Buck Boost Transformers

Buck Boost Transformer Selection Guide

You should have the following information before selecting a buck boost transformer.

Line Voltage - The voltage that you want to buck (decrease) or boost (increase). This can be found by measuring the supply line voltage with a voltmeter.

Load Voltage - The voltage at which your equipment is designed to operate. This is listed on the nameplate of the load equipment.

Load KVA or Load Amps - You do not need to know both - one or the other is sufficient for selection purposes. This information usually can be found on the nameplate of the equipment that you want to operate.

Frequency - The supply line frequency must be the same as the frequency of the equipment to be operated - either 50 or 60 cycles.

Phase - The supply line should be the same as the equipment to be operated - either single or three phase.

4 Step Selection

1) From the top row of the chart, find the high and low voltage combination that is closest to the one you require.

2) Move down that column to the kVA or Ampere rating equal to or greater than the rating required by the load.

3) From the far left column, obtain the transformer catalog model.

4) The corresponding connection diagram is indicated at the top of each Voltage/kVA column. Click on the link view the diagram.


Product Selection - 1 Phase Buck Boosts

Group 1 - 120x240 Input, 12/24 Output

Models by Brand Line Voltage 96 100 100 105 110 110 115 115 120 120 200 208 220 220 240 240
Load Voltage 115 110 120 115 121 132 126 138 132 144 220 229 231 242 252 264
Connection Diagram 2 1 2 1 1 2 1 2 1 2 4 4 3 4 3 4
1 Acme
Federal
GE
HV Amps 2.08 4.17 2.08 4.17 4.17 2.08 4.17 2.08 4.17 2.08 2.08 2.08 4.17 2.08 4.17 2.08
kVA 0.24 0.46 0.25 0.48 0.504 0.28 0.53 0.29 0.55 0.3 0.49 0.48 0.96 0.504 1.05 0.55
LV Amps 2.5 4.58 2.5 4.58 4.58 2.5 4.58 2.5 4.58 2.5 2.29 2.29 4.38 2.29 4.38 2.29
2 GE HV Amps 3.13 6.25 3.13 6.27 6.26 3.13 6.25 3.13 6.25 3.13 3.13 3.12 6.23 3.13 6.25 3.13
kVA 0.36 0.69 0.38 0.72 0.76 0.41 0.79 0.43 0.83 0.45 0.69 0.72 1.44 0.76 1.58 0.83
LV Amps 3.75 6.88 3.75 6.86 6.89 3.75 6.85 3.75 6.88 3.75 3.44 3.44 6.55 3.44 6.56 3.44
3 Acme
Federal
GE
HV Amps 4.17 8.33 4.17 8.33 8.33 4.17 8.33 4.17 8.33 4.17 4.17 4.17 8.33 4.17 8.33 4.17
kVA 0.48 0.91 0.5 0.961 1.01 0.55 1.05 0.57 1.1 0.6 0.917 0.953 1.92 1.01 2.1 1.1
LV Amps 5 9.17 5 9.17 9.17 5 9.17 5 9.17 5 4.58 4.58 8.75 4.58 8.75 4.58
4 Acme
Federal
GE
HV Amps 6.25 12.5 6.25 12.5 12.5 6.25 12.5 6.25 12.5 6.25 6.25 6.25 12.5 6.25 12.5 6.25
kVA 0.72 1.38 0.75 1.44 1.51 0.825 1.58 0.83 1.65 0.9 1.38 1.43 2.88 1.51 3.15 1.65
LV Amps 7.5 13.8 7.5 13.8 13.8 7.5 13.8 7.5 13.8 7.5 6.88 6.88 13.1 6.88 13.1 6.88
5 Acme
Federal
GE
HV Amps 10.4 20.8 10.4 20.8 20.8 10.4 20.8 10.4 20.8 10.4 10.4 10.4 20.8 10.4 20.8 10.4
kVA 1.2 2.29 1.25 2.4 2.52 1.37 2.63 1.44 2.75 1.5 2.29 2.38 4.81 2.52 5.25 2.75
LV Amps 12.5 22.9 12.5 22.9 22.9 12.5 22.9 12.5 22.9 12.5 11.4 11.4 21.8 11.4 21.8 11.4
6 Acme
Federal
GE
HV Amps 20.8 41.6 20.8 41.7 41.7 20.8 41.7 20.8 41.7 20.8 20.8 20.8 41.7 20.8 41.7 20.8
kVA 2.39 4.58 2.5 4.8 5.04 2.75 5.26 2.87 5.5 3 4.58 4.77 9.62 5.04 10.5 5.5
LV Amps 24.9 45.8 25 45.8 45.8 25 45.8 25 45.8 25 22.9 22.9 43.7 22.9 43.7 22.9
7 Acme
Federal
GE
HV Amps 31.2 62.5 31.2 62.5 62.5 31.2 62.5 31.2 62.5 31.2 31.2 31.2 62.5 31.2 62.5 31.2
kVA 3.6 6.87 3.75 7.2 7.56 4.12 7.89 4.31 8.25 4.5 6.87 7.15 14.4 7.56 15.7 8.25
LV Amps 37.5 68.7 37.5 68.7 68.7 37.5 68.7 37.5 68.7 37.5 34.4 34.4 65.6 34.4 65.6 34.4
8 Acme
Federal
GE
HV Amps 41.7 83.3 41.7 83.3 83.3 41.7 83.3 41.7 83.3 41.7 41.7 41.7 83.3 41.7 83.3 41.7
kVA 4.8 9.17 5 9.6 10.1 5.5 10.5 5.75 11 6 9.17 9.53 19.2 10.1 21 11
LV Amps 50 91.7 50 91.7 91.7 50 91.7 50 91.7 50 45.8 45.8 87.5 45.8 87.5 45.8
9 Acme
Federal
GE
HV Amps 62.5 125 62.5 125 125 62.5 125 62.5 125 62.5 62.5 62.5 125 62.5 125 62.5
kVA 7.2 13.7 7.5 14.4 15.1 8.25 15.8 8.62 16.5 9 13.7 14.3 28.9 15.1 31.5 16.5
LV Amps 75 137 75 137 137 75 137 75 137 75 68.7 68.8 131 68.7 131 68.7
10 Acme
Federal
GE
HV Amps 83.3 166 83.3 166 166 83.3 166 83.3 166 83.3 83.3 83.3 166 83.3 166 83.3
kVA 9.58 18.3 10 19.2 20.1 11 21 11.5 22 12 18.3 19 38.5 20.1 42 22
LV Amps 99.8 183 100 183 183 100 183 100 183 100 91.7 91.7 175 91.6 175 91.6
11 Acme
Federal
GE
HV Amps 125 250 125 250 250 125 250 125 250 125 125 125 250 125 250 125
kVA 14.4 27.5 15 28.8 30.2 16.5 31.5 17.2 33 18 27.5 28.6 57.7 30.2 63 33
LV Amps 150 275 150 275 275 150 275 150 275 150 137 137 262 137 262 137
12 Acme
Federal
GE (60 Hz)
GE (50 Hz)
HV Amps 208 417 208 417 417 208 417 208 417 208 208 208 417 208 417 208
kVA 24 45.8 25 48 50.4 27.5 52.7 28.7 55 30 45.8 47.7 96.3 50.4 105 55
LV Amps 250 458 250 458 458 250 458 250 458 250 229 229 438 229 438 229
13 Acme HV Amps 313 625 313 627 626 313 625 313 625 313 313 312 623 313 625 313
kVA 36 69 38 72 76 41 79 43 83 45 69 71 144 76 158 83
LV Amps 375 688 375 686 689 375 685 375 688 375 344 344 655 344 656 344
14 Acme HV Amps 417 834 417 836 835 417 833 417 833 417 417 416 831 417 833 417
kVA 48 91.7 50 96.1 101 55 105 57.5 110 60 91.7 95.3 192 101 210 110
LV Amps 500 917 500 915 918 500 913 500 917 500 459 458 873 459 875 458

