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Home » AC Fundamentals » Why Transformer Rating In kVA, Not in KW?

Why Transformer Rating In kVA, Not in KW?

 In Simple words, 
There are two type of losses in a transformer;
1. Copper Losses
2. Iron Losses or Core Losses or  Insulation Losses
Copper losses ( I²R)depends on Current which passing through transformer winding while Iron Losses or Core Losses or  Insulation Losses depends on Voltage.
That’s why the Transformer Rating may be expressed in kVA,Not in kW.Why Transformer Rating In kVA, Not in KW?
Also read here about : 
How to Calculate/Find the Rating of Transformer (Single Phase and Three Phase)? 

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  1. electrical knowledge

    Great post, now im knowing why tranformer rating in KVA..

  2. Again I am little bit confuse about the unit of Transformer.<br /><br />We write, P=VxI and whatever product of VxI comes out to be, we just denote it with unit &quot;WATT&quot;. Why not &quot;VA&quot; as in Tranformers? <br /><br />then in case of Transformer due to CU loss (I) and Iron loss (V) we write its unit &quot;VA&quot;. Why not &quot;WATT&quot; as in case of P=VI?

  3. this is the question I am asking that why the unit of Apparent power (S)=VI is VA? Why not to write Watts? <br /><br />So what if it is apparent power and we are writing it as VA and in case of active power we write Watts. Why? Couldn&#39;t get the logic behind.<br /><br />I read your post it is really good but unit is still not clear to me logically.

    • Dear Mukesh Khatri@<br />See…. Watt is a unit of real power or actual power…i.e. the power which we utilize in our home [ Electrical Appliances operate on Real Power] <br />in other hand… Apparent Power [ S= VxI = VA and its formula <br />Apparent Power = √ (True power2 + Reactive Power2)] is not real power..its complex power [ … I.e we can&#39;t use Reactive power but only real power in

    • Muhammed Ibrahim

      hi mukesh,i see you are confused about kva and kw.just keep in mind that where ever there is a generation point of electricity its not loaded with anything so its calculated in kva,but when we put on the load on to it then we calculate it on kw becuase of the power at the consuming end,if you are not consuming the electricity then there is no kw.<br />

    • Real Power=Voltage*Current*Cos(Pi).Watts<br />Reactive Power=Voltage*Current*Sin(Pi)<br />Apparent Power=Resultant of Real and reactive power so cos(pi) and Sin(pi) will become one.hence we will get apparent power Volt-Amps only

    • it is the totoal power for real and reactive power so it is to the v-i

  4. my question in simple words is how you come to know that P=VxI= VA?<br /><br />If you have the proof of denoting VxI with unit VA then what was the necessity of using unit the VA inspite of Watts as a unit for apparent power? <br /><br />What is the problem with the unit WATT that we don&#39;t mention it as a unit of apparent power when we clearly see it is product of VxI ..

    • Dear Mukesh Khatri@<br />See…. Watt is a unit of real power or actual power…i.e. the power which we utilize in our home [ Electrical Appliances operate on Real Power] <br />in other hand… Apparent Power [ S= VxI = VA and its formula <br />Apparent Power = √ (True power2 + Reactive Power2)] is not real power..its complex power [ … I.e we can&#39;t use Reactive power but only real power in

    • I know, Apparent power(s) is sum of route of Active Power(wattful) and Reactive Power(wattless).<br /><br />I am afraid how does this justify that the unit of Apparent Power will be &quot;VA&quot;? <br /><br />You mean to say it is complex power and we utilize real power not reactive power therefore Apparent power&#39;s unit should be &quot;VA&quot;.<br /><br />Does common sense accept this

    • Look…There are three kinds of Power. Active Power, Re-active Power and Apparent power. now use common sense,,,is it right to just use the same unit for these three different quantities. The same question rises here that we also know that Re-active power = P = VxI…But the unit is in VARS….Why??? Should was not it in Watts. Also this is not a technical answer [ as we have already given

    • See @Mukesh…Apparent power is the product of voltage(V) &amp; current(I). Now, the unit of voltage is volt &amp; unit of current is ampere. So the unit of apparent power is volt-ampere.<br /><br />Now to get back to watts…when the apparent power is further multiplied with power factor(cosine of the angle between voltage &amp; current in an AC circuit) the result is active power which is

    • great sir tq now 1ly iam clear about this tx a lot <br />

  5. great work by your team

  6. great explanation….hats of to u…….<br />

  7. dear sir;<br /> I need a clarification regarding..<br /> 1.why we should not connect earth to the ground?<br /> to calculate the bus bar size?

