SECTION
BUCK-BOOST TRANSFORMERS
ACME ELECTRIC
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MILWAUKEE, WI
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800.334.5214
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acmetransformer.com
93
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.
28. I have noted the reprint of the N.E.C.
(National Electrical Code), Article 450-4 shown in
the previous question covering 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. T111683 has a primary of 120 x
240V and a secondary of12 x 24V. 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
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.
Steps for Selecting the Proper Buck-Boost Transformer
Four Step Selection
1. A series of LINE VOLTAGE and LOAD
VOLTAGE combinations are listed across the top of
each selection chart. Select a LINE VOLTAGE and LOAD
VOLTAGE combination from ANY of the charts that comes
closest to matching the LINE VOLTAGE and LOAD
VOLTAGE of your application.
2. Read down the column you have selected until you reach
either the LOAD kVA or LOAD AMPS of the equipment you
want to operate. You probably will not find the exact value of
LOAD kVA or LOAD AMPS so go to the next higher rating.
3. From this point, read across the column to the far left-
hand side and you have found the catalog number of
the exact buck-boost transformer you need. Refer to the
catalog number listing on page 103 for dimensions.
4. CONNECT the transformer according to the connection
diagram specified at the bottom of the column where you
selected YOUR LINE VOLTAGE and LOAD VOLTAGE
combination. Connection diagrams are found at the end of
this section.
This same connection information is packed with each
buck-boost transformer.
