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How to Calculate/Find the Rating of Transformer in kVA (Single Phase and Three Phase)?

How to Calculate/Find the Rating of Transformer in kVA (Single Phase and Three Phase)?

We know that, Transformer always rated in kVA.  Below are the two simple formulas to find the rating of Single phase and Three phase Transformers.

Find Rating of Single Phase Transformer

Rating of Single Phase Transformer:
P = V x I.
Rating of a single phase transformer in kVA
kVA= (V x I) / 1000

Rating of a Three Phase Transformer

Rating of a Three Phase Transformer:
P = √3. V x I
Rating of a Three phase transformer in kVA
kVA = (√3. V x I) /1000
But Wait, A question is raised here… Look at the General nameplate rating of a 100kVAtransformer.How to Calculate/Find the Rating of Transformer in kVA (Single Phase and Three Phase)?
Did you notice something????Anyway, I don’t care what is your answer 😉  but lets me try to explain.
Here is the rating of Transformer is 100kVA.
But Primary Voltages or High Voltages (H.V) is 11000 V = 11kV.
And Primary Current on High Voltage side is 5.25 Amperes.
Also Secondary voltages or Low Voltages (L.V) is 415 Volts
And Secondary Current (Current on Low voltages side) is 139.1 Amperes.
In simple words,
Transformer rating in kVA = 100 kVA
Primary Voltages = 11000 = 11kV
Primary Current = 5.25 A
Secondary Voltages = 415V
Secondary Current = 139.1 Amperes.
Now calculate for the rating of transformer according to
P=V x I (Primary voltage x primary current)
P = 11000V x 5.25A = 57,750 VA = 57.75kVA
Or P = V x I (Secondary voltages x Secondary Current)
P= 415V x 139.1A = 57,726 VA = 57.72kVA
Once again, we noticed that the rating of Transformer (on Nameplate) is 100kVA but according to calculation…it comes about 57kVA…
The difference comes due to ignorance of that we used single phase formula instead of three phase formula.
Now try with this formula 
P = √3 x V x I
P=√3 Vx I (Primary voltage x primary current)
P =√3 x 11000V x 5.25A = 1.732 x 11000V x 5.25A = 100,025 VA = 100kVA
Or P = √3 x V x I (Secondary voltages x Secondary Current)
P= √3 x 415V x 139.1A = 1.732 x 415V x 139.1A= 99,985 VA = 99.98kVA
Consider the (next) following example.
Voltage (Line to line) = 208 V.
Current (Line Current) = 139 A
Now rating of the three phase transformer
P =  √3 x V x I
P = √3 x 208 x 139A = 1.732 x 208 x 139
P = 50077 VA = 50kVA
Note: This post has been made on the request of our Page fan Anil Vijay.

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  1. Why is transformer rating always written in KVA? We know that the unit of Power is Watt. then why don't we write transformer rating in Watts?

      • Ariffin nasir

        Both consider rating important only its concerned the power factor fluactuation to be maltiply.

      • naushad hussain

        just because we don’t know that this transformer where use there are three type of load no:1 resistive load no:2 capacitive load no:3 inductive load if transformer not in KVA you can not use all three type of load so we make transformer in KVA so that we can use in three type of load.

    • The two types of transformer are core loss and ohmic losses.<br />The core loss depends on transformer voltage and ohmic loss depends on transformer current.As these losses depends on transformer voltage and current and are almost unaffected by the load pf, transformer rated output is expressed in VA or in KVa.

    • because the power factor of the load is not known so transformer rating is given in KVA

    • Usman Shabbir B.Tech(Hons)

      Because it has three resins:
      1. Transformer winding consists Copper and copper losses are occurs in winding due to current(I).
      2. Transformer core obliviously made by Ferromagnetic material and core losses are occurs in core due to voltage(V).
      3. Transformer transmit high electrical energy so reduce the unite of voltages level engineers used kilo(K) unit for transformer.
      So, that transformer rated in KVA mean Kilo volt Ampere.