  8. Fist the understading P=VA=WATT is wrong,<br />P=VA Cos(phi) = watt for AC power<br />S=VA = Apparent Power <br />One can write P=VA only withe explanation that power factor Cos (phi) is unity.<br /><br />For DC curcuit P=VA=Watt since there is no power factor as well as no apparenat power.<br />

  9. W and VA are the same, they are both power. VA unit is just used for apparent power to discriminate from the real power unit (W), if we use the unit watt(W, for both apparent and real power, how would somebody what power you are referring to? this is to avoid confusion. The same logic on big engines, they have this ratings: BHP, KW

  10. Why motor rating is in KW? not in VA..

    • simply put: its bcoz of power factor, the mptor is at the load side hence PF is taken into consideration, when u check on the name plate you will find the following rated parameters:<br />Voltage<br />Amperage<br />Power Factor….thus<br />Voltage*Amperage*PF =Kw

  11. Why motor rating in kw nt in kva,there are also cu and iron loss

  12. please help to solve this problem:<br /><br />A 440V, 60hz, six-pole, 3-phase induction motor is taking 50kVA at 0.8 power factor and is running at a slip of 2.5 %. The stator copper losses are 0.5kW and rotational losses are 2.5kW. Calculate: <br />a) the rotor copper losses<br />b) the shaft power<br />c) the efficiency<br />d) the shaft torque

  13. whats a explanation by faruq sir…you clear the concept of watt and volt-amp

  14. If i want to install a 250 kva generator set, how can i choose the rating of the transformer t be installed? Can anybody help me please!

  15. Pls Answer this Q…. What is the current of an 237V, 328.4kW three-phase load with cosFI=0.99 ?

  16. P=VI<br />Power=Voltage*Current

  17. Quote &quot;we can&#39;t use Reactive power but only real power in watts but we pay for both real and reactive power&quot;<br />Electrical Technology kindly clarify, i read somewhere that our energy meters only charge us based on the Real power(Kw) consumed bt here you&#39;ve stated that even the reactive power is paid for….

  18. Can you please explain the effect of low/high impedance of transformer?

  19. why we need to connect neutral to earth in a Transformer??

  20. why in theTransformer, Neutral is Connected to Earth??

    • One thing, since the earth is a good conductor, it can be used as a common line in distribution system. So the metal usage for the neutral wire can be minimized by earthing the neutral wire in many places of a distribution network.<br />The other thing is safety related. The leakage current of consumer equipments and machinery can be bypassed to earth only if the neutral point of the transformer

  21. i like to know about multiplication factor

  22. The answer to the common man:<br />The power indicated in Watts (W) is the power consumed by an equipment.<br />But the power indicated in Volt-Ampere (VA) is the capacity of an equipment to deliver the power.<br />You can connect a 1000W heater to a 1KVA transformer. Here the transformer is just transferring 1000W power to heater.

  23. very nice explenation

  24. why the rating of generator and transformer are same plz explain the real logic behind it..

  25. Transformer rating Depends upon the power factor of the load but this is not specified at the time of manufacturing so rating of transformer and generator in KVA but in motor nameplate manufacturer always mentioned the pf thats why its rating is in kw.

  26. Tranformer rating depand on the cu-losses and core(iron) losses.So tranformer rating has been kVA.

  27. Great post thank u

  28. why are transformer in KVA? need a simple and easy to understanding answer, thanks

  29. Rating depends on maximum current flowing through conductor without overheating and damage of conductor wire. If heavy load connected to a thin wire, when we switch on, wire get melt due to high current required for heavy load flowing through thin conductor generate heat and wire get melts.
    Same thing happens in transformer since transformer connected to load (grid) having resistance capacitance and inductance three basic loads and all are parallel in power distribution system. All three basic circuit element taking current from transformer winding but resistance only dissipate power, reactive component only store energy in form of charge storage in capacitor and magnetic flux in case of inductor so it is called reactive power. Total current drown from transformer coil is vector sum of active plus reactive current which is greater then current that is dissipated in pure resistor and rating of any conductor (transformer coil) depends on current flowing through it,since voltage of line is fixed(220,440 etc)and In KVA, V is fixed(line voltage or generating voltage) A denote the maximum current that can withdrawn by load without damage of transformer conductor.we can not say rating as KW because it only denote the product of voltage V and current that flowing through resistor only which is always less then A. But A is the actual current(reactor+resister) flowing through transformer coil, and this A define rating of transformer conductor and hence transformer. That,s why its rating is KVA.

  30. Iron losses and copper losses are common for all electrical machines (be it motor/ alternator or transformer). That is not the reason for transformer to be rated in VA. The real reason is that all AC sources (transformer or alternators) are rated in VA (or KVA or MVA) is because the power factor of current is decided by the load connected. And since the load power factor is not known (while the transformer is being manufactured), they are rated considering resistive load (PF = 1). If the load is inductive + resistive (for example, induction motor), some part of power from transformer will go into supplying the reactive power (VAR) and remaining for supplying the active power (W). But the power supplied from transformer (apparant power, VA) will still be vector sum of VAR and W.

  31. Very useful things and comprehensive study material. I like it so much, my fav among all the educational sites.



    very nice in explanation
    thank you ……………………….

  34. Guyz, even motor and generator has both iron loss and copper loss so we should also rate them in KVA right??

  35. Thanks for your responsible

  36. Qasem Almansuri

    Very nice post

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