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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 two simple formulas to find the rating of Single phase and Three phase Transformers.
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:
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 J  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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31 comments

  1. Why is transformer rating always written in KVA? We know that the unit of Power is Watt. then why don&#39;t we write transformer rating in Watts?

  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. <br /><br /><a href="http://www.epsgroupaustralia.com.au&quot; rel="nofollow"> High Voltage Transformers </a><br />

  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.<br />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.<br />Points to be remembered:-<br />-&gt;KVA is always greater then KW<br />because KW only have the effect of resistor.<br /><br />-&gt; 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 <br />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 side.so 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, <br />I want to know about how to select step down transformer rating. 415/230V and VA rating. ..?<br />..I mean how va 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 assingment frm my teacher <br />Vo=0 to 15 volts<br /> and iL=load current is 3 amp.<br />and design the transformer

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

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