How to Select a Transformer

How to Select a Transformer
■ Transformer size is determined by the KVA of the load.
■ Load voltage, or secondary voltage, is the voltage needed to operate the load.
■ Line voltage, or primary voltage, is the voltage from the source.
■ Single-Phase has two lines of AC power.
■ Three-Phase has three lines of AC power, with each line 120 degrees out of phase with the
other two.
■ KVA is kilovolt ampere or thousand volt ampere. This is how transformers are rated.
NOTE: If motors are started more than once per hour, increase minimum transformer KVA by 20%.
To determine the size of the transformer you
need, use this handy formula, or refer to the
Technical Reference Section beginning on page
93 for more information.
Determine the Load Voltage
Load Voltage = __________________________
Determine the Load Current (Amps)
Load Current/Amps = ____________________
Determine the Line Voltage
Line Voltage = __________________________
Determine if your application is single-phase or
three-phase, and use the corresponding formula
in the column at right.
The KVA of the transformer should be equal to or
greater than the KVA of the load to handle present requirements and to account for future
expansion.
Or, use our Product Specifier at
www.JeffersonElectric.com for quick and easy
transformer selection.
Transformer Selection Formulas
Single-Phase Transformers
Volts x Amps
= ____ KVA
1000
Plug your numbers into the formula:
Volts
x Amps
= ____ KVA
1000
For single-phase transformers, see the following pages:
Encapsulated ...................... 9-16
Ventilated .......................... 17-20
Buck Boost ........................ 59-68
Three-Phase Transformers
Volts x Amps x 1.732 = ____ KVA
1000
Plug your numbers into the formula:
Volts
x Amps
x 1.732 = ____ KVA
1000
For three-phase transformers, see the following pages:
Encapsulated .................... 21-28
Ventilated .......................... 29-42
Buck Boost ........................ 59-68
4
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Size and Select a Transformer
Full Load Currents/Single-Phase Transformers
Voltage (Line-to-Line)
KVA
Rating
120V
208V
240V
277V
480V
600V
.050
.075
.100
.150
.250
.500
.750
1
1.5
2
3
5
7.5
10
15
25
37.5
50
75
100
167
.42
.63
.83
1.25
2.08
4.16
6.25
8.3
12.5
16.7
25
41.7
62.5
83.4
125
208
312
417
625
834
1396
.24
.36
.48
.72
1.20
2.40
3.60
4.8
7.2
9.6
14.4
24
36.1
48
72
120
180
240
361
480
805
.21
.31
.42
.63
1.04
2.08
3.13
4.2
6.2
8.3
12.5
20.8
31.2
41.6
62.5
104
156
208
312
416
698
.18
.27
.36
.54
.90
1.8
2.7
3.6
5.4
7.2
10.8
18.0
27
36
54
90
135
180
270
361
602
.10
.16
.21
.31
.52
1.04
1.56
2.1
3.1
4.2
6.2
10.4
15.6
20.8
31.2
52
78
104
156
208
349
.08
.13
.17
.25
.42
.83
1.25
1.7
2.5
3.3
5.0
8.3
12.5
16.7
25
41.7
62.5
83.5
125
167
279
Full Load Currents/Three-Phase Transformers
Voltage (Line-to-Line)
KVA
Rating
208V
240V
480V
600V
3
6
9
15
30
45
75
112.5
150
225
300
500
750
8.3
16.6
25
41.6
83
125
208
312
416
625
830
1390
2080
7.2
14.4
21.6
36.0
72
108
180
270
360
542
720
1200
1800
3.6
7.2
10.8
18.0
36
54
90
135
180
271
360
600
900
2.9
5.8
8.7
14.4
29
43
72
108
144
217
290
480
720
For Other Single-Phase KVA Ratings or Voltages
Amperes
=
KVA x 1000
Volts
For Other Three-Phase KVA Ratings or Voltages
Amperes
=
KVA x 1000
Volts x 1.732
Source: EASA Handbook
www.Jeffer sonElectric.com • 1-800-892-3755
5
Size and Select a Transformer
Single-Phase AC Motors
Full Load Currents (Amperes)
HP
115V
230V
1/6
1/4
1/3
1/2
3/4
1
11/2
2
3
5
71/2
10
4.4
5.8
7.2
9.8
13.8
16
20
24
34
56
80
100
2.2
2.9
3.6
4.9
6.9
8
10
12
17
29
40
50
Electrical Relationships
Three-Phase Transformers AC Motors
Full Load Currents (Amperes)
Rating
115V
230V
460V
575V
1/2
3/4
1
11/2
2
3
5
71/2
10
15
20
25
30
40
50
60
75
100
125
150
200
4
5.6
7.2
10.4
13.6
2
2.8
3.6
5.2
6.8
9.6
15.2
22
28
42
54
68
80
104
130
154
192
248
312
360
480
1
1.4
1.8
2.6
3.4
4.8
7.6
11
14
21
27
34
40
52
65
77
96
124
156
180
240
0.8
1.1
1.4
2.1
2.7
3.9
6.1
9
11
17
22
27
32
41
52
62
77
99
125
144
192
NOTE: If motors are started more than once per hour, increase minimum transformer KVA by 20%.
6
1-800-892-3755 • www.Jeffer sonElectric.com