    • Why does my .5 kva transformer say its rated for 31.25 amps but wen i try and size my transformer needed it says i need 7.68 kVa transformer for 32 amps at 227 volts

  2. dear in last consideration u shows that line to line volts is 208V then after calculation over value is came 50 KVA but actual value is 100 KVA have. plz Kindly explain it

  3. Your article looks great!Thanks for you sharing. I love it. High Voltage Transformers

  4. If we dont know the line current,how 2 calculate the rating?

  5. kW=kva×pf<br />is it right?

    • Yes it is 100% right. When considering the apparent power we also considered the effect of inductor and capacitor which always will be minimized by using power factor correction technique. Points to be remembered: KVA is always greater then KW because KW only have the effect of resistor. KVA=KW if and only if the power factor is one so the load is pure

  6. Sridhar Chary

    sir could u explain DYn-11…..

  7. Hello Sir, Can u explain why in transformer,the power factor is not considering.? And why in others.?

    • Correct me if I&#39;m wrong &#39; The power factor is determined by the load, not the transformer. The PF on the load side of the transformer will be the same as it is on the primary, except at very light loads where the magnetizing current could have some effect. Having said that, the impedance in the primary circuit could effect the PF of the system. In other words, for a given load (with

  8. Kanishka Kani

    hi sir, how do the transformer designer calculate the turns, emf, loss, flux, area if we say we need1600KVA

  9. Kanishka Kani

    hi sir, how does transformer designer calculate flux, emf, loss, turns if v just say the rating eg,1600Kva if u reply with a example calculation it will be comfortable

  10. Hi Sir, what size transformer do I need for 5A,230V Load?

  11. dear 220kv/500kv transmission poll install in the forest.and we don&#39;t know the load and the production tell the method of load and production identification?<br />

  12. hi sir, i want to design a 500VA rating of transformer.but i dont understand how to select the v/g &amp; c/n value? and no. of turn on each side of transformer?<br />pls kindely rply me<br />

  13. Sir ,why we dont consider power factor for transfrmer but for generator or motor?

  14. Sir ,why we dont consider power factor for transfrmer but for generator or motor

  15. Dear Sir, I want to know about how to select step down transformer rating. 415/230V and VA rating. ..?..I mean how we can be calculated. .based on the which factors. .

  16. Can you explain me how to calculate the output current of 3 phase trasformer? can you explain me with formula?

  17. Sir, how to calculate the rating of VCB for HT side,ACB for LT side and what are the CT meter ratio required,the load the shop is 410 kw ( 500 kva transformer)

  18. high sir i got one assignment from my teacher Vo=0 to 15 volts and iL=load current is 3 and design the transformer

  19. Any one have complete designing calculation of distribution transformer of 25KVA

  20. Can any one explains about Cable cross bonding and transposition clearly..

    Thanks in advance.
    Sk Anwar Basha.

  21. P unit is KW
    S unit is kVA, MVA.
    P unit is kVAR.
    Why you write P (kVA)? Please explain? Do you know Transformer formula? S2 = P2 + Q2 ?

  22. Sorry, Q unit is kVAR.

  23. Durga chararn behera

    send me star-delta starter control wiring diagram with remote control &indication wiring.

  24. Durga chararn behera

    calculation how much kvar capacitor will connected to100hp load

  25. is there another reason for why transformer rating in KVA and what is the necessity of power factor

  26. Hi Sir,
    could you please explain me how to calculate knee point voltage Vk of a CT, say 132/33kv, 30 MVA Transformer feeder. %z= 8.5

  27. Sir 500VA Control transformer take how many ampere and how to calculate it????

    • VA = Voltage x Amps for single phase
      KVA = Voltage x Amps for single phase/1000

      VA = square root of 3 x Voltage x Amps for three phase
      KVA = square root of 3 x Voltage x Amps for three phase/1000

  28. respected sir:
    if we make a transformar that have;
    pri: 230v

    sec: 12v and 35am

    how much turns required for this

    if we use the core size(2*3) tell me with exemple

  29. A.Hafeez Khan

    Dear sir I am facing a problem with electricity board of Talangan State.
    Kindly guide me to come out with this problem,

    I had applied 18 nos 5kw each domastic services & 2nos each 15&20kw commercial services total load 125 kW for my Residential/Commercial Apartment, department asking me for 160kva but I’m arguing for 100kva. my calculation is for domestic 18×05=90KW ÷ 2 ÷ 0.9 = 50,
    commercial 15+20=35KW ÷ 1.5 ÷ 0.9 =25.92,
    Total KVA 75.92
    The formula which I mention is belongs TSSPDCL.
    Why should I take 160kva if 100 kva is sufficient to me.

