SECTION
DRY-TYPE DISTRIBUTION TRANSFORMERS
ACME ELECTRIC
•
MILWAUKEE, WI
•
800.334.5214
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acmetransformer.com
11
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%.
1 Phase kVA = Volts x Amps
1000
NOTE:
If motors are started more than once per
hour, increase minimum transformer kVA by 20%.
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 volt-
age 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 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:
kVA = Volts x Amps
1000
(2)
To determine
Amperes
when kVA and volts are known:
Amps = kVA x 1000
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 kVA required.
1.3 amps x 120 volts = .156 kVA
1000
For each lighting fixture
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 trans-
former 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.
TABLE 1
Full Load Current in Amperes–
Single Phase Circuits
Min.
Horse- 115 V 208 V 230 V Trans-
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
TABLE 2
Full Load Amperes
Single Phase A.C. Motors
①
kVA 120V 208V 240V 277V 380V 440V 480V 600V
.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