Group 2 - 120x240 Input, 16/32 Output

Models by Brand Line Voltage 95 101 106 110 110 115 115 120 120 203 208 220 220 230 240 240
Load Voltage 120 115 120 125 139 130 146 136 152 230 236 235 249 261 256 272
Connection Diagram 2 1 1 1 2 1 2 1 2 4 4 3 4 4 3 4
1 Acme
Federal
GE
HV Amps 1.56 3.13 3.13 3.13 1.56 3.13 1.56 3.13 1.56 1.56 1.56 3.13 1.56 1.56 3.13 1.56
kVA 0.188 0.36 0.38 0.39 0.22 0.41 0.23 0.44 0.24 0.36 0.37 0.73 0.39 0.41 0.8 0.43
LV Amps 1.98 3.54 3.54 3.54 1.98 3.54 1.98 3.54 1.98 1.77 1.77 3.33 1.77 1.77 3.33 1.77
2 GE HV Amps 2.35 4.68 4.69 4.67 2.35 4.7 2.34 2.69 2.34 2.34 2.34 0.47 2.35 2.34 0.47 2.34
kVA 0.282 0.54 0.56 0.58 0.33 0.61 0.34 0.64 0.36 0.54 0.55 0.11 0.58 0.61 0.12 0.64
LV Amps 2.97 5.33 5.31 5.31 2.97 5.32 2.97 5.31 2.97 2.66 2.66 0.5 2.66 2.66 0.5 2.66
3 Acme
Federal
GE
HV Amps 3.12 6.25 6.25 6.25 3.12 6.25 3.12 6.25 3.12 3.12 3.12 6.25 3.12 3.12 6.25 3.12
kVA 0.376 0.72 0.75 0.78 0.44 0.82 0.46 0.85 0.48 0.72 0.74 1.47 0.78 0.82 1.6 0.85
LV Amps 3.96 7.08 7.08 7.08 3.96 7.08 3.96 7.08 3.96 3.54 3.54 6.67 3.54 3.54 6.67 3.54
4 Acme
Federal
GE
HV Amps 4.69 9.38 9.38 9.38 4.69 9.38 4.69 9.38 4.69 4.69 4.69 9.38 4.69 4.69 9.38 4.69
kVA 0.564 1.07 1.13 1.17 0.653 1.22 0.683 1.28 0.713 1.08 1.1 2.2 1.17 1.22 2.4 1.28
LV Amps 5.94 10.6 10.6 10.6 5.94 10.6 5.94 10.6 5.94 5.31 5.31 10 5.31 5.31 10 5.31
5 Acme
Federal
GE
HV Amps 7.81 15.6 15.6 15.6 7.81 15.6 7.81 15.6 7.81 7.81 7.81 15.6 7.81 7.81 15.6 7.81
kVA 0.94 1.8 1.87 1.95 1.09 2.04 1.14 2.12 1.19 1.8 1.84 3.67 1.95 2.04 4 2.12
LV Amps 9.9 17.7 17.7 17.7 9.87 17.7 9.9 17.7 9.9 8.85 8.85 16.7 8.85 8.85 16.7 8.85
6 Acme
Federal
GE
HV Amps 15.6 31.2 31.2 31.2 15.6 31.2 15.6 31.2 15.6 15.6 15.6 31.2 15.6 15.6 31.2 15.6
kVA 1.88 3.59 3.75 3.9 2.18 4.07 2.28 4.25 2.37 3.59 3.68 7.33 3.9 4.07 8 4.25
LV Amps 19.8 35.4 35.4 35.4 19.8 35.4 19.8 35.4 19.8 17.7 17.7 33.3 17.7 17.7 33.3 17.7
7 Acme
Federal
GE
HV Amps 23.4 46.8 46.8 46.9 23.4 46.9 23.4 46.9 23.4 23.4 23.4 46.9 23.4 23.4 46.9 23.4
kVA 2.82 5.39 5.63 5.84 3.27 6.1 3.41 6.37 3.56 5.39 5.53 11 5.84 6.11 12 6.37
LV Amps 29.7 53.1 53.1 53.1 29.7 53.1 29.7 53.1 29.7 26.6 26.6 50 26.6 26.6 50 26.6
8 Acme
Federal
GE
HV Amps 31.2 62.5 62.5 62.5 31.2 62.5 31.2 62.5 31.2 31.2 31.2 62.5 31.2 31.2 62.5 31.2
kVA 3.76 7.18 7.5 7.79 4.35 8.15 4.55 8.5 4.75 7.19 7.37 14.7 7.79 8.15 16 8.5
LV Amps 39.6 70.8 70.8 70.8 39.6 70.8 39.6 70.8 39.6 35.4 35.4 66.7 35.4 35.4 66.7 35.4
9 Acme
Federal
GE
HV Amps 46.9 93.7 93.7 93.7 46.9 93.7 46.9 93.7 46.9 46.9 46.8 93.7 46.9 46.9 93.7 46.9
kVA 5.64 10.8 11.2 11.7 6.53 12.2 6.83 12.7 7.12 10.8 11 22 11.7 12.2 24 12.7
LV Amps 59.4 106 106 106 59.4 106 59.4 106 59.4 53.1 53.1 100 53.1 53.1 100 53.1
10 Acme
Federal
GE
HV Amps 62.5 125 125 125 62.5 125 62.5 125 62.5 62.5 62.5 125 62.5 62.5 125 62.5
kVA 7.5 14.4 15 15.6 8.71 16.3 9.1 17 9.5 14.4 14.7 29.3 15.6 16.3 32 17
LV Amps 79.1 142 142 142 79.2 142 79.2 142 79.2 70.8 70.8 133 70.8 70.8 133 70.8
11 Acme
Federal
GE
HV Amps

