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.
Transformer Sizing for Single-Phase Loads
1step
Calculate Electrical Load
- 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.
2step
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
3step
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:
4step
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
1table
Full-Load Currents for Single-Phase Transformers
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
Transformer Sizing for Three-Phase Loads
1step
Calculate Electrical Load
- 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.
2step
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
3step
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:
4step
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
2table
Full-Load Currents for Single-Phase Transformers
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.
After determining your kVA and load amps, you can confidently order your transformer with us!