**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?

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

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

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

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

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.