
    LV Side I²R losses = I²R · 0.5 · 3

    Thank you in advanced.

    • Hannah Duong
      Feb 04, 2022

      IEC60076-1, Clause 10.2 specified that: “The resistance of each winding, the terminals between which it is measured”.
      It means in short-circuit test to get resistance values, they will measure the resistance between two transformer terminals (A-B, B-C & C-A).

      HV – Delta Connection: Rwinding = Rmeasured x 1.5
      [Because they measured terminal – terminal while 1Rwinding // (Rwinding + Rwinding)]

      LV – Star connection: Rwinding = Rmeasured x 0.5
      [Because they measured terminal – terminal while Rwinding + Rwinding]

      HV – Delta connection: Current through winding Iw = I/sqrt(3)
      LV – Star connection: Current through winding Iw = I

      So now we calculate the copper loss (resistance loss): 3 windings so that we have to multiple with 3

      HV – Delta connection: 3 x Iw x Iw x Rw = 3 x I/sqrt(3) x I/sqrt(3) x R x 1.5 = I x I x R x 1.5

      LV – Star connection: 3x Iw x Iw x Rw = 3x I x I x R x 0.5 = I x I x 0.5 x 3

  29. venjateswarlu
    Jul 30, 2015

    Dear sir,
    For finding i^2R losses on hv side =I^2R.1.5
    What does 1.5 means

    • Fernando Pereira Andrade
      Oct 17, 2016

      Dear Venjateswar,

      Did you find any answer for this question ?

      Best regards,

  30. hafizah
    Jun 09, 2015


    what is V1 and V for the percentage impedance and reactance?


    • John Njuguna
      Nov 16, 2017

      The V1 is the voltage that, applied to the HV side of the Tx whose voltage rating is V, shall circulate the rated current on the LV side of the Tx with it’s terminal short circuited. This voltage is expressed as a %age.

  31. hafizah
    Jun 09, 2015

    Is the measured loss a physically measured value? Is there a way of estimating this number based on the transformer size?

  32. Wyn
    Feb 12, 2015

    Edvard / Jignesh
    well done gents this is practical and useful I like it!.
    Here is a suggestion for you ready reckoner spread sheets for estimating:
    overhead power lines e.g. 415V AC distance covered number of bends and calculate the number of posts, cross arms, insulators, conductor sizes, tension, weight of the conductors….
    same for 22kV or 33kv or underground with approximate schedules of rates so I can enter local steel costs ideal pole spacing typical sag heights….

  33. deepak
    Jan 03, 2015

    can you provide spreadsheet for transformer losses calculation of 1000kva

    • Edvard
      Jan 03, 2015

      Can’t you use this spreadsheet? Just edit field rated power in kVA.

  34. Mario Tovar
    Oct 13, 2014

    How can we convert this losses to thermal losses?

  35. sagheer ahmad
    Oct 12, 2014

    EEP provides much more information about any given electrical article which is guide line for technical people.

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