SECTION
BUCK-BOOST TRANSFORMERS
ACME ELECTRIC
•
MILWAUKEE, WI
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800.334.5214
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acmetransformer.com
92
Sound Levels
22. 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).
Cost and Life Expectancy
23. 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.
24. 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.
National Electrical Code
25. Your catalog indicates that a buck-boost
transformer is suitable for connecting as an
AUTOTRANS-FORMER. 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). This
is illustrated in the drawing below.
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 auto-
transformers.
Buck-boost transformers are frequently field-connected as
autotransformers.
26. 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.
27. When a buck-boost transformer is
connected as an autotransformer such as
boosting 208V to 230V, the kVA is greatly
increased. What is the procedure for determining
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.
400VOLTS
LINETOGROUND
(POTENTIALSAFETY
HAZARD)
480V
INPUT
240VOUTPUT
LINETOLINE
PRIMARY
(INPUT)
PRIMARY
(INPUT)
SECONDARY
(OUTPUT)
SECONDARY
(OUTPUT)
AUTOTRANSFORMER
ISOLATION
TRANSFORMER
(ALSO CALLED
“INSULATING TRANSFORMER”)
SHUNTWINDING(S)
B
A
Diagram 450-4
