**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

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.

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?

Dear Mukesh Khatri@ Read This Article Why Transformer rating in kVA and not in kW?

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

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

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.

Sincerely,

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

Thanks

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

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)

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

thanks teacher

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

It is already given on every transformer's nameplate

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.

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

sir could u explain DYn-11…..

Wait for the upcoming posts… Thanks

http://en.m.wikipedia.org/wiki/Vector_group

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

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

Correct me if I'm wrong ' 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

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

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

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

dear 220kv/500kv transmission poll install in the forest.and we don't know the load and the production side.so tell the method of load and production identification?<br />

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

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

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

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. .

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

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)

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

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

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

Thanks in advance.

Sk Anwar Basha.

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 ?

Sorry, Q unit is kVAR.

yes oyu correctly htk…i agree..

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

calculation how much kvar capacitor will connected to100hp load

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

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

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

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

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

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

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.

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?

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…

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

13.8kv is primary and 220/380 is secondry

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.

Regards

Dejan

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

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 ?

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 .

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

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

Thank you to anil and vijay sir.

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.

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

My query is for determining the capacity

What do you mean by Transformer Capacity?

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

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

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.

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)

Where

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

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.

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

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

Its really satisfaction to concept thakyou sir..

Sir why KVA is divided by 1000

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

who is the owner of this website ?

whoever he is, a simply genius of electrical engineering. simply awesome.

Thanks for appreciation …

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

Thanks..@RAJ LAHARE

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

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

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

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

Hey transfermoer rating in kVA

Dear Harish ,

Your query is not clear so no comment can be given RAJ LAHARE

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

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

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

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