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# how we can calculate transformer capacity in kva?

10/04/2008 1:56 PM

pls explain how we calculate transformer capacity in practically. if manufacturer made any transformer of rating (40kva , 33/0.415kv) instead of making 50kva, 33/0.415kv?

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Guru

Join Date: Sep 2006
Location: CHINA
Posts: 2958
#1

### Re: how we can calculate transformer capacity in kva?

10/04/2008 9:08 PM

What are you want to know? its vagua.

if you hope to know how estimate transformer's capacity. a simple way is to see its volume.

40kva is a smaller than 50kva, generally spealing.

if they use a different core, its another things.

Power-User

Join Date: Dec 2007
Location: Sydney Australia
Posts: 482
#2

### Re: how we can calculate transformer capacity in kva?

10/04/2008 9:51 PM

Transformer Capacity is not a calculation. It will be on the Specification Plate of the transformer.

At a Unity PF (i.e PF=1) kVA and kW is the same 40kVA=40kW

at a lower PF (e.g. PF=0.85) 40kVA=34kW (i.e. 40kW*.85)

so the PF of your load will determine how may kW you get out of the transformer the closer to Unity the better.

This is why many Large installations use Power Factor Correction.

Regards,
Sapper

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#3

### Re: how we can calculate transformer capacity in kVA?

10/05/2008 1:33 AM

Your question is some what vague, assuming you are a consumer, not a manufacturer of transformers. Manufacturers rate transformers with their kVA and voltage, based on a number of manufacturing factors, but I will discuss three of the most relevant --- the insulation system, the conductor system, and cooling.

The insulation system, i.e., bushings, winding-to-winging, and layer-to-layer insulation, determine the maximum voltage at which the transformer can operate. In your example 33 kV on the primary side and 415 V on the secondary side.

The conductor system, i.e., bushings and wire sizes, help determine a design ampacity.

The cooling system, air-cooled or oil-cooled, is also used to determine the ampacity of the transformer. Unless indicated otherwise, transformers are rated for a load factor of 100%. The current flowing through the unit will generate heat ((I^2)R), that must be dissipated. For an oil filled transformer this design limit will often be 55 or 65 deg C., dictating an ampacity limit. (note power factor does not mater at this point, just the current level, "I").

From these limiting factors the manufacturer will design their unit, say for 7200 volts primary, 8 amps. This would result in a kVA capacity of:

7200 V * 8 amps = 57600 VA or 57.6 kVA

But, transformers are rated in prescribed sized, like 5, 10, 15, 25, 50, 75, 100 kVA, so a rating of 50 kVA would be applied to the nameplate.

From the user standpoint, the only way I know to validate the manufacturer ratings is to perform a heat-run test, where the unit is loaded to rated current for a length of time with temperature readings taken to validate the heat-rise.

Member

Join Date: Dec 2006
Posts: 6
#4

### Re: how we can calculate transformer capacity in kva?

10/05/2008 2:56 PM

Dear you have not mentioned the input phases of the transformer. Let it assume 3 phase 33kv primary and 415vac 3 phase output with neutral. As you are aware of the

Input/Output Voltage and Rating of the Transformer. You can calculate the Input/Output Current, as well as Voltage using the following formulas:

Power in 3 phase circuits:

P = E x I X 1.73

Where as:

P = Power

E= Voltage

I = Current

1.73 = Square Root of 3 (for the 3 phases)

Now in reverse You have the capacity of the transformer in KVA instead of current

and input and output voltage. so you can calculate the Input and output current of the transformer i.e. in your case:

I = VA/V/1.73

I = Current

VA = Volt Ampere (Sum of the Vols time Amperes)

V = Volts

1.73 = (Square Root of the 3)

Now you can caulate the power of any transformer Let us calculate the input/output current of your 40 KVA transformer;

INPUT I.E. PRIMARY CURRENT PER PHASE:

40000/33000/1.73 = 0.7006 Amperes per Phase

OUTPUT I.E. SECONDARY CURRENT PER PHASE:

40000/415/1.73 = 55.1742 AMPS PER PHASE

Now you can remember the Power formula i.e. P= E x I x 1.73 and recheck your reseults of input current:

INPUT AND OUTPUT POWER OF YOUR 40 KVA TRANSFORMER:

INPUT KVA:

33000 X 0.7006 X 1.73 = 39997.254 VA OR 39.997 KVA WHICH IS exteremly close to

the 40KVA.

OUTPUT KVA:

415 x 55.1742 x 1.73 = 39612 VA or 39.612 KVA which is exteremly close to 40 KVA

Hoping that this will help you out in future while you are going to deal with transformers. If you are dealing with single phase transformers the neglet 1.73.

