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How to Size a Single Phase and Three Phase Transformer in kVA? Calculator

How to Calculate the Rating of Single Phase & Three Phase Transformers in kVA

We know that a transformer is always rated in kVA. Below are the two simple formulas which can be used to find and calculate the rating of Single Phase and Three Phase Transformers.

In any case, both the voltage and currents should be form an individual side either primary or secondary respectively ( e.g. Primary voltage x primary current or secondary voltage x secondary current). To property size a transformer for a required home appliance, you should select the supply voltage and secondary load current in amperes.

Sizing a Single Phase Transformer

Rating of Single Phase Transformer:

P = V x I


  • V = Primary or Secondary Voltage
  • I = Primary or Secondary Current
  • P = Power of transformer in VA (Volt-Amps)

Rating of a single phase transformer in kVA

kVA= (V x I) / 1000


Suppose a 1-phase transformer having the secondary voltage and current of 240V and 62.5A respectively. Calculate the size of the  single-phase transformer.


  • Secondary Voltage: 240V
  • Load Current: 62.5A

Transformer Rating = P = V x I

Putting the values:

P = 240V x 62.5A

P = 15000 VA = 15 kVA

Sizing a Three Phase Transformer

Rating of a Three Phase Transformer:

P = √3 x V x I


  • V = Primary or Secondary Voltage
  • I = Primary or Secondary Current
  • √3 = 1.732
  • P = Power of transformer in VA (Volt-Amps)

Rating of a Three phase transformer in kVA

kVA = (√3 x V x I) / 1000


A 3-phase transformer having the primary voltage and current of 7200V and 4A respectively. Calculate the size of the three phase transformer.


  • Primary Voltage: 7200V
  • Primary Current: 4A

Transformer Rating = P = √3 x V x I

Putting the values:

P = 1.732 x 7200V x 4A

P = 49,881 VA ≈ 50 kVA

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Transformer Rating in kVA Calculator

The following calculator will calculate the rating of the transformer in kVA, Primary or secondary voltage and current respectively. Just enter any two values and click on calculate to find the desired value.

Enter Any Two Values:
Transformer Phases No of Phases
Voltage Volts
Current Amperes
kVA kilovolt-Amperes

Now, look at the General nameplate rating of a 100kVA transformer.

How to Size a Transformer - Calculate the Rating of Transformer

The nameplate clearly shows the rating of transformer is 100 kVA.

The primary voltages or high voltages (HV) is 11000 V = 11kV.

And the primary current on the high voltage (HV) side is 5.25 Amperes.

Also secondary voltage or low voltages (LV) is 415 Volts

And secondary current (load current on low voltages (LV) 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

As you noticed, the rating of the transformer (on the nameplate) is 100 kVA but according to the calculation, the calculated value is  57kVA.

Well, The difference comes due to ignorance of the fact that we used a single phase formula instead of three phase formula. It means it is a three phase transformer and we will use the associated formula for it. 

Let’s clear it using the three phase transformer rating formula.

P = √3 x V x I

P = √3 V x 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 ≈ 100kVA

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

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That’s it. Now you know how to properly size a three phase and single phase transformer with suitable rating in VA or kVA for home appliances or any other load applications.

The following table shows the dry transformer data of standard kVA rating, Voltage and Current rating for both single phase and three phase transformers.

Single Phase Transformer Three Phase Transformer
KVA Rating Amperes KVA Rating Amperes
120V 240V 600V 120V 240V 480V 600V
0.75 6.25 3.13 1.25 3 8.33 7.22 3.61 2.89
1 8.33 4.17 1.67 9 25 21.7 10.8 8.66
1.5 12.5 6.25 2.5 15 41.6 36.1 18 14.4
2 16.7 8.33 3.33 20 55.5 48.1 24.1 19.2
3 25 12.5 5 25 69.4 60.1 30.1 24.1
5 41.6 20.8 8.33 30 83.3 72.2 36.1 28.9
7.5 62.5 31.3 12.5 37.5 104 90.2 45.1 36.1
10 83.3 41.7 16.7 45 125 108 54.1 43.3
15 125 62.5 25 75 208 180 90.2 72.2
25 208 104 41.7 100 278 241 120 96.2
37.5 313 156 62.5 112.5 312 271 135 108
50 417 208 83.3 150 416 361 180 144
75 625 313 125 225 625 541 271 217
100 833 417 167 300 833 722 361 289
167 1392 696 278 500 1388 1203 601 481
250 2083 1042 417 750 2082 1804 902 722

Here is the table in the image format if you need to download as a reference.