93.7

187 187 187 93.7 187 93.7 187 93.7 93.7 93.7 187 93.7 93.7 187 93.7
kVA 11.3 21.5 22.5 23.4 13 24.4 13.6 25.5 14.2 21.6 22.1 44 23.4 24.4 48 25.5
LV Amps 119 212 212 212 119 212 119 212 119 106 106 200 106 106 200 106
12 Acme
Federal
GE (60 Hz)
GE (50 Hz)
HV Amps 156 312 312 312 156 312 156 312 156 156 156 312 156 156 313 156
kVA 18.8 35.9 37.5 39 21.8 40.7 22.8 42.5 23.8 35.9 36.8 73.3 39 40.7 80 42.5
LV Amps 198 354 354 354 198 354 198 354 198 177 177 333 177 177 333 177
13 Acme HV Amps 235 468 469 467 235 470 234 469 234 234 234 47 235 234 47 234
kVA 28 54 56 58 33 61 34 64 36 54 55 11 58 61 12 64
LV Amps 297 533 531 531 297 532 297 531 297 266 266 50 266 266 50 266
14 Acme HV Amps 313 624 626 623 313 627 312 625 313 313 312 63 313 312 63 313
kVA 37.6 71.8 75.1 77.9 43.5 81.5 45.5 85 47.5 71.9 73.7 14.7 77.9 81.5 16 85
LV Amps 396 711 708 708 395 709 396 708 396 354 354 67 354 354 67 354

Group 3 - 240x480 Input, 24/48 Output

Models by Brand Line Voltage 200 208 220 230 240 380 416 430 435 436 440 440 450 456 460 480
Load Voltage 220 229 242 276 264 418 458 473 457 480 462 484 473 479 483 504
Connection Diagram 1 1 1 2 1 4 4 4 3 4 3 4 3 3 3 3
1 Acme
Federal
GE
HV Amps 2.08 2.08 2.08 1.04 2.08 1.04 1.04 1.04 2.08 1.04 2.08 1.04 2.08 2.08 2.08 2.08
kVA 0.46 0.48 0.5 0.29 0.55 0.44 0.48 0.49 0.95 0.5 0.96 0.5 0.98 1 1.01 1.05
LV Amps 2.29 2.29 2.29 1.25 2.29 1.15 1.15 1.15 2.19 1.15 2.19 1.15 2.19 2.19 2.19 2.19
2 GE HV Amps 3.14 3.11 3.13 1.58 0.31 1.56 1.56 1.57 3.12 1.56 3.13 1.57 3.12 3.13 3.12 3.13
kVA 0.69 0.713 0.758 0.435 0.083 0.653 0.713 0.743 1.425 0.75 1.45 0.758 1.48 1.5 1.51 1.58
LV Amps 3.45 3.43 3.44 1.89 0.34 1.72 1.71 1.73 3.28 1.72 3.29 1.72 3.28 3.29 3.28 3.28
3 Acme
Federal
GE
HV Amps 4.17 4.17 4.17 2.08 4.17 2.08 2.08 2.08 4.17 2.08 4.17 2.08 4.17 4.17 4.17 4.17
kVA 0.92 0.95 1.01 0.58 1.1 0.87 0.95 0.99 1.9 1 1.93 1.01 1.97 2 2.01 2.1
LV Amps 4.58 4.58 4.58 2.5 4.58 2.29 2.29 2.29 4.38 2.29 4.38 2.29 4.38 4.38 4.38 4.38
4 Acme
Federal
GE
HV Amps 6.25 6.25 6.25 3.13 6.25 3.13 3.13 3.13 6.25 3.13 6.25 3.13 6.25 6.25 6.25 6.25
kVA 1.38 1.43 1.51 0.86 1.65 1.31 1.43 1.48 2.85 1.5 2.89 1.51 2.95 2.99 3.02 3.15
LV Amps 6.88 6.88 6.88 3.75 6.88 3.44 3.44 3.44 6.56 3.44 6.56 3.44 6.56 6.56 6.56 6.56
5 Acme
Federal
GE
HV Amps 10.4 10.4 10.4 5.21 10.4 5.21 5.21 5.21 10.4 5.21 10.4 5.21 10.4 10.4 10.4 10.4
kVA 2.29 2.38 2.52 1.44 2.75 2.18 2.38 2.46 4.76 2.5 4.81 2.52 4.92 4.99 5.03 5.25
LV Amps 11.5 11.5 11.5 6.3 11.5 5.73 5.73 5.73 10.9 5.73 10.9 5.73 10.9 10.9 10.9 10.9
6 Acme
Federal
GE
HV Amps 20.8 20.8 20.8 10.4 20.8 10.4 10.4 10.4 20.8 10.4 20.8 10.4 20.8 20.8 20.8 20.8
kVA 4.58 4.77 5.04 2.88 5.5 4.35 4.77 4.93 9.52 5 9.63 5.04 9.84 9.98 10.1 10.5
LV Amps 22.9 22.9 22.9 12.5 22.9 11.5 11.5 11.5 21.9 11.5 21.9 11.5 21.9 21.9 21.9 21.9
7 Acme
Federal
GE
HV Amps 31.3 31.3 31.3 15.6 31.3 15.6 15.6 15.6 31.3 15.6 31.3 15.6 31.3 31.3 31.3 31.3
kVA 6.88 7.15 7.56 4.31 8.25 6.53 7.15 7.39 14.3 7.49 14.4 7.56 14.8 15 15.1 15.8
LV Amps 34.4 34.4 34.4 18.8 34.4 17.2 17.2 17.2 32.8 17.2 32.8 17.2 32.8 32.8 32.8 32.8
8 Acme
Federal
GE
HV Amps 41.7 41.7 41.7 20.8 41.7 20.8 20.8 20.8 41.7 20.8 41.7 20.8 41.7 41.7 41.7 41.7
kVA 9.17 9.53 10.1 5.75 11 8.71 9.53 9.85 19 9.99 19.3 10.1 19.7 20 20.1 21
LV Amps 45.8 45.8 45.8 25 45.8 22.9 22.9 22.9 43.8 22.9 43.8 22.9 43.8 43.8 43.8 43.8
9 Acme
Federal
GE
HV Amps 62.5 62.5 62.5 31.3 62.5 31.3 31.3 31.3 62.5 31.3 62.5 31.3 62.5 62.5 62.5 62.5
kVA 13.8 14.3 15.1 8.6 16.5 13.1 14.3 14.8 28.5 15 28.9 15.1 29.5 29.9 30.2 31.5
LV Amps 68.8 68.8 68.8 37.5 68.8 34.4 34.4 34.4 65.6 34.4 65.6 34.4 65.6 65.6 65.6 65.6
10 Acme
Federal
GE
HV Amps 83.3 83.3 83.3 41.7 83.3 41.7 41.7 41.7 83.3 41.7 83.3 41.7 83.3 83.3 83.3 83.3
kVA 18.3 19.1 20.2 11.5 22 17.4 19.1 19.7 38.1 20 38.5 20.2 39.4 39.9 40.3 42
LV Amps 91.7 91.7 91.7 50 91.7 45.8 45.8 45.8 87.5 45.8 87.5 45.8 87.5 87.5 87.5 87.5
11 Acme
Federal
GE
HV Amps 125 125 125 62.5 125 62.5 62.5 62.5 125 62.5 125 62.5 125 125 125 125
kVA 27.5 28.6 30.3 17.3 33 26.1 28.6 29.6 57.1 30 57.8 30.3 59.1 59.9 60.4 63
LV Amps 138 138 138 75 138 68.8 68.8 68.8 131 68.8 131 68.8 131 131 131 131
12 Acme
Federal
HV Amps 208 208 208 104 208 104 104 104 208 104 208 104 208 208 208 208
kVA 45.8 47.7 50.4 28.8 55 43.5 47.7 49.3 95.2 50 96.3 50.4 98.4 99.8 101 105
LV Amps 229 229 229 125 229 115 115 115 219 115 219 115 219 219 219 219
13 Acme HV Amps 314 311 313 158 31 156 156 157 312 156 313 157 312 313 312 313
kVA 69 71 76 44 8 65 71 74 143 75 145 76 148 150 151 158
LV Amps 345 343 344 189 34 172 171 173 328 172 329 172 328 329 328 328
14 Acme HV Amps 418 415 417 210 42 208 207 209 416 208 418 209 416 418 416 417
kVA 92 95 101 58 11 87 95 99 190 100 193 101 197 200 201 210
LV Amps 460 457 459 252 46 229 228 230 437 229 439 230 438 439 437 438