Best Regards,

Younis Sabir

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Location: Ohio
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#6

### Re: how we can calculate transformer capacity in kva?

10/06/2008 7:52 AM

In my earlier discussion I did not get into all the multiplication by the square root of three math, as I figured the Guest is referring to a single-phase transformer. At least for those of us in areas more represented by IEEE standards (as opposed to IEC and other standards), 50 kVA would be a preferred rating for a single-phase transformer, not a three-phase transformer.

<<Balance of table truncated>>

IEEE Standard C57.12.00 - IEEE Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers, pp. 10, 2000.

Anonymous Poster
#12

### Re: how we can calculate transformer capacity in kva?

10/02/2010 12:58 PM

daku

Off Topic (Score 5)
Power-User

Join Date: Aug 2006
Posts: 183
#5

### Re: how we can calculate transformer capacity in kva?

10/06/2008 2:41 AM

Dear Guest

Your question is not clear and appears to be incomplete as well. I think, u wish to know "How to find out the exact capacity of a transformer". Thus if the nameplate is 50 kVA then "How can I find out if the transformer confirms to this capacity". Please correct me if I am wrong in my assessment about your question.

If this is the question, then proceed as follows.

(a) Check & note the secondary voltage at nominal Pri. input voltage.

(b) Load the secondary to rated secondary current, as mentioned on the nameplate, for MIN. OF 16-20 HOURS. During this time, CONTINOUSLY MONITOR Ambient temp., temp. of coil & core and Oil ( if oil cooled ). TEMP. RISE ( difference between measured temp. & ambient ) should be LESS THAN THE MAX. PERMISSIBLE TEMP. RISE FOR THE CLASS OF INSULATION MENTIONED ON NAME PLATE. If during the test, temp. rise IS MORE THAN THAT MENTIONED ON NAME PLATE, then stop the load test immediately OTHERWISE TRANSFORMER WILL BURN.

Multiply the secondary voltage with sq. root of 3 and current to get the capacity

Thanks and regards

Ashok Toshniwal, Bangalore, India

Anonymous Poster
#7

### Re: how we can calculate transformer capacity in kva?

02/18/2010 8:07 AM

Anonymous Poster
#8

### Re: how we can calculate transformer capacity in kva?

04/29/2010 12:17 PM

The question is quite in order... Especially during field inspection of transformer, we've seen that a manufacturer may produce a transformer of 260KVA but wrote 300kVA on the name plate. I understand that the correct rating of the transformer can be determined from short circuit test by measuring the short circuit current and taking into consideration of the per unit impedance of the unit. Then you can match your result to the expected values as per the name plate. Hope this opens a new vista into the discussion. Regards, Eze

Anonymous Poster
#11

### Re: how we can calculate transformer capacity in kva?

09/08/2010 6:25 AM

pls expantiate with illustration.

Anonymous Poster
#9

### Re: how we can calculate transformer capacity in kva?

08/12/2010 11:57 PM

I would like to know that in our project have many electrical equipment such: air conditioner about 1,500,000btu; game machines 450 sets (used 3 amp/set) and have the lights and any plug

Anonymous Poster
#10

### Re: how we can calculate transformer capacity in kva?

08/13/2010 7:28 AM

I have one 200 va transformer it have 1 pri.winding and 4 tapping like this 380v,400v,420v,500v. i have connected it to 420v. also have two sec.winding, first is connected to 220v and sec. one is connected to 24v. which is used for control wiring.but my problem is that it was burnt out, and i want both secondary power distribution in va.

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#13

### Re: how we can calculate transformer capacity in kva?

11/24/2012 3:20 PM

i would like to ask more about transformers because i am making my design and i wonder what will be the rating of my transformer will be installed or what will be my basis to install a rating of an transformer that fits to my design, my design is in a single phase having 4 ACU, 1 motor pump, 5 ckts of lighting outlet, 3 CO's, 2 refregeration outlets and having a total FULL LOAD CURRENT of 206.44

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#14

### Re: how we can calculate transformer capacity in kva?

07/15/2015 2:42 PM

Transformer always rated in kVA instead of kW. In your case, The rating of transformer will be considered as Transformer Capacity.

If you want to know that how to find the rating of transformer, Here is a simple calculation for both 1-Phase & 3-Phase Transformers.

you may Increase the Transformer Capacity, Improve the Power factor as

The Transformer Capacity = Transformer Rating x P.f.

Let me know if you want to know more.

Thanks

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