Dry transformer data of standard kVA rating, Voltage and Current rating for 1-phase & 3-Phase Transformers
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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?

      1. 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.

          1. So the thing is when they design transformer they never consider power factor because they consider the user will connect resistive load that is default. But in the case of motor or any other appliances they consider power factor in to account. For instance, synchronous motor uses for power factor improvement and induction motor draws lagging current.

      2. When input power in electrical and output is also in electrical the the device , transformer, generator etc capacity are measured in kVA , but when input power is electrical and output is mechanical energy like motors its capacity is measured in ke.

      3. The transformer losses are mainly depend in voltage and current so the voltage=v ,current,=A and the rating in terms of transformer is kilobits so we can say transformer rating in kvA

    1. 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.

    2. 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.


    3. 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

    4. cz transformer rating depends on temprateure and tempreture is depneds on loss
      as you know that their is two type of loss copper and iron
      copper>> Current (A)
      Iron>> Volt(V)
      So if it depends of powe then we can say that it will in in KW but it denpends on Loss So we used in KVA

    5. transformer rating is related to its losses. there are copper loss and core loss in trasformr. copper loss is depends on current and core loss is depends on voltge,so the rating of transformer is said to be the product of voltage and current. VA OR KVA

  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

    1. Dear Waqas Ahmed<br />Here are two examples.<br />The first one is for 100kVA which shown in the image.<br />The Second one is for 50kVA (where line to line voltage is 208V)

    1. This system arrangement is very common, both at the utilization level as 480 Y/277 V and 208 Y/120 V, and also<br />on most utility distribution systems.

    1. 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

    1. The “D” indicates that this is a three phase transformer and the primary is connected in DELTA then the “y” indicates that the secondary is connected in STAR connection “N” indicates that the star point is taken out from the transformer and is known as Neutral . The “11” indicates that the vector of secondary voltage is lagging by 30 degrees from the vector of primary voltage RAJ LAHARE

    1. 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

    1. you can find the loss with the help of open circuit(eddy current and hysterisis) and short circuit test(copper loss) and other parameter can also be calculated with these test.

  3. 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

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

  5. 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 />

  6. 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. .

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

  8. 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)

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

  10. 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 ?

  11. 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

    1. 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

  12. 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

  13. 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

  14. 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

    1. Dear, The proper load of transformer is not defined (it may inductive, capacitive, resistive or max load…Thats why it is rated in kVA insted or kW.

      1. hi sir can show me how to ansenser this pleas Work out the transformer primary and secondary currents assuming an 11kV/400-230V, 3-phase, 50
        Hz supply transformer and suggest suitable types and ratings for the primary and secondary protecti

  15. 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?

    1. In your case, the transformer rating looks like, 220kV is primary while 13.8kV is secondary… (In case of Step down i.e. distribution side transformer…

    1. 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

      1. Hi ,

        I am having a motor 460V, 10HP , It will be great if you could please let me know the rating of the transformer which I need to use for connecting it to our supply voltage of 600V. I would also like to know the steps involved in calculating the rating of step down transformers for motors.


  16. 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.

  17. 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

    1. 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

  18. 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 ?

    1. 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 .

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

  20. 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.

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

    1. What do you mean by Transformer Capacity? Do you want to know about the weight or T/F or Rating of Transformer?

  22. 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

    1. 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.

  23. 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

    1. 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.

      1. 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

  24. 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.