Product Selection - 3 Phase Buck Boosts

Group 1 - 120x240 Input, 12/24 Output

Models by Brand Line Voltage 189 198 208 208 220 227 228 232 240 189Y
109
199Y
115
208Y
120
379Y
219
416Y
240
416Y
240
Load Voltage 208 208 218 229 242 250 239 255 264 208Y
120
239Y
138
229Y
132
417Y
241
436Y
252
457Y
264
Quantity Required 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3
Connection Diagram 7 8 8 7 7 7 8 7 7 10 9 10 12 11 12
1 Acme
Federal
GE
HV Amps 2.08 4.17 4.17 2.08 2.08 2.08 4.17 2.08 2.08 4.17 2.08 4.17 2.08 4.17 2.08
kVA 0.76 1.51 1.58 0.83 0.88 0.91 1.73 0.93 0.96 1.5 0.86 1.65 1.51 3.15 1.65
LV Amps 2.29 4.38 4.38 2.29 2.29 2.29 4.38 2.29 2.29 4.58 2.5 4.58 2.29 4.38 2.29
2 GE HV Amps 5.44 10.85 10.87 5.44 5.42 5.43 10.86 5.44 5.43 10.82 5.43 10.81 5.41 10.84 5.42
kVA 1.13 2.26 2.37 1.25 1.31 1.36 2.6 1.39 1.43 2.25 1.3 2.48 2.26 4.73 2.48
LV Amps 5.99 11.4 11.39 5.99 5.97 5.98 11.38 5.98 5.97 11.9 6.52 11.9 5.96 11.36 5.95
3 Acme
Federal
GE
HV Amps 4.17 8.33 8.33 4.17 4.17 4.17 8.33 4.17 4.17 8.33 4.17 8.33 4.17 8.33 4.17
kVA 1.51 3.01 3.16 1.66 1.75 1.81 3.46 1.85 1.91 3 1.73 3.3 3.01 6.3 3.3
LV Amps 4.58 8.75 8.75 4.58 4.58 4.58 8.75 4.58 4.58 9.17 5 9.17 4.58 8.75 4.58
4 Acme
Federal
GE
HV Amps 6.25 12.5 12.5 6.25 6.25 6.25 12.5 6.25 6.25 12.5 6.25 12.5 6.25 12.5 6.25
kVA 2.26 4.51 4.73 2.48 2.62 2.71 5.19 2.77 2.86 4.5 2.59 4.95 4.52 9.45 4.95
LV Amps 6.88 13.1 13.1 6.88 6.88 6.88 13.1 6.88 6.88 13.8 7.5 13.8 6.88 13.1 6.88
5 Acme
Federal
GE
HV Amps 10.4 20.8 20.8 10.4 10.4 10.4 20.8 10.4 10.4 20.8 10.4 20.8 10.4 20.8 10.4
kVA 3.76 7.51 7.89 4.13 4.37 4.51 8.64 4.61 4.77 7.5 4.31 8.25 7.53 15.75 8.25
LV Amps 11.5 21.9 21.9 11.5 11.5 11.5 21.9 11.5 11.5 22.9 12.5 22.9 11.5 21.9 11.5
6 Acme
Federal
GE
HV Amps 20.8 41.7 41.7 20.8 20.8 20.8 41.7 20.8 20.8 41.7 20.8 41.7 20.8 41.7 20.8
kVA 7.51 15 15.8 8.26 8.74 9.02 17.3 9.21 9.53 15 8.6 16.5 15.1 31.5 16.5
LV Amps 22.9 43.8 43.8 22.9 22.9 22.9 43.8 22.9 22.9 45.8 25 45.8 22.9 43.8 22.9
7 Acme
Federal
GE
HV Amps 31.3 62.5 62.5 31.3 31.3 31.3 62.5 31.3 31.3 62.5 31.3 62.5 31.3 62.5 31.3
kVA 11.3 22.5 23.7 12.4 13.1 13.5 25.9 13.8 14.3 22.5 12.9 24.8 22.6 47.3 24.8
LV Amps 34.4 65.6 65.6 34.4 34.4 34.4 65.6 34.4 34.4 68.8 37.5 68.8 34.4 65.6 34.4
8 Acme
Federal
GE
HV Amps 41.7 83.3 83.3 41.7 41.7 41.7 83.3 41.7 41.7 83.3 41.7 83.3 41.7 83.3 41.7
kVA 15 30 31.5 16.5 17.5 18 34.6 18.4 19.1 30 17.3 33 30.1 63 33
LV Amps 45.8 87.5 87.5 45.8 45.8 45.8 87.5 45.8 45.8 91.7 50 91.7 45.8 87.5 45.8
9 Acme
Federal
GE
HV Amps 62.5 125 125 62.5 62.5 62.5 125 62.5 62.5 125 62.5 125 62.5 125 62.5
kVA 22.5 45 47.3 24.8 26.2 27 51.8 27.6 28.6 45 25.9 49.5 45.2 94.5 49.5
LV Amps 68.8 131 131 68.8 68.8 68.8 131 68.8 68.8 138 75 138 68.8 131 68.8
10 Acme
Federal
GE
HV Amps 83.3 167 167 83.3 83.3 83.3 167 83.3 83.3 167 83.3 167 83.3 167 83.3
kVA 30 60 63.1 33 34.9 36.1 69.1 36.8 38.1 60 34.5 66 60.3 126 66
LV Amps 91.7 175 175 91.7 91.7 91.7 175 91.7 91.7 183 100 183 91.7 175 91.7
11 Acme
Federal
GE
HV Amps 125 250 250 125 125 125 250 125 125 250 125 250 125 250 125
kVA 45 90 94.6 49.5 52.4 54.1 103.7 55.3 57.2 90 51.8 99 90.4 189 99
LV Amps 138 263 263 138 138 138 263 138 138 275 150 275 138 263 138
12 Acme
Federal
GE (60 Hz)
GE (50 Hz)
HV Amps 208 417 417 208 208 208 417 208 208 417 208 417 208 417 208
kVA 75 150 157.6 82.6 87.3 90.1 172.8 92.1 95.3 150 86.3 165 151 315 165
LV Amps 229 438 438 229 229 229 438 229 229 458 250 458 229 438 229
13 Acme HV Amps 544 1085 1087 544 542 543 1086 544 543 1082 543 1081 541 1084 542
kVA 113 226 237 125 131 136 260 139 143 225 130 248 226 473 248
LV Amps 599 1140 1139 599 597 598 1138 598 597 1190 652 1190 596 1136 595
14 Acme HV Amps 726 1447 1450 725 723 724 1448 725 723 1442 724 1441 722 1445 722
kVA 151 301 316 166 175 181 346 185 191 300 173 330 301 630 330
LV Amps 799 1520 1519 798 795 797 1518 797 796 1587 869 1587 794 1514 793

