Acme Electric DRY-TYPE DISTRIBUTION TRANSFORMERS Selection Table
SECTION
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SECTION
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DRY-TYPE DISTRIBUTION TRANSFORMERS
DRY-TYPE DISTRIBUTION TRANSFORMERS
Steps for Selecting the Proper Transformer
SINGLE PHASE LOADS
1. Determine electrical load
A. Voltage required by load.
B. Amperes or kVA capacity required by load.
C. Frequency in Hz (cycles per second).
D. Verify load is designed to operate on a single phase supply.
All of the above information is standard data normally obtained from equipment
nameplates or instruction manuals.
2. Determine supply voltage
A. Voltage of supply (source).
B. Frequency in Hz (cycles per second).
The frequency of the line supply and electrical load must be the same. Select
single phase transformer designed to operate at this frequency, having a primary
(input) equal to the supply voltage and a secondary (output) equal to the voltage required by the load.
3. If the load nameplate expresses a rating in kV A, a transf ormer can be
directly selected from the charts. Choose from a group of transformers with
primary and secondary voltages matching those you have just determined.
A. Select a transformer with a standard kVA capacity equal to or greater than
that needed to operate the load.
B. Primary taps are available on most models to compensate for line voltage
variations. (Refer to question #2 in the Transformer Questions and Answers
Section on page 6.)
C. When load ratings are given only in amperes, tables 1 and 2 or the following
formulas may be used to determine proper kVA size for the required transformer.
(1) To determine kVA when volts and amperes are known:
1000
(2) To determine Amperes when kV A and volts ar e known:
Volts
Single Phase Example
Question: Select a transformer to meet the following conditions. Load is single
phase lighting using incandescent lamps. Each fixture requires 1.3 amps @ 120
volts, 1 phase, 60 Hz, power factor of unity. The installation requires 52-100 watt
fixtures. The desired circuit distributing power to the light fixtures is 120/240 volt,
three wire, single phase. The supply voltage is 460 volt, 3 phase.
Answer: Compute the kV A r equir ed.
1000
Always use amps x volts to compute VA, never use lamp wattage. .156 kVA/
Fixture x 52 Fixture = 8.11 kVA. The two sizes (kVA) nearest 8.11 kVA are 7.5 kVA
and 10 kVA. Use the 10 kVA. This will not overload the transformer and allows some
capacity, 1.89 kVA, for future loads. Since the supply is 460 V (not 480 V) use
the 456 V tap. This will produce approximately 120 volts on output. If the tap is not
used, the output will be 115 V compared to the desired 120 V. Note the transformer selected is single phase but the supply is 480 V, 3 phase. Single phase is
obtained by using any 2 wires of the 3 phase supply.
Volts x Amps
kV A =
Amps =
kV A x 1000
1.3 amps x 120 volts
For each lighting fixture
= .156 kV A
TABLE 1
Full Load Current in Amperes–
Single Phase Circuits
kVA 120 V 208 V 240 V 277 V 380 V 440V 480 V 600 V
.050 0.4 0.2 0.2 0.2 0.1 0.1 0.1 0.1
.100 0.8 0.5 0.4 0.3 0.2 0.2 0.2 0.2
.150 1.2 0.7 0.6 0.5 0.4 0.3 0.3 0.3
.250 2.0 1.2 1.0 0.9 0.6 0.5 0.5 0.4
.500 4.2 2.4 2.1 1.8 1.3 1.1 1.0 0.8
.750 6.3 3.6 3.1 2.7 2.0 1.7 1.6 1.3
1 8.3 4.8 4.2 3.6 2.6 2.3 2.1 1.7
1.5 12.5 7.2 6.2 5.4 3.9 3.4 3.1 2.5
2 16.7 9.6 8.3 7.2 5.2 4.5 4.2 3.3
3 25 14.4 12.5 10.8 7.9 6.8 6.2 5.0
5 41 24.0 20.8 18.0 13.1 11.3 10.4 8.3
7.5 62 36 31 27 19.7 17 15.6 12.5
10 83 48 41 36 26 22.7 20.8 16.7
15 125 72 62 54 39 34 31 25
25 208 120 104 90 65 57 52 41
37.5 312 180 156 135 98 85 78 62
50 416 240 208 180 131 114 104 83
75 625 360 312 270 197 170 156 125
100 833 480 416 361 263 227 208 166
167 1391 802 695 602 439 379 347 278
250 2083 1201 1041 902 657 568 520 416
TABLE 2
Full Load Amperes
Single Phase A.C. Motors
MIN.
HORSE POWER FORMER
KVA
1/6 4.4 2.4 2.2 .53
1/4 5.8 3.2 2.9 .70
1/3 7.2 4.0 3.6 .87
1/2 9.8 5.4 4.9 1.18
3/4 13.8 7.6 6.9 1.66
1 16 8.8 8 1.92
1.5 20 11.0 10 2.40
2 24 13.2 12 2.88
3 34 18.7 17 4.10
5 56 30.8 28 6.72
7.5 80 44 40 9.6
10 100 55 50 12.0
①
When motor service factor is greater than 1,
increase full load amps proportionally.
Example: If service factor is 1.15, increase
above amp values by 15%.