    Dear sir I am waiting for your replay for this particular formula if I get any book or however you please guide me
    Thank you

  30. sir as in this case you showed us max load of transformer 100 kva but i want to ask you how much load it will support actually on site. what is the safety factor or service factor of transformer.lets suppose we have a load of 1200 kva now tell me appropriate size of transformer for this load thanks

  31. In a numerical probem, transformer rating is given as 220/13.8 KV. I want to know which is primary and which is its secondary?

  32. dear i am asking for load .how much kva rating of transformer for 200kva load

    • The capacity of transformer required for 200 Kva load can be correctly ascertained only after receiving the individual loading pattern . For example the total load of 200 Kva from small motors like 5 , 10 HP ( KVA ) each the transformer of 200 KVA only is enough.But If a individual load is more tha 150 KVA You will need at least 300 KVA capacity transformer. RAJ LAHARE

  33. 13.8kv is primary and 220/380 is secondry

  34. Hello, I am facing a little problem with transformer calculations.
    I have 3ph 600V 200A feed , and trying to figure out size of transf. for 208V 1phase, there is many explanations but none on 3phase to single phase calculation. Can you help me please.

  35. A contractor has supplied a transformer 11/0.433 KV, 160 KVA . I suspect that the transformer is not of that rating. How can I check / Test this

    • the input voltage rating need not be be exceeded nor should the output current(amp) but lets say the transformer is rated at input of 11 kv with an output of 235 and you have 5.5 kv coming in and you want 120 this is what you should expect 118 this would be fine i did not go into current ratings but i suspect this more or less what your contractor did your transformer will have costed a bit more but will last allot longer likely for a hospital or something that needs a high reliability

  36. We have been supplied a Transformer of 160 KVA 11/0.433 rating. How we can check / test that the transformer is of Given rating ?

  37. for 600 KVA transformer what will be the line current or individual phase current?

    • For proper answer you will have to give following additional information. Whether the transformer is single phase or 3 phase. What is the primary and secondary voltage Please clarify .

  38. Dear Nirmal, This is not reply to my query.Your answer gives the loading on the transformer and NOT the Rating / capacity of transformer.

  39. obiukwu osita nemson

    How can i calculate transformer capacity with out looking at main plate of the transformer.

  40. Thank you to anil and vijay sir.

  41. The formula will give the load in VA or KVA on the single phase transformer and can not give the capacity of the transformer. The Capacity means How much load can be put on the said transformer without over heating and with voltage drop within limit .Wish proper guidance.

  42. how can we determine the KVA rating of a transformer from test

  43. My query is for determining the capacity

  44. My query is for determining the capacity of a available transformer.But I am not getting any proper answer to my query.

  45. daer all The word Capacity is very clear. It can never be the weight. The capacity means for a given transformer how much electrical load can be put on this transformer. Say for a transformer of 230 / 12 volts and 60 VA capacity ( Single phase) The max load can be 5 Amps on 12 V side if the load is exceeded the transformer will get overheated and may burn off. So if I want to verify this capacity , what is the laboratory method ? My origional question was regarding a transformer supplied by a contractor saying 160 KVA capacity but at about just 120 kva load this transformer gets over heated than the specified temperature. So the query. RAJ LAHARE

    • 160kVA Transformer Capacity overheated on 120kVA load? There may be different factors.
      Did you check the Transformer power factor (As improved P.F increased the T/F Capacity as well) or did you verify the proper cooling of the transformer?
      In your case, Transformer Load capacity will be considered as Transformer Rating in VA or kVA.
      Also, The Transformer load capacity is the Rating of the Transformer x P.f.