    1. kW = kVA x Cos(Ø) = Where Cos(Ø) = Pf…
      Now Transformer rating in kVA = kW/Cos(Ø)
      Now put your actual power factor (for instance pf=09)
      kVA = 125kW/0.9
      kVA = 139kVA
      for additional load in the future… 150kVA would be better than.. Thanks

  25. 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

    1. It is actually VA (Volt-Ampere) .. Instead of 1000VA, We divide it by 1000, So that, It becomes 1kVA, where k = 1000.

      1. Draw a schematic single line diagram for the distribution of the complex in question, showing where
        appropriate the transformer, generating set/sets and UPS systems

    1. 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

  26. 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.

    1. 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

  27. 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

    1. 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

    1. 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

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

    1. 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

  29. 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

    1. 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

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

  31. 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.

  32. 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

    1. 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.


      1. 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

    2. Your answer may be right only if you are assuming a single phase transformer.
      Remember he did not specify, so inorder to guide him correctly, you may need to be sure of the number of phases.

    1. 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 .

  33. 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.

  34. 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.

  35. 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

  36. 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

    1. 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.

  37. 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.

  38. 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.

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

  40. 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

    1. 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???

    2. 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.

  41. My power transformer is 10 MVA capacity what is the Maximum Amps load can be connect in 33 kv, 11kv or 415 V, with formula

    1. Hi Akhatar Ali,
      For 33KV: 10MVA/1.73*33KV=10*1000000VA/1.73*33*1000V=10000A/1.73*33= Simply further
      For 11KV: 10MVA/1.73*11KV… simply further.
      Note: 1.73 is gotten from Root three (root 3)
      Take care!

      1. Dear Akhatar Ali.
        As the thumb Rule the full load current on 33 KV side for 10 MVA load is 175 Amps. for 11 KV it will be 525 Amps. and for LT 440 volts it will be approx 13125 Amps. RAJ LAHARE

  42. pls , what is responsible for high voltage in a distribution transfomer, 11/.415kv, 100kVA after one of the phases coil that got burnt, has just been replaced

  43. how do i determine the correct rating of fuse of 11/.415kv,500KVA transformer, both primary and secondary side

  44. You mentioned Line – Line voltage at bottom of the article.. But you’ve done as 208V for 3-Ph Instead of 415V.
    for 415V;
    P= 1.732 X 415 X 139.1 = 99982 VA = 99.98 KVA
    So Please correct it.

    Best Regards.

  45. I have a transformer with 110/220 input primary,secondary has:0v 33v,0v 13v,27v 0v 27v,9v 0v 9v.The CD player consumption is 25watts.How do I get the amps from the secondary?.Do I have to divide with 25watts with all of the secondary winding?.

  46. Dear sir ,

    What you write here for calculation of power in single and three phase transformer p= √3×v×I is this is right because we know active power denotation is by s.

    So please correct the formula
    For single phase
    S= v×I
    Three phase

    Thanks & regards
    Indra naryan

  47. Apparent power denotation by s in kvA so calculation of power in kva we have to denote s =√3xvxI in three phase transformer
    Not p = √3xvxI
    So we have to correct the formula


  48. I have a 3 phase 75 KVA transformer being fed from a 80 KW standby diesel. the transformer is reverse fed…that is the diesel is putting out 208 volts delta 125 amp on the high voltage side and the low side it is 3 phase 4 wire at 600/347 volts. My question is ..what amperage could I get from the 600/347 side ?

    1. Dear Bill, Your question is rather confusing, for Better and correct technical answer please send the rating plate details of both the, transformer and the Diesel Generator. RAJ LAHARE

  49. Dear Bill,
    Your query is rather confusing. Please give the Rating plate details of both, the Transformer and the Generator, If possible the schematic diagram of the system so That better answer can be given. RAJ LAHARE

  50. i have a question regarding transformer
    transformer having primary voltage of 11000 v ,than why the fuse of 5A is used ?

  51. Dear All , If you check the second example provided that states 50KVA transformer and the voltage taken is 208 Volts is probably not correct . The reason why i am saying is there is no 50KVA transformer with a voltage of 208 Volts. 208Volts is a single phase voltage as per rules of SEC and U.S Standards. 50KVA is a three phase not a single phase .

