Selecting a Transformer

Selecting a Transformer

A guide to understanding how to size a single or three phase transformer.


Single Phase

A single phase transformer is designed to transform single phase or three phase input (source) voltage to the single phase output (load) voltage required by your equipment. To select the correct single phase transformer, you must first determine:

1) The equipment being installed operates on a single phase supply (see your equipment nameplate or installation manual).
2) The Primary Voltage of the transformer. This is the same as the line input (or source) voltage, typically 480 or 600 volts AC.
3) The Secondary Voltage of the transformer. The equipment being installed will have a specified supply voltage (see equipment nameplate or installation manual). The transformer you select must have a secondary voltage equal to the required supply voltage of the equipment, typically 120/240 VAC.

4) The Frequency in hertz (cycles per second) of the input (source) voltage must be the same as the operating frequency of the equipment being supplied. The transformer selected must operate at the same frequency. Typical operating frequency is 60 Hz.
5) The Total VA of the load is determined by the product of the voltage supplied across the load and the current passing through it. This is normally expressed in VA (Volt-Amperes) or kVA (kilo Volt-Amperes) on the equipment nameplate. The total load is often a combination of various loads (i.e. lights, heaters, motors). You must calculate these individual loads and add them together to obtain the total load of the transformer. The transformer you select must have a kVA rating equal to or greater than the load on the transformer.

How to use the full load chart to find kVA

A) Determine the secondary voltage of your transformer.
B) Sum up the total amperes required by the load.
C) From the full load current table below, select a transformer under the corresponding secondary voltage, with a standard kVA capacity and amperage equal to or higher than the sum required by the load.

Calculating kVA

To calculate kVA when volts and amperes are known:

Single Phase kVA Equation

Full Load Current Table (1 Phase Transformer)

kVA Current in Amperes
120V 240V 416V 480V 600V 2400V 4160V
0.25 2.08 1.04 0.6 0.52 0.41 - -
0.5 4.16 2.08 1.2 1.04 0.83 - -
0.75 6.25 3.13 1.8 1.56 1.25 - -
1 8.33 4.17 2.4 2.08 1.67 - -
1.5 12.5 6.25 3.6 3.13 2.5 - -
2 16.7 8.33 4.81 4.17 3.33 - -
3 25 12.5 7.21 6.25 5 1.25 0.72
5 41.6 20.8 12 10.4 8.33 2.08 1.2
7.5 62.5 31.2 18 15.6 12.5 3.12 1.8
10 83.3 41.6 24 20.8 16.6 4.16 2.4
15 125 62.5 36 31.2 25 6.25 3.6
25 208 104 60 52 41.6 10.4 6
37.5 312 156 90.1 78.1 62.5 15.6 9.01
50 416 208 120 104 83.3 20.8 12
75 625 312 180 156 125 31.2 18
100 833 416 240 208 166 41.6 24
150 1250 625 360 312 250 62.5 36
167 1391 695 401 347 278 69.5 40.1
250 2083 1041 600 520 416 104 60
333 2775 1387 800 693 555 138 80

Single Phase AC Motor Full Load Running Currents in Amperes and Recommended Transformer Ratings

Horsepower Full Load Current (Amps) Minimum Transformer KVA
110-120V 208V 220-240V*
0.5 9.8 5.4 4.9 1.5
0.75 13.8 7.6 6.9 2
1 16 8.8 8 3
1.5 20 11 10 3
2 24 13.2 12 5
3 34 18.7 17 5
5 56 30.8 28 7.5
7.5 80 44 40 15
10 100 55 50 15
15 135 74.8 68 25
20 - - 88 25
25 - - 110 37.5
30 - - 136 37.5
40 - - 176 50
50 - - 216 75

kVA ratings include 10% overcapacity for frequent motor starts.

*For 200 volt motors, increase 220-240V ratings by 15%.


