Transformer Sizing Guide

How to Calculate the kVA Rating for Single and Three-Phase Transformers? | Transformer Sizing Guide – Step by Step

Because the transformer capacity is measured in KVA, we must determine the KVA of the loads in order to size the transformer.

Step 1

• Voltage and Amperes required by the load.
• Frequency in Hz (50 or 60 Hz)
• Make sure the load is designed to run on a single-phase supply.

In general, all the above information is derived from equipment nameplates or instruction manuals.

Step 2

Determine Supply Voltage

• Supply voltage (Source).
• Frequency in Hz (50 or 60 Hz).

Both the line supply and electrical load must have the same frequency. You must select a single-phase transformer that operates at this frequency and has a primary (input) voltage equal to the supply voltage and a secondary (output) voltage equal to the load voltage.

3-phase: 600v, 480v, 208v, 240v

1-phase: 480v, 240v, 120v

Step 3

Calculate KVA Rating

Table 1 or the following formula can be used to determine the proper KVA size for the required transformer.

To determine KVA when load volts and amperes are known:

Step 4

Single-Phase Transformer Sizing Example

For example, let’s use Load voltage = 240 volts and Load current = 175 Amps. Using the above formula,

We round this figure up to a standard Single-Phase size of 50 kVA (as shown in the chart below).

3-phase kVA calc: voltage X amperage X 1.732 / 1,000

1-phase kVA calc: voltage X amperage / 1,000

KVA

120 V

208 V

240 V

480 V

600 V

1.5

12.5

7.2

6.2

3.1

2.5

2

16.7

9.6

8.3

4.2

3.3

3

25

14.4

12.5

6.2

5.0

5

41.7

24

20.8

10.4

8.3

7.5

62.5

36.1

31.2

15.6

12.5

10

83.4

48

41.6

20.8

16.7

15

125

72

62.5

31.2

25

25

208

120

104

52

41.7

37.5

312

180

156

78

62.5

50

417

240

208

104

83.5

75

625

361

312

156

125

100

834

480

416

208

167

125

1042

600

520

260

208

167.5

1396

805

698

349

279

200

1666

960

833

416

333

250

2080

1200

1040

520

417

333

2776

1600

1388

694

555

500

4170

2400

2080

1040

835

1

8.3

4.8

4.2

2.1

1.7

Step 1

• Voltage and Amperes required by the load.
• Frequency in Hz (50 or 60 Hz)
• Make sure the load is designed to run on a single-phase supply.

In general, all the above information is derived from equipment nameplates or instruction manuals.

Step 2

Determine Supply Voltage

• Supply voltage (Source).
• Frequency in Hz (50 or 60 Hz).

Both the line supply and electrical load must have the same frequency. You must select a single-phase transformer that operates at this frequency and has a primary (input) voltage equal to the supply voltage and a secondary (output) voltage equal to the load voltage.

3-phase: 600v, 480v, 208v, 240v

1-phase: 480v, 240v, 120v

Step 3

Calculate KVA Rating

Table 2 or the following formula can be used to determine the proper KVA size for the required transformer.

To determine KVA when load volts and amperes are known:

Step 4

Three-Phase Transformer Sizing Example

For example, let’s use Load voltage = 208 volts and Load current = 175 Amps. Using the above formula,

We round this figure up to a standard Three-Phase size of 75 kVA (as shown in the chart below).

3-phase kVA calc: voltage X amperage X 1.732 / 1,000

1-phase kVA calc: voltage X amperage / 1,000

KVA

208 V

240 V

480 V

600 V

2400 V

4160 V

50

139

120

60

48

12

7

75

208

180

90

72

18

10.4

112.5

312

270

135

108

27

15.6

150

416

360

180

144

36

20.8

225

625

542

271

217

54

31.2

300

830

720

360

290

72

41.5

500

1390

1200

600

480

120

69.4

600

1665

1443

722

577

144

83

750

2080

1800

900

720

180

104

1000

2775

2400

1200

960

240

139

1500

4150

3600

1800

1440

360

208

2000

5550

4800

2400

1930

480

277

3

8.3

7.2

3.6

2.9

0.72

0.42

6

16.6

14.4

7.2

5.8

1.44

0.83

9

25

21.6

10.8

8.7

2.16

1.25

15

41.6

36

18

14.4

3.6

2.1

22.5

62

54

27

21.6

5.4

3.1

25

69

60

30

24

6

3.5

30

83

72

36

29

7.2

4.15

37.5

104

90

45

36

9

5.2

45

125

108

54

43

10.8

6.25

Important Note!

In most cases, the load will never be entirely three-phase equipment. There will always be lighting, receptacles, and other single-phase loads that need to be supplied in addition to the three-phase load. In all such cases where there are both three-phase loads and single-phase loads, the transformer KVA must be determined based on the largest KVA load on any one of the transformer phase windings.