  52. I was able to carry out short circuit Voltage Impedance test on a 7.5MVA, 33/11KVA transformer. From the test I was able to ascertain the actual impedance of the transformer using, Measured Sec. Voltage/transformer sec. voltage X Transformer Sec. Current/ Measured Secondary Current X 100%. Using the same test result, One should be able to determine the Rating(Capacity of the transformer). Can anybody help with the formula to use for this calculation?. Thank you

  53. I would like some technical assistance regarding transformer. My question is i have a transformer of 2000kva, Its voltage ratio is 6/0.415 KV now i want to use this transformer on 6.6KV instead on 6KV. What will be the impact on transformer and can we use it on continuous operation. Secondly can we use it by changing its tap on 6.6kv. Kindly assist me. Thanx.

  54. Dear Mr. Mukhtar,
    Send the complete Name Plate details of Your Transformer so That Correct technical Guidance can be given. You May mail the same directly to me . “rajlahare@gmail.com”

  55. Dear Mukhtar,
    You Know that the The Rating of Primary Voltage is of most importance for the health of transformer and the same should never be increased. with 6.0 KV rating of transformer and If you feed 6.6 KV to it “about 10 % higher” the transformer will FAIL. Further your output voltage will also be 415 + 41.5 =456.5 volts this will damage your Equipment connected on LV side of transformer. So for proper Guidance you may send Transformer”s number plate/ name plate details directly to me “rajlahare@gmail .com “

  56. are the formulas same for step up and step down transformer? in the name plate pic how come we know it is step up or step down?

    1. On the transformer name plate, they clearly mentioned the primary and secondary (HV & LV) Voltages. If Primary (HV) Are higher then the LV, It is Step down and vice versa.

  57. There are always minimum 2 windings for a transformer. 1 primery winding and 2 secondary winding. If voltage of primery winding is more than that of secondary winding then the transformer is step down type.

  58. Very fascinating facts about generating Power, My wish is also to have sustainable Power generating systems which is eco friendly and safe for Our Planet and also can be there for the future generations,generating Power is the way to go, but also be more responsible in sustaining it. Thank you for the space to share, wishing you the best of luck, CHEEEERS

  59. Hello. Im try to understand the behavior of a transformer 11kv/415v. When : 1. Its dedicated to a single consumer ( quarry and big industrial plant) 2. Its total power its smaller than the consumer needs( the transformer its 1000kva and the consumer typically needs its 1200kva) The loads are instantly dramatically change because of heavy loads machines( rocks crushers) and the plenty of smaller motors. Thanks and i hope read an article about that. P. S. I can give more data and power quality measurements about the occasions.

    1. Calculate transformer current rating capacity
      And learn how to choose Transformer for
      entire building as per our load
      KVA x 10^3
      I= __________
      V x 1.732

  60. Hello,
    I had to select two transformers(step up and step down) to power a dc load. In between the step down transformer there is a rectifier circuit which outputs 50-150V DC . the power of dc load is around 800kw. How can i calculate the ratting of transformers? Your suggestions are highly appreciated.

  61. Hi, please clarify the doubt. we have a 500 KVA step down transformer Pri-480-440V and Sec-400-380V. so can I use it as step up transformer Pri-380V and Sec-480V.

  62. Whether this is single phase or three phase ? If three phase what is the vector group ? If single phase it can be used as step up. As the transformation ratio is low I think this may be autotransformer. Please mail schematic diagram as printed on nameplate of transformer for better solution .

  63. Because of power factor.
    We take motor rating in kw because there power factor is necessary but in transformer no need of power factor thus why we take it kva

  64. My input Ac voltage is 120 volt which is coming from a 3 phase 400kva step down transformer.I want to install a 10 kva single phase step up transformer in my house to get 220 volt for my domestic appliances to operate properly.please let me know how much extra load of this10 kva step up transforner will be put on the mains 3 phase transformer.hope i will get a good response and better solution Thanks

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