Three Phase

A three phase transformer is designed to transform a three phase input (source) voltage to the single phase and three phase output (load) voltages required by your equipment.

In order to select the correct three phase transformer you must first determine:

1) The equipment being installed operates on a three phase supply. Note: If both single phase and three phase equipment makes up the load, the single phase and three phase equipment makes up the load, the single phase equipment is connected to only one phase of the transformer.
2) The Primary Voltage of the transformer. This is the same as the line input (or source) voltage, typically 480 or 600 volts AC.
3) The Secondary Voltage of the transformer. This is the transformer's output voltage and must bt the same as the voltage required by the equipment being installed (see equipment nameplate, typically 208Y/120 volts).
4) The Frequency in hertz (cycles per second) of the input (source) voltage must be the same as the operating frequency of the equipment being supplied. The transformer selected must operate at the same frequency. Typical operating frequency is 60 Hz.
5) The Total VA of the load is determined by the product of the voltage supplied across the load and the current passing through it. This is normally expressed in VA (Volt-Amperes) or kVA (kil Volt-Amperes) on the equipment nameplate.

The total load is often as combination of various loads (i.e. lights, heaters, motors). You must calculate these individual loads and add them together to obtain the total load of the transformer.

The transformer you select must have a kVA rating equal to or greater than the load requirement. Note: The three phase transformer must be selected so that any one phase is not overloaded. If you are connecting a single phase load to one phase of the three phase transformer, you must calculate the load as if it were loading all three phases.

Calculating kVA

To calculate kVA when volts and amperes are known:

Three phase kVA equation

Full Load Current Table - 3 Phase Transformer?

kVA Current in Amperes
208V 240V 380V 416V 480V 600V 2400V 4160V
2 5.55 4.81 3.03 2.77 2.4 1.92 0.48 0.27
3 8.32 7.21 4.55 4.16 3.6 2.88 0.72 0.41
6 16.6 14.4 9.11 8.32 7.21 5.77 1.44 0.83
9 24.9 21.6 13.6 12.4 10.8 8.66 2.16 1.24
15 41.6 36 22.7 20.8 18 14.4 3.6 2.08
30 83.2 72.1 45.5 41.6 36 28.8 7.21 4.16
45 124 108 68.3 62.4 54.1 43.3 10.8 6.24
75 208 180 113 104 90.2 72.1 18 10.4
112.5 312 270 170 156 135 108 27 15.6
150 416 360 227 208 180 144 36 20.8
225 624 541 341 312 270 216 54.1 31.2
300 832 721 455 416 360 288 72.1 41.6
450 1249 1082 683 624 541 433 108 62.4
500 1387 1202 759 693 601 481 120 69.3
600 1665 1443 911 832 721 577 144 83.2
750 2081 1804 1139 1040 902 721 180 104

Three Phase AC Motor Full Load Running Currents in Amperes and Recommended Transformer Ratings

Horsepower Full Load Current (Amps) Minimum Transformer kVA
110-120V 208V 220-240V* 440-480V 550-600V
0.5 4 2.2 2 1 0.8 3
0.75 5.6 3.1 2.8 1.4 1.1 3
1 7.2 4 3.6 1.8 1.4 3
1.5 10.4 5.7 5.2 2.6 2.1 3
2 13.6 7.5 6.8 3.4 2.7 6
3 19.2 10.7 9.6 4.8 3.9 6
5 30.4 16.7 15.2 7.6 6.1 9
7.5 44 24 22 11 9 15
10 56 31 28 14 11 15
15 84 46 42 21 17 30
20 108 59 54 27 22 30
25 136 75 68 34 27 45
30 160 88 80 40 32 45
40 208 114 104 52 41 75
50 260 143 130 65 52 75
60 - 170 154 77 62 75
75 - 211 192 96 77 112.5
100 - 273 248 124 99 150

kVA ratings include 10% overcapacity for frequent motor starts.

* For 200 volt motors, increase 220-240V ratings by 15%.