Group 2 - 120x240 Input, 16/32 Output

Models by Brand Line Voltage 184 195 208 208 225 230 234 234 240 240 184Y
106
416Y
240
379Y
219
416Y
240
416Y
240
Load Voltage 209 208 222 236 240 245 250 265 272 256 208Y
120
236Y
136
431Y
249
443Y
256
471Y
272
Quantity Required 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3
Connection Diagram 7 8 8 7 8 8 8 7 7 8 10 10 12 11 12
1 Acme
Federal
GE
HV Amps 1.56 3.13 3.13 1.56 3.13 3.13 3.13 1.56 1.56 3.13 3.13 3.13 1.56 3.13 1.56
kVA 0.57 1.13 1.21 0.64 1.3 1.33 1.36 0.72 0.74 1.39 1.13 1.28 1.17 2.4 1.28
LV Amps 1.77 3.33 3.33 1.77 3.33 3.33 3.33 1.77 1.77 3.33 3.54 3.54 1.77 3.33 1.77
2 GE HV Amps 4.06 8.15 8.14 4.07 8.13 8.14 8.13 4.08 4.08 8.14 8.11 8.1 4.05 8.13 4.06
kVA 0.85 1.7 1.81 0.96 1.95 2 2.03 1.08 1.11 2.09 1.69 1.91 1.75 3.6 1.91
LV Amps 4.61 8.69 8.69 4.62 8.67 8.67 8.69 4.62 4.63 8.69 9.17 4.6 4.61 8.65 4.6
3 Acme
Federal
GE
HV Amps 3.13 6.25 6.25 3.13 6.25 6.25 6.25 3.13 3.13 6.25 6.25 6.25 3.13 6.25 3.13
kVA 1.13 2.26 2.41 1.28 2.6 2.66 2.71 1.44 1.48 2.78 2.25 2.55 2.33 4.8 2.55
LV Amps 3.54 6.67 6.67 3.54 6.67 6.67 6.67 3.54 3.54 6.67 7.08 7.08 3.54 6.67 3.54
4 Acme
Federal
GE
HV Amps 4.69 9.38 9.38 4.69 9.38 9.38 9.38 4.69 4.69 9.38 9.38 9.38 4.69 9.4 4.69
kVA 1.7 3.38 3.61 1.92 3.9 3.99 4.06 2.16 2.21 4.16 3.38 3.83 3.5 7.2 3.83
LV Amps 5.31 10 10 5.31 10 10 10 5.31 5.31 10 10.6 10.6 5.31 10 5.31
5 Acme
Federal
GE
HV Amps 7.81 15.6 15.6 7.8 15.6 15.6 15.6 7.81 7.81 15.6 15.6 15.6 7.81 15.6 7.81
kVA 2.83 5.63 6.01 3.19 6.5 6.64 6.76 3.59 3.69 6.93 5.63 6.38 5.84 12 6.38
LV Amps 8.85 16.7 16.7 8.9 16.7 16.7 16.7 8.85 8.85 16.7 17.7 17.7 8.85 16.7 8.85
6 Acme
Federal
GE
HV Amps 15.6 31.3 31.3 15.6 31.3 31.3 31.3 15.6 15.6 31.3 31.3 31.3 15.6 31.3 15.6
kVA 5.65 11.3 12 6.38 13 13.3 13.5 7.18 7.37 13.9 11.3 12.8 11.7 24 12.8
LV Amps 17.7 33.3 33.3 17.7 33.3 33.3 33.3 17.7 17.7 33.3 35.4 35.4 17.7 33.3 17.7
7 Acme
Federal
GE
HV Amps 23.4 46.9 46.9 23.4 46.9 46.9 46.9 23.4 23.4 46.9 46.9 46.9 23.1 46.9 23.4
kVA 8.47 16.9 18 9.57 19.5 19.9 20.3 10.8 11.1 20.8 16.9 19.1 17.5 36 19.1
LV Amps 26.6 50 50 26.6 50 50 50 26.6 26.6 50 53.1 53.1 26.6 50 26.6
8 Acme
Federal
GE
HV Amps 31.3 62.5 62.5 31.3 62.5 62.5 62.5 31.3 31.3 62.5 62.5 62.5 31.3 62.5 31.3
kVA 11.3 22.5 24 12.8 26 26.6 27 14.4 14.7 27.7 22.5 25.5 23.3 48 25.5
LV Amps 35.4 66.7 66.7 35.4 66.7 66.7 66.7 35.4 35.4 66.7 70.8 70.8 35.4 66.7 35.4
9 Acme
Federal
GE
HV Amps 46.9 93.8 93.8 46.9 93.8 93.8 93.8 46.9 46.9 93.8 93.8 93.8 46.9 93.8 46.9
kVA 16.9 33.8 36 19.1 39 39.8 40.5 21.5 22.1 41.6 33.8 38.3 35 72 38.3
LV Amps 53.1 100 100 53.1 100 100 100 53.1 53.1 100 106 106 53.1 100 53.1
10 Acme
Federal
GE
HV Amps 62.5 125 125 62.5 125 125 125 62.5 62.5 125 125 125 62.5 125 62.5
kVA 22.6 45 48 25.5 52 53.1 54 28.7 29.4 55.4 45 51 46.7 96 51
LV Amps 70.8 133.3 133.3 70.8 133.3 133.3 133.3 70.8 70.8 133.3 142 142 70.8 133 70.8
11 Acme
Federal
GE
HV Amps 93.75 187.5 187.5 93.75 187.5 187.5 187.5 93.75 93.75 187.5 188 188 93.8 188 93.8
kVA 33.9 67.6 72.1 38.3 78 79.7 81.1 43.1 44.2 83.1 67.5 76.5 70 144 76.5
LV Amps 106.3 200 200 106.3 200 200 200 106.3 106.3 200 213 213 106 200 106
12 Acme
Federal
GE (60 Hz)
GE (50 Hz)
HV Amps 156.25 312.5 312.5 156.25 313 312.5 312.5 156.25 156.25 312.5 313 313 156 313 156
kVA 56.4 112.6 120.1 63.8 129.9 132.8 135.1 71.8 73.6 138.6 113 128 117 240 128
LV Amps 177.1 333.3 333.3 177.1 333.3 333.3 333.3 177.1 177.1 333.3 354 354 177 333 177
13 Acme HV Amps 406 815 814 407 813 814 813 408 408 814 811 810 405 813 406
kVA 85 170 181 96 195 200 203 108 111 209 169 191 175 360 191
LV Amps 461 869 869 462 867 867 869 462 463 869 917 460 461 865 460
14 Acme HV Amps 541 1087 1086 542 1083 1086 1084 543 544 1086 1082 1081 541 1084 541
kVA 113 226 241 128 260 266 271 144 148 278 225 255 233 480 255
LV Amps 614 1159 1159 615 1156 1157 1158 615 617 1158 1223 613 615 1154 613

