How to Size a Transformer? Calculate the Rating of Transformer

How to Size a Single Phase and Three Phase Transformer in kVA? Calculator

How to Calculate the Rating of Single Phase & Three Phase Transformers in kVA

We know that a transformer is always rated in kVA. Below are the two simple formulas which can be used to find and calculate the rating of Single Phase and Three Phase Transformers.

In any case, both the voltage and currents should be form an individual side either primary or secondary respectively ( e.g. Primary voltage x primary current or secondary voltage x secondary current). To property size a transformer for a required home appliance, you should select the supply voltage and secondary load current in amperes.

Sizing a Single Phase Transformer

Rating of Single Phase Transformer:

P = V x I

Where:

• V = Primary or Secondary Voltage

• I = Primary or Secondary Current

• P = Power of transformer in VA (Volt-Amps)

Rating of a single phase transformer in kVA

kVA= (V x I) / 1000

Example:

Suppose a 1-phase transformer having the secondary voltage and current of 240V and 62.5A respectively. Calculate the size of the  single-phase transformer.

Solution:

• Secondary Voltage: 240V

• Load Current: 62.5A

Transformer Rating = P = V x I

Putting the values:

P = 240V x 62.5A

P = 15000 VA = 15 kVA

Sizing a Three Phase Transformer

Rating of a Three Phase Transformer:

P = √3 x V x I

Where:

• V = Primary or Secondary Voltage

• I = Primary or Secondary Current

• √3 = 1.732

• P = Power of transformer in VA (Volt-Amps)

Rating of a Three phase transformer in kVA

kVA = (√3 x V x I) / 1000

Example:

A 3-phase transformer having the primary voltage and current of 7200V and 4A respectively. Calculate the size of the three phase transformer.

Solution:

• Primary Voltage: 7200V

• Primary Current: 4A

Transformer Rating = P = √3 x V x I

Putting the values:

P = 1.732 x 7200V x 4A

P = 49,881 VA ≈ 50 kVA

Related Posts:

Transformer Rating in kVA Calculator

The following calculator will calculate the rating of the transformer in kVA, Primary or secondary voltage and current respectively. Just enter any two values and click on calculate to find the desired value.

Enter Any Two Values:

Transformer Phases

No of Phases

Voltage

Volts

Current

Amperes

kVA

kilovolt-Amperes

      

Now, look at the General nameplate rating of a 100kVA transformer.

The nameplate clearly shows the rating of transformer is 100 kVA.

The primary voltages or high voltages (HV) is 11000 V = 11kV.

And the primary current on the high voltage (HV) side is 5.25 Amperes.

Also secondary voltage or low voltages (LV) is 415 Volts

And secondary current (load current on low voltages (LV) 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

As you noticed, the rating of the transformer (on the nameplate) is 100 kVA but according to the calculation, the calculated value is  57kVA.

Well, The difference comes due to ignorance of the fact that we used a single phase formula instead of three phase formula. It means it is a three phase transformer and we will use the associated formula for it. 

Let’s clear it using the three phase transformer rating formula.

P = √3 x V x I

P = √3 V x 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 ≈ 100kVA

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

Related Posts:

That’s it. Now you know how to properly size a three phase and single phase transformer with suitable rating in VA or kVA for home appliances or any other load applications.

The following table shows the dry transformer data of standard kVA rating, Voltage and Current rating for both single phase and three phase transformers.

Single Phase Transformer

Three Phase Transformer

KVA Rating

Amperes

KVA Rating

Amperes

120V

240V

600V

120V

240V

480V

600V

0.75

6.25

3.13

1.25

3

8.33

7.22

3.61

2.89

1

8.33

4.17

1.67

9

25

21.7

10.8

8.66

1.5

12.5

6.25

2.5

15

41.6

36.1

18

14.4

2

16.7

8.33

3.33

20

55.5

48.1

24.1

19.2

3

25

12.5

5

25

69.4

60.1

30.1

24.1

5

41.6

20.8

8.33

30

83.3

72.2

36.1

28.9

7.5

62.5

31.3

12.5

37.5

104

90.2

45.1

36.1

10

83.3

41.7

16.7

45

125

108

54.1

43.3

15

125

62.5

25

75

208

180

90.2

72.2

25

208

104

41.7

100

278

241

120

96.2

37.5

313

156

62.5

112.5

312

271

135

108

50

417

208

83.3

150

416

361

180

144

75

625

313

125

225

625

541

271

217

100

833

417

167

300

833

722

361

289

167

1392

696

278

500

1388

1203

601

481

250

2083

1042

417

750

2082

1804

902

722

Here is the table in the image format if you need to download as a reference.

Related Posts:

URL Copied