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Q u e s t i o n s a n d A n s w e r s
24. How do you select a transformer to operate in an ambient higher than 40° centigrade?
When the ambient exceeds 40°C use the following chart for de-rating standard transformers.
Maximum Ambient Temperature Maximum Percentage of Loading
40°C (104°F) 100%
50°C (122°F) 92%
60°C (140°F) 84%
Instead of ordering custom built transformers to operate in ambients higher than 40°C, it is more economical to use a standard
transformer of a larger kVA rating.
25. Can transformers listed in this catalog be reconnected as autotransformers to increase their kVA rating?
Several standard single phase transformers listed in this catalog can be connected as autotransformers. The kVA capacity will
be greatly increased when used as an autotransformer, in comparison to the nameplate kVA as an insulating transformer.
Examples of autotransformer applications are changing 600 volts to 480 volts in either single phase or three phase; changing 480
volts to 240 volts single or three phase or vice versa; or the developing of a fourth wire (neutral) from a 480 volt three phase three
wire system for obtaining 277 volts single phase. This voltage is normally used for operating fluorescent lamps or similar devices
requiring 277 volts. For further details showing kVA and voltage combinations for various autotransformer connections refer to
Page 33 and 34 in this catalog.
26. Are ACME Transformers shown in this catalog U.L. Listed?
All of the transformers, with few exceptions, are listed by Underwriters’ Laboratories and have met their rigorous requirements.
We are also prepared to have transformers, which are not presently listed, submitted for listing to Underwriters’ upon the
customer’s request. Please contact the factory for details.
27. Is CSAcertification available for transformers shown in this catalog?
Most ACME transformers shown in this catalog are certified by Canadian Standards Association. They have been designed and
tested in accordance with the latest specifications. Please contact the factory if further details are required.
28. What is BIL and how does it apply to transformers listed in this catalog?
BIL is an abbreviation for Basic Impulse Level. Impulse tests are dielectric tests that consist of the application of a high frequency
steep wave front voltage between windings, and between windings and ground. The Basic Impulse Level of a transformer is a
method of expressing the voltage surge (lightning, switching surges, etc.) that a transformer will tolerate without breakdown. All
transformers manufactured in this catalog, 600 volts and below, will withstand the NEMA standard BIL rating, which is 10 KV. This
assures the user that he will not experience breakdowns when his system is properly protected with lightning arrestors or similar
surge protection devices.
29. What is polarity, when associatedwith a transformer?
Polarity is the instantaneous voltage obtained from the primary winding in relation to the secondary winding. Transformers 600 volts
and below are normally connected in additive polarity — that is, when tested the terminals of the high voltage and low voltage
windings on the left hand side are connected together, refer to diagram below. This leaves one high voltage and one low voltage
terminal unconnected. When the transformer is excited, the resultant voltage appearing across a voltmeter will be the sum of the
high and low voltage windings. This is useful when connecting single phase transformers in parallel for three phase operations.
Polarity is a term used only with single phase transformers.
H1 H2
x2 x1
120 VOLT
OUTPUT
240 VOLT
INPUT
VOLT
METER
360
VOLT
READING
ADDITIVE
POLARITY