Group 3 - 240x480 Input, 24/48 Output

Models by Brand Line Voltage 173 200 208 362 380 400 419 436 457 480 480 346Y
200
379Y
219
400Y
231
416Y
240
Load Voltage 208 240 250 380 418 440 440 480 480 528 504 416Y
240
456Y
263
480Y
277
499Y
288
Quantity Required 2 2 2 2 2 2 2 2 2 2 2 3 3 3 3
Connection Diagram 5 5 5 6 7 7 8 7 8 7 8 9 9 9 9
1 Acme
Federal
GE
HV Amps 1.04 1.04 1.04 2.08 1.04 1.04 2.08 1.04 2.08 1.04 2.08 1.04 1.04 1.04 1.04
kVA 0.38 0.44 0.46 1.44 0.76 0.8 1.67 0.87 1.82 0.96 1.82 0.75 0.82 0.87 0.9
LV Amps 1.25 1.25 1.25 2.29 1.15 1.15 2.29 1.15 2.29 1.15 2.19 1.25 1.25 1.25 1.15
2 GE HV Amps 2.7 2.72 2.73 5.68 2.71 2.71 5.68 2.72 5.67 2.71 5.42 2.7 2.7 2.7 2.71
kVA 0.563 0.653 0.683 2.16 1.13 1.19 2.5 1.31 2.72 1.43 2.73 1.13 1.23 1.3 1.35
LV Amps 3.25 3.26 3.28 5.97 2.98 2.98 5.96 2.99 5.96 2.98 5.69 3.25 3.25 3.24 3.25
3 Acme
Federal
GE
HV Amps 2.08 2.08 2.08 4.17 2.08 2.08 4.17 2.08 4.17 2.08 4.17 2.08 2.08 2.08 2.08
kVA 0.75 0.87 0.91 2.88 1.51 1.59 3.33 1.74 3.63 1.91 3.64 1.5 1.64 1.73 1.8
LV Amps 2.5 2.5 2.5 4.58 2.29 2.29 4.58 2.29 4.58 2.29 4.38 2.5 2.5 2.5 2.5
4 Acme
Federal
GE
HV Amps 3.13 3.13 3.13 6.25 3.13 3.13 6.25 3.13 6.25 3.13 6.25 3.13 3.13 3.13 3.13
kVA 1.13 1.3 1.36 4.32 2.27 2.39 4.99 2.6 5.45 2.86 5.46 2.25 2.47 2.6 2.7
LV Amps 3.75 3.75 3.75 6.88 3.44 3.44 6.88 3.44 6.88 3.44 6.56 3.75 3.75 3.75 3.75
5 Acme
Federal
GE
HV Amps 5.21 5.21 5.21 10.4 5.21 5.21 10.4 5.21 10.4 5.21 10.4 5.21 5.21 5.21 5.21
kVA 1.88 2.17 2.26 7.19 3.78 3.97 8.32 4.33 9.07 4.77 9.1 3.75 4.11 4.33 4.56
LV Amps 6.25 6.25 6.25 11.5 5.73 5.73 11.5 5.73 11.5 5.73 10.9 6.25 6.25 6.25 6.25
6 Acme
Federal
GE
HV Amps 10.4 10.4 10.4 20.8 10.4 10.4 20.8 10.4 20.8 10.4 20.8 10.4 10.4 10.4 10.4
kVA 3.75 4.34 4.51 14.4 7.55 7.94 16.6 8.66 18.1 9.53 18.2 7.5 8.22 8.66 9
LV Amps 12.5 12.5 12.5 22.9 11.5 11.5 22.9 11.5 22.9 11.5 21.9 12.5 12.5 12.5 12.5
7 Acme
Federal
GE
HV Amps 15.6 15.6 15.6 31.3 15.6 15.6 31.3 15.6 31.3 15.6 31.3 15.6 15.6 15.6 15.6
kVA 5.62 6.5 6.76 21.6 11.3 11.9 25 13 27.2 14.3 27.3 11.3 12.3 13 13.5
LV Amps 18.8 18.8 18.8 34.4 17.2 17.2 34.4 17.2 34.4 17.2 32.8 18.8 18.8 18.8 18.8
8 Acme
Federal
GE
HV Amps 20.8 20.8 20.8 41.7 20.8 20.8 41.7 20.8 41.7 20.8 41.7 20.8 20.8 20.8 20.8
kVA 7.5 8.67 9.01 28.7 15.1 15.9 33.3 17.3 36.3 19.1 36.4 15 16.4 17.3 18
LV Amps 25 25 25 45.8 22.9 22.9 45.8 22.9 45.8 22.9 43.8 25 25 25 25
9 Acme
Federal
GE
HV Amps 31.3 31.3 31.3 62.5 31.3 31.3 62.5 31.3 62.5 31.3 62.5 31.3 31.3 31.3 31.3
kVA 11.2 13 13.5 43.1 22.6 23.8 49.9 26 54.4 28.6 54.6 22.5 24.7 26 27
LV Amps 37.5 37.5 37.5 68.8 34.4 34.4 68.8 34.4 68.8 34.4 65.6 37.5 37.5 37.5 37.5
10 Acme
Federal
GE
HV Amps 41.7 41.7 41.7 83.3 41.7 41.7 83.3 41.7 83.8 41.7 83.3 41.7 41.7 41.7 41.7
kVA 15 17.3 18 57.5 30.2 31.8 66.5 34.6 72.6 38.1 72.8 30 32.9 34.6 36
LV Amps 50 50 50 91.7 45.8 45.8 91.7 45.8 91.7 45.8 87.5 50 50 50 50
11 Acme
Federal
GE
HV Amps 62.5 62.5 62.5 125 62.5 62.5 125 62.5 125 62.5 125 62.5 62.5 62.5 62.5
kVA 22.5 26 27 86.2 45.3 47.6 99.8 51.9 109 57.2 109 45 49.3 51.9 54
LV Amps 75 75 75 138 68.8 68.8 138 68.8 138 68.8 131 75 75 75 75
12 Acme
Federal
HV Amps 104 104 104 208 104 104 208 104 208 104 208 104 104 104 104
kVA 37.5 43.3 45 144 75.4 79.4 166 86.5 181 95.3 182 75 82.2 86.6 90
LV Amps 125 125 125 229 115 115 229 115 229 115 219 125 125 125 125
13 Acme HV Amps 270 272 273 568 271 271 568 272 567 271 542 270 270 270 271
kVA 56 65 68 216 113 119 250 131 272 143 273 113 123 130 135
LV Amps 325 326 328 597 298 298 596 299 596 298 569 325 325 324 325
14 Acme HV Amps 361 363 364 758 361 361 757 363 756 362 722 361 360 360 361
kVA 75 87 91 288 151 159 333 174 363 191 364 150 164 173 180
LV Amps 434 435 438 796 397 398 795 399 794 398 758 434 433 433 433

FAQ


Buck-boost transformers are small single phase transformers designed to reduce (buck) or raise (boost) line voltage from 5 - 20%. The most common example is boosting 208 volts to 230 volts, usually to operate a 230 volt motor such as an airconditioner compressor, from a 208 volt supply line.