115 V 208 V 230 V
1 Phase kVA =
NOTE: If motors are started more than once per
hour, increase minimum transformer kVA by 20%.
①
TRANS-
Volts x Amps
1000
ACME ELECTRIC • MILWAUKEE, WI • 800.334.5214 • acmepowerdist.com
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SECTION
1
SECTION
DRY-TYPE DISTRIBUTION TRANSFORMERS
DRY-TYPE DISTRIBUTION TRANSFORMERS
THREE PHASE LOADS
1. Determine electrical load
A. Voltage required by load.
B. Amperes or kVA required by load.
C. Frequency in Hz (cycles per second).
D. Verify load is designed to operate on three phase.
All the above information is standard data normally obtained from equipment
nameplates or instruction manuals.
2. Determine supply voltage
A. Voltage of supply (source).
B. Frequency in Hz (cycles per second).
The frequency of the line supply and electrical load must be the same. A three
phase transformer is selected which is designed to operate at this frequency
having a primary (input) equal to the supply voltage and a secondary (output)
equal to the voltage required by the load.
3. If the load nameplate expresses a rating in kVA, a transformer can be
directly selected from the charts. Choose from the group of transformers
with primary and secondary voltages matching that which you have
just determined.
A. Select a transformer with a standard kVA capacity equal to or greater than
that needed to operate the load.
B. Primary taps are available on most models to compensate for line voltage
variations. (Refer to question #2 in the Transformer Questions and Answers
Section on page 6.)
C. When load ratings are given only in amperes, tables 3 and 4 or the following
formulas may be used to determine proper kVA size for the required transformer.
(1) To determine three phase kVA when volts and amperes are known:
Three Phase kVA =
Volts x Amps x 1.73
1000
(2) To determine Amperes when kVA and volts are known:
Amps =
Three Phase Example
Question: Select a transformer to fulfill the following conditions. Load is a three phase
induction motor, 25 horsepower @ 240 volts, 60 Hz and a heater load of 4 kilowatts @
240 volts single phase. The supply voltage is 480Y/277, three phase, 4 wire.
Answer: Compute the kV A requir ed. Motor—From table 4 the current is 68 amps.
240 volts x 68 amps x 1.73
1000
(The kVA can also be obtained from table 4).
Heater — 4 kVA
A three phase transformer must be selected so that any one phase is not overloaded.
Each phase should have the additional 4 kVA rating required by the heater even though
the heater will operate on one phase only. So, the transformer should have a minimum
kVA rating of 28.2 + 4 + 4 + 4 or 40.2 kVA. Refer to the appropriate selection chart.
A 480 delta primary — 240 delta secondary transformer may be used on a 4 wire,
480Y/277 volt supply. The fourth wire (neutral) is not connected to the transformer. To
not overload the transformer, a 45 kVA transformer should be selected.
NOTE: Any two wires of the 240 volts, 3 phase developed by the secondary of the
transformer may be used to supply the heater. Any 2 wires of a 3 phase system is
single phase.
3 Phase kVA x 1000
Volts x 1.73
= 28.2 kVA
TABLE 3
Full Load Current in Amperes–
Three Phase Circuits
kVA 208 V 240 V 380 V 440 V 480 V 600 V
3 8.3 7.2 4.6 3.9 3.6 2.9
4.5 12.5 10.8 6.8 5.9 5.4 4.3
6 16.6 14.4 9.1 7.8 7.2 5.8
9 25 21.6 13.7 11.8 10.8 8.6
15 41 36 22.8 19.6 18.0 14.4
22.5 62 54 34.2 29 27 21.6
30 83 72 45.6 39 36 28
45 124 108 68.4 59 54 43
75 208 180 114 98 90 72
112.5 312 270 171 147 135 108
150 416 360 228 196 180 144
225 624 541 342 294 270 216
300 832 721 456 392 360 288
500 1387 1202 760 655 601 481
750 2081 1804 1139 984 902 721
1000 2775 2405 1519 1312 1202 962
TABLE 4
Full Load Amperes
Three Phase A.C. Motors
MIN.
HORSE- 208 V 230 V 460 V 575 V TRANS POWER FORMER
KVA
1/2 2.2 2.0 1.0 0.8 0.9
3
/4 3.1 2.8 1.4 1.1 1.2
1 4.0 3.6 1.8 1.4 1.5
2 7.5 6.8 3.4 2.7 2.7
3 10.7 9.6 4.8 3.9 3.8
5 16.7 15.2 7.6 6.1 6.3
10 31 28 14 11 11.2
15 46 42 21 17 16.6
20 59 54 27 22 21.6
25 75 68 34 27 26.6
30 88 80 40 32 32.4
40 114 104 52 41 43.2
50 143 130 65 52 52
60 170 154 77 62 64
75 211 192 96 77 80
100 273 248 124 99 103
125 342 312 156 125 130
150 396 360 180 144 150
200 528 480 240 192 200
①
When motor service factor is greater than 1,
increase full load amps proportionally.
Example: If service factor is 1.15, increase
above amp values by 15%.
3 Phase kVA =
Volts x Amps x 1.73
1000
NOTE: If motors are started more than once per
hour, increase minimum transformer kVA by 20%.
①
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ACME ELECTRIC • MILWAUKEE, WI • 800.334.5214 • acmepowerdist.com
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