  46. Dear Sir,
    I specifically say that the load is 120 KVA and not KW so the question of Power factor do not come in picture. Still to communicate that the PF is 0.99 . The Transformer is outdoors like any other transformers so cooling etc do not come in picture . I suspect that the supplier must have supplied lower capacity transformer with rating plate of 160 Kva and I want to test / investigate this . RAJ LAHARE

    • For This, You need to know the two pieces of information

      1. the load line-to-line voltage (V)
      2. the maximum load phase current (I)

      kVA Capacity = ( √3 x V x I x cosΦ ) / 1000
      Do this simple calculation then
      Or You need This tool to do the job.

      • As per basics of transformer , The power capacity of transformer ( VA ) is based on the cross section and the permeability of the transformer core. Then the voltage is based on the Number of Turns . Based on the Voltage and the VA capacity the current is arrived at and then only considering standard losses the current is calculated which inturn will give the cross section of the winding conductor. Here the transformer is already received and is sealed. I know very well to ascertain the capacity by opening the transformer and then measuring the Core area, primary and secondary winding cross section area and then measuring number of turns each of primary and secondary. BUT I DO NOT WANT TO DO ALL THIS EXERCISE AND WANT TO CHECK THE CAPACITY OF SUPPLIED TRANSFORMER WITHOUT OPENING THE TRANSFORMER. I HOPE AT LEAST WITH ALL THIS CLARIFICATION I WILL GET PROPER GUIDANCE . I ONCE AGAIN SAY THAT I DOUBT ABOUT THE SUPPLIED TRANSFORMER IS NOT OF ORDERED ( 160 kva) CAPACITY. RAJ LAHARE

  47. Dear Sir,
    I need complete calculation with formula. Now my doubt is I have total 125kw load so how much rating of transformer I require. please inform it’s urgent to me.

  48. Dear Sudarshan,
    In my earlier mail I had given a comment and also suggested that the capacity of transformer is dependent on the max individual load so If you can give your list of 125 KW load the correct capacity can be calculated. RAJ LAHAREthe

  49. Devbansh yadav

    Its really satisfaction to concept thakyou sir..

  50. Sir why KVA is divided by 1000

  51. who is the owner of this website ?
    whoever he is, a simply genius of electrical engineering. simply awesome.

  52. for a 3 phase transformer, is there always line to line voltage written on name plate for star connection?

    • Yes, for three phase transformers the voltage given on name plate is phase to phase NO LOAD that is open circuit voltage. On load ,depending on the impedance transformer the secondary voltage drops down to some extent. RAJ LAHARE

  53. It is really good job done by this website thanking you.

  54. HOW TO Convart H.T amper to L.T Amper..formola

  55. Dear Sir
    Suppose someone have given transformer capacity, LV side voltage, HV voltage and believe power factor is 1, than how to calculate LV site current capacity and Hv side current capacity?
    Please awaiting for kind reply.

    • For this additional DATA is required 1) Whether the transformer is single phase or poly phase 2) If poly phase what is vector group on primary and secondary side then only correct solution can be given . In any query if correct input is given the answers can be faster. RAJ LAHARE

  56. Primary side voltage is 230 v, secondary side voltage is 50 vhow to calculate KVA..This is a old transformer no name plate details in this transformer

    • Dear Chokklingam,
      As per your query this seems to be single phase transformer, There is no name plate as per your statement. Can you send the details as the cross section area of the core and the material used for the core. further whether the transformer is core type or shell type ,then the rating can be calculated. RAJ LAHARE

  57. Hey transfermoer rating in kVA

  58. 500 kva tarsformer fuse range

    • For deciding the fuse rating we must know the voltage ratio of the transformer And the full load current is always written on the rating plate. For the 500 KVA 11 KV /0.433 KV three phase DYn11 Transformer HV Full load current is @ 26.65 Amps and Lv side current is @ 667.5 Amps. RAJ LAHARE

  59. i want to select frequency drive for transformer application.transformer rating is 500 va .what will be rating of drive pls help