Buck-boosts are a standard type of single phase distribution transformers, with primary voltages of 120, 240 or 480 volts and secondaries typically of 12, 16, 24, 32 or 48 volts. They are available in sizes ranging from 50 volt amperes to 10 kilo-volt amperes.

Buck-boost transformers are shipped ready to be connected for a number of possible voltage combinations.

A typical buck boost application is 120 volts in, 12 volts out for low voltage lighting or control circuitry. In most applications, this low voltage transformer is field connected as an autotransformer. (See question 2 for the definition of an autotransformer). Buck-boost transformers provide tremendous capabilities and flexibility in KVA sizes and input/output voltage combinations. Basically, you get 75 different transformers, all in one convenient package.

Other buck boost applications are, where (A) low supply voltage exists because equipment is installed at the end of a bus system; (B) the supply system is operating at or over its design capacity; and (C) where overall consumer demands may be so high the utility cuts back the supply voltage to the consumer causing a "brownout."

Why Use Buck-Boost Instead of Another Type Transformer?

Take a look at the advantages and disadvantages of using a buck boost transformer (autotransformer) compared to a standard isolation transformer of the proper size and voltage combination.

As you can see, the advantages are many, the economies great. Buck- boost transformers are readily available from the stock of your nearest Power Distribution Products Distributor.

ADVANTAGES DISADVANTAGES
More efficient No circuit isolation
Smaller & lighter Cannot create a neutral
5-10 times increase in KVA Application voltage and the KVA don't match the nameplate voltages and KVA
Versatile, many applications -
Lower cost -

Proper Voltage Is Critical

With nearly two-thirds of all electrical loads being A.C. motor loads, maintenance of the proper voltage to that motor is very important. If the supply line voltage is not maintained, motor winding current is increased causing reduced motor torque and escalating motor temperature, all of which results in the rapid loss of insulation life expectancy.

In addition to motor loads, the detrimental effects of low voltage on both resistive heating loads and incandescent lighting output is illustrated in the chart.

Anytime you have a lower than standard voltage, equipment damage and failure can result.

Buck-boost transformers are an economical way to correct this potentially very serious problem. Anytime a line voltage change in the 5-20% range is required, a buck boost transformer should be considered as your first line of defense.

3. How does a buck boost transformer differ from an insulating transformer?

A buck boost transformer IS an insulating type transformer when it is shipped from the factory. When it is connected at the job site, a lead wire on the primary is connected to a lead wire on the secondary - thereby changing the transformer's electrical characteristics to those of an autotransformer. The primary and secondary windings are no longer "insulated" and secondary windings are no longer "insulated" and its KVA capacity is greatly increased.

4. What is the difference between a buckboost transformer and an autotransformer?

When a primary lead wire and secondary lead wire of a buckboost transformer are connected together electrically, in a recommended voltage bucking or boosting connection, the transformer is in all respects, an autotransformer. However, if the interconnection between the primary and secondary winding is not made, then the unit is an insulating type transformer.

5. Why are they used?

Electrical and electronic equipment is designed to operate on standard supply voltage. When the supply voltage is constantly too high or too low, (usually more than 55%), the equipment fails to operate at maximum efficiency. A buck and boost transformer is a simple and ECONOMICAL means of correcting this off-standard voltage.

6. What are the most common applications for buck boost transformers?

Boosting 208V to 230V or 240V and vice versa for commercial and industrial air conditioning systems; boosting 110V to 120V and 240V to 277V for lighting systems; voltage correction for heating systems and induction motors of all types. Many applications exist where supply voltages are constantly above or below normal.

7. Can buck boost transformers be used to power low voltage circuits?

Yes, low voltage control, lighting circuits, or other low voltage applications requiring either 12V, 16V, 24V, 32V or 48V. The unit is connected as an insulating transformer and the nameplate KVA rating is the transformer's capacity.


8. Why do buck boost transformers have 4 windings?

To make them versatile! A four winding buck boost transformer (2 primary and 2 secondary windings) can be connected eight different ways to provide a multitude of voltage and KVA outputs. A two winding (1 primary & 1 secondary) buck boost transformer can be connected only one way.

9. Will a buck boost transformer stabilize voltage?

No. The output voltage is a function of the input voltage. If the input voltage varies, then the output voltage will also vary by the same percentage.


Load Data

10. Are there any restrictions on the type of load that can be operated from a buck boost transformer?

No, there are no restrictions.

11. Why can a buck boost transformer operate a KVA load many times larger than the KVA rating on its nameplate?

Since the transformer has been auto-connected in such a fashion that the 22V secondary voltage is added to the 208V primary voltage, it produces 230V output.

The autotransformer KVA is calculated:

KVA = Output Volts x Secondary Amps
1000
KVA = 230 V x 41.67 Amps = 9.58 KVA
1000

12. Can buck boost transformers be used on motor loads?

Yes, either single or three phase. Refer to the motor data charts in Section I for determining KVA and Amps required by NEMA standard motors.

13. How are single phase and three phase load Amps and load KVA calculated?

Single phase Amps = KVA x 1000
Volts
Three phase Amps = KVA x 1000
Volts x 1.73
Single phase KVA = Volts x Amps
1000
Three phase KVA = Volts x Amps x 1.73
1000

Three-Phase

14. Can buck boost transformers be used on three-phase systems as well as single phase systems?

Yes. A single unit is used to buck or boost single phase voltage - two or three units are used to buck or boost three phase voltage. The number of units to be used in a three -phase installation depends on the number of wires in the supply line. If the three-phase supply is 4 wire Y, use three buck boost transformers. If the 3 - phase supply is 3 wire Y (neutral not available), use two buck boost transformers. Refer to three-phase selection charts.

15. Should buck boost transformers be used to develop a three-phase 4 wire Y circuit from a three-phase 3 wire delta circuit?

No. A three phase "wye" buck boost transformer connection should be used only on a 4 wire source of supply. A delta to wye connection does not provide adequate current capacity to accommodate unbalanced currents flowing in the neutral wire of the 4 wire circuit.

