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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. 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?

  2. Duaa Alqaruty
    Sep 20, 2018

    Great work, keep it up ^_^

  3. 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?

  4. sam oketch
    Apr 18, 2018

    The spreadsheet for transformer losses calculations is a good work

  5. mani
    Feb 04, 2018

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

  6. Genaro Lucas Jr
    Nov 25, 2017

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

  7. 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.

  8. 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

  9. 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,

  10. 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.

  11. 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?

  12. 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….

  13. 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.

  14. Mario Tovar
    Oct 13, 2014

    How can we convert this losses to thermal losses?

  15. 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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