    • Dear Bhushan,
      It may be kept in mind that the Transformers are the sours of electrical energy from the system at given voltage. And the basic design of the transformers are related with the frequency, wave form, Permeability of the core and the voltage, And the Drives are designed for limiting the inrush currents of the motors Say it is a current control device. so there no meaning in design of frequency drive for a transformer. RAJ LAHARE

  60. i have a microwave transformer that i want to reuse as a welder i have 56 windings on the primary of 12 gauge the core is laminated soft iron in the shape of a box with a bar that divides the primary and secondary over dimensions 4.25 x 6.25 x 2.25 with core wall thickness of 3/4 and the dividing wall between the coils is 5/8 of in square

    i was thinking of using some simple 4 gauge thwn wire covered in a high temp sleeve as the secondary cooling it off with a 200 cfm fan while placed in a metal case that is just over 1/4 in wider on 2 sides and above and front and back is also 1/2 in biggerbellow 1/2 in the fan will make a tight seal i am wanting to do resistance spot welding any advice on changes in design and duty cycle ratings

    • Dear Tom, When You give dimenssions You are also expected to give the Units such as whether the measurements are in MM or CM or Inches..Secondly what is the cross section Area of the CORE And what is rated Primary Voltage and secondary voltage of this transformer. On hearing this solution can be suggested. Can You please send the photograph of your existing transformer ? RAJ LAHARE

  61. Dear Tom, This is NOT technical reply to my Query. I will like to have perfect TECHNICAL solution to my query. RAJ LAHARE

  62. Jaywant Jadhav

    Earting & Transformer

  63. Hello i want to know HV and LV coil resiatance and inductance values of 11kV/440V rating 100kVA transformer

  64. Tony Ohaegbulam

    How do I calculate the fuse rating of a power supply circuit with 500VA, 220V AC/ 15V AC step-down transformer, if after rectification, filtration/smoothening, and 12V DC regulation; the load is a purely resistive load of 100watts, 12V DC. Thanks.

  65. andrew magunda

    thanks for the content because i really liked it

  66. Dear Tony, Your transformer is 500 VA with 220 volts promary so the primary fuse rating will be 2.27 Amps say 2 Amps. Now you are using resistive load at 12 volts DC and 100 watts so your secondary side current willbe 8.33 Amps so you may use 8 Amps fuse. For full load of 500 watts at 12 volts the current will be approx 41 Amps so the fuse required will be 40 Amps. RAJ LAHARE

    • hi Raj,

      If the load for a building 39kw 3/phase and 10kw for s/phase.
      Please give me the formula for total load and required transformer rating.


      • Dear Jan,
        For technically correct calculation of the requirement of transformer some additional DATA will be required that is The details of 39 KW 3 phase load bASICALY what is the highest idivisual load.And what is the source voltage. Please. RAJ LAHARE

  67. how to design transformer increas or decraes

  68. how to find primary and secondary amp of a 1500 kva transformer (11kv/400v). Pls reply

    • Dear jkjk,
      As we know Kva = root 3* V * I *1000 so for your problem
      1500 = 1.7321 * 11000 * I so
      I = 1500* 1000 /1,7321/11000
      = 78.72 Amps
      You may caThis will be full load current of transformer Now You can calculate current on 400 volts side.
      Remember 11 KV voltage is phase to phase voltage.If find difficult may ask again .

  69. I agree with Mr. HTK. The KVA should be for appearance power S NOT in P active power, however if you want to calculate in P active power, in this case you have to take into consideration the power factor and the result will be in KW not in KVA.

  70. I have a power system,in the following order, with a source of 132kV, 2No, 140 MVA, 132/33kV transformers, 33kV Bus, 8 transformers of 1.6MVA, 33Kv/415V, 8 loads of 0.8 MW, PF=0.9, connected to the transformers at 415V side. 2 transformers of 50MVA, 33kV/11kV, connected from the 33kV bus to 11kV bus, 4 transformers of 5 MVA, 11kV/3.3kV, to which 4 loads of 2.5 MW are connected with PF=0.9, 4 transformers of 2MVA, 11kV/415V, to which 4 loads of 1MW each are connected with PF=0.9.