3 PHASE CONNECTIONS
INPUT (SUPPLY SYSTEM) DESIRED OUTPUT CONNECTION
DELTA
3 wire
WYE
3 or 4 wire
DO NO USE
OPEN DELTA
3 wire
WYE
3 or 4 wire
DO NOT USE
WYE
3 or 4 wire
CLOSED DELTA
3 wire
DO NOT USE
WYE
4 wire
WYE
3 or 4 wire
OK
WYE
3 or 4 wire
OPEN DELTA
3 wire
OK
CLOSED DELTA
3 wire
OPEN DELTA
3 wire
OK

16. Why isn't a closed delta buck boost connection recommended?

A closed delta buck boost auto transformer connection requires more transformer KVA than a "wye" or open delta connection and phase shifting occurs on the output. Consequently the closed delta connection is more expensive and electrically inferior to other three-phase connections.


17. How does the installer or user know how to connect a buck boost transformer?

The connection chart (for Acme and Federal) packed with each unit shows how to make the appropriate connections.

18. Can 60 Hertz buck boost transformers be used on a 50 Hertz service?

No. Acme buck boost transformers should be operated only at the frequencies recommended. However, units recommended for 50 cycle operation are suitable for 60 cycle operation but not vice versa.


Selection

19. How do you select a buck boost transformer?

Refer to the selection steps below for easy 4-step selection, then go to the charts.


20. Why are buck boost transformers shipped from the factory as insulating transformers and not preconnected at the factory as autotransformers?

A four winding buck boost transformer can be auto connected eight different ways to provide a multitude of voltage and KVA output combinations. The proper transformer connection depends on the user's supply voltage, load voltage and load KVA. Consequently, it is more feasible for the manufacturer to ship the unit as an insulating transformer and allow the user to connect it on the job site in accordance with the available supply voltage and requirements of his load.

21. Why is the isolation transformer KVA rating shown on the nameplate instead of the autotransformer KVA rating?

The KVA rating of a buck boost transformer when auto connected depends on the amount of voltage buck or boost. Since the amount of voltage buck or boost is different for each connection, it is physically impossible to show all of the various voltage combinations and attainable KVA ratings on the nameplate. A connection chart showing the various attainable single phase and three-phase connections is packed with each unit.


Safety

22. Do buck boost transformers present a safety hazard usually associated with autotransformers?

No. Most autotransformers, if they are not of the buck boost variety, change voltage from one voltage class to another. (Example 480V to 240V) In a system where one line is grounded, the user thinks he has 240V; yet due to the primary and secondary being tied together, it is possible to have 480V to ground from the 240V output. A buck boost transformer only changes the voltage a small amount, such as 208V to 240V. This small increase does not represent a safety hazard, as compared to a buck of 480V to 240V.


Sound Levels

23. Are buck boost transformers as quiet as standard isolation transformers?

Yes. However, an auto-connected buck boost transformer will be quieter than an isolation transformer capable of handling the same load. The isolation transformer would have to be physically larger than the buck boost transformer, and small transformers are quieter than larger ones. (Example) 1 KVA - 40 db; 75 KVA - 50 db. (db is a unit of sound measure).


24. How does the cost of a buck boost transformer compare to that of an insulating transformer - both capable of handling the same load?

For the most common buck boost applications, the dollar savings are generally greater than 75% compared to the use of an insulating type distribution transformer for the same application.

25. What is the life expectancy of a buck boost transformer?

The life expectancy of a buck boost transformer is the same as the life expectancy of other dry type transformers.


26. It is indicated that a buck boost transformer is suitable for connecting as an autotransformer. What is the definition of an autotransformer and how does it differ from an isolation transformer?

An autotransformer is a transformer in which the primary (input) and the secondary (output) are electrically connected to each other. An isolation transformer, also known as an insulating transformer, has complete electrical separation between the primary (input) and the secondary (output).

An autotransformer changes or transforms only a portion of the electrical energy it transmits. The rest of the electrical energy flows directly through the electrical connections between the primary and secondary. An isolation transformer (insulating transformer) changes or transforms all of the electrical energy it transmits.

Consequently, an autotransformer is smaller, lighter in weight, and less costly than a comparable KVA size insulating transformer. Please refer to Question 27 for additional information on autotransformers.

Buck-boost transformers are frequently field-connected as autotransformers.

27. Buck-boost transformers are almost always installed as auto-transformers. Does the N.E.C. (National Electrical Code) permit the use of autotransformers?

Yes. Please refer to N.E.C. Article 450-4, "Autotransformers 600 Volts, Nominal, or Less." Item (a) explains how to overcurrent protect an autotransformer; item (b) explains that an insulating transformer such as a buck boost transformer may be field connected as an autotransformer.

28. When a buck boost is connected as an autotransformer such as boosting 208V to 230V, the KVA is greatly increased. How do you determine the size (ampere rating) of the overcurrent protective device (such as a fuse or circuit breaker)?

The National Electrical Code Article 450-4 addresses overcurrent protection of autotransformers. A copy is reproduced below for easy reference.

450-4. Autotransformers 600 Volts, Nominal, or Less.

(a) Overcurrent Protection. Each autotransformer 600 volts, nominal, or less shall be protected by an individual overcurrent device installed in series with each ungrounded input conductor. Such overcurrent device shall be rated or set at not more than 125 percent of the rated full-load input current of the autotransformer. An overcurrent device shall not be installed in series with the shunt winding (the winding common to both the input and the output circuits) of the autotransformer between Points A and B as shown in Diagram 450-4.

Exception: Where the rated input current of an autotransformer is 9 amperes or more and 125 percent of this current does not correspond to a standard rating of a fuse or non-adjustable circuit breaker, the next higher standard rating described in Section 240-6 shall be permitted. When the rated input current is less than 9 amperes, an overcurrent device rated or set at not more than 167 percent of the input current shall be permitted.

(b) Transformer Field-Connected as an Autotransformer. A transformer field-connected as an autotransformer shall be identified for use at elevated voltage.

29. The reprint of the N.E.C. (National Electrical Code), Article 450-4 shown in the previous question covers autotransformer overcurrent protection. Could you explain this article in detail by citing an example?

An example of an everyday application is always a good way to explain the intent of the "Code."

Example: A 1 KVA transformer Catalog No. Acme has a primary of 120 x 240V and a secondary of 12 x24V. It is to be connected as an autotransformer at the time of installation to raise 208V to 230V single phase.

When this 1 KVA unit is connected as an autotransformer for this voltage combination, its KVA rating is increased to 9.58 KVA (may also be expressed as 9,580 VA). This is the rating to be used for determining the full load input amps and the sizing of the overcurrent protect device (fuse or breaker) on the input.

Full Load Input Amps= 9,580 Volt Amps =46 Amps
208 Volts

When the full load current is greater than 9 amps, the overcurrent protective device (usually a fuse or non-adjustable breaker) amp rating can be up to 125 percent of the full load rating of the autotransformer input amps.

Max. amp rating of the
overcurrent device
= 46 amps x 125% = 57.5 amps

The National Electrical Code, Article 450-4 (a) Exception, permits the use of the next higher standard ampere rating of the overcurrent device. This is shown in Article 240-6 of the N.E.C.

Max. size of the fuse or circuit breaker = 60 amps