    My question is, what is the total connected load at 132kV receiving point, do we need to consider the transformers as load, or just the main load are enough??

    I’m giving the list of transformers and load again,
    Transformers: 140MVA (2No), 50MVA (2No), 1.6MVA (8No), 5MVA (4No), 2MVA (4No)
    Loads: 0.8MW (8No), 2.5MW (4No), 1MW (4No), all at POWER FACTOR = 0.9

    Please help me calculate the total load at 132kV receiving point.

  71. How to calculate the total load received at the source point? suppose I have the following in my power system:
    Transformers: 140 MVA, 132/33KV (2 NO). 50MVA, 33/11KV (2 NO). 1.6MVA, 33KV/415V (8 NO). 5MVA, 11/3.3KV (4 NO). 2MVA, 11KV/415V (4 NO)
    LOADS: 0.8 MW, PF=0.9 (8 NO), 2.5MW, PF=0.9 (4 NO). 1MW, PF=0.9 (4 NO).

    Receiving point:- 132kV source generator

  72. Dear Abdullah,
    Coinsidently Similar question is asked by Mr, Thaamir, For exact calculations of this type You are also expected to give additional information such as 1) whether all the transformers of same voltage rating are connected in parallel or are separate. 2) Percentage impedance of each transformer. How ever roughly the Total load on 132 System will be Approximately 25.47 MVA considering standard Transformation loss of 5 % at each level. If You can give thiven. RAJ LAHAREe clarification of above 2 points still correct answer can be g

    • Dear Mr.Raj,

      1) 1.6 MVA transformers are connected in parallel in pairs. there are 8 such transformers, so there are 4 pairs. similarly, two 5MVA are connected in parallel, so there are 2 pairs. Similarly, two 2MVA transformers are connected in parallel, so there are 2 pairs. the 140 MVA and 50 MVA transformers are connected separately.

      2) the percentage impedance are:
      140 MVA TF, Z%=12.5
      50 MVA TF, Z%=12.5
      1.6 MVA TF, Z%= 6
      2 MVA TF, Z%= 6
      5 MVA TF, Z%= 8

      Thank you in advance for your timely help, if you give your email id, i can send you my system, so that it will be clear to you.

  73. Nnabugwu Kingsley

    Load is calculated in ampere or megawatt not in MVA. For example, if you are calculating a load on 33kv, the total current flow on it is the load. Let say 50A. Then , if you want to convert the load to megawatt, you divide 50A by 20. If it is on 11kv, the 50A load will be converted to megawatt by dividing it by 60. If it is on 132kv, the 50A load will be converted to megawatt by dividing it by 5.

  74. Nnabugwu Kingsley

    Hi Abdullah,
    Coming to your question, first of all, you have to consider transformation ratio of your voltage level(i.e 132kv/11kv,132kv/33kv,33kv/11kv), then, with respect to my recent post, convert all the megawatt loads to current loads(i.e the megawatt loads for 11kv convert them to current loads by multiplying the total sum by 60, while those of 33kv megawatt loads will be also converted to current loads by multiplying the total sum by 20). After that, divide the current converted load of 33kv by 4 to get the current load on the 132kv and divide the converted current load of 11kkv by 12 to get the current load on the 132kv. Then, sum your answers to get the total current load of the 132kv. To convert it to megawatt load, divide the 132kv current load by 5. Thanks.

  75. Gubendiran Singaram

    dear all,
    tel me some explanation about leakage current in 100kVA distribution transformer and what is the maximum leakage current allowable in transformer

  76. Dear Gubendiran, Please refer to ISS 2026 and ISS 1180. You will get all the information. These ISS publications are available on internet. raj lahare

    • Gubendiran singaram

      thanks for the reply sir..
      i seen the both standards there are no specific value of leakage current.i want standard value of specific leakage current.standards only says how to find and measure leakage currents.i think 100mA for 100kVA distribution but how???

    • Gubendiran singaram

      dear sir,
      Thanks for the reply.
      i seen both standards there are no permissible or maximum allowable leakage current in directly mention.could you please answer me.

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