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#101699 - 11/29/02 11:20 AM Questions & Answers about Buck-Boost Transformers
Tony Moscioni Offline
Registered: 05/15/01
Posts: 144
Questions & Answers about Buck-Boost Transformers

The following pages contain the answers to the most commonly asked questions about dry type distribution transformers. If you need further technical information about Acme transformers please contact our technical services department at 1-800-334-5214. Our highly trained representatives will be happy to answer your application questions.
1) What is a buck-boost transformer?
Buck-boost transformers are small single phase transformers designed to reduce (buck) or raise (boost) line voltage from 5-20%. The most common example is boosting 208 volts to 230 volts, usually to operate a 230 volt motor such as an air-conditioner compressor, from a 208 volt supply line.
Buck-boosts are a standard type of single phase distribution transformer, with primary voltages of 120, 240 or 480 volts and secondaries typically of 12, 16, 24, 32 or 48 volts. They are available in sizes ranging from 50 volt amperes to 10 kilo-volt amperes.
Buck-boost transformers are shipped ready to be connected for a number of possible voltage combinations.
2) How does a buck-boost transformer differ from an insulating transformer?
A buck-boost transformer IS an insulating type transformer when it is shipped from the factory. When it is connected at the job site, a lead wire on the primary is connected to a lead wire on the secondary - thereby changing the transformer's electrical characteristics to those of an autotransformer. The primary and secondary windings are no longer "insulated" and secondary windings are no longer "insulated" and its KVA capacity is greatly increased.
3) What is the difference between a buck-boost transformer and an autotransformer?
When a primary lead wire and secondary lead wire of a buck-boost transformer are connected together electrically, in a recommended voltage bucking or boosting connection, the transformer is in all respects, an autotransformer. However, if the interconnection between the primary and secondary winding is not made, then the unit is an insulating type transformer.
4) Why are they used?
Electrical and electronic equipment is designed to operate on standard supply voltage. When the supply voltage is constantly too high or too low, (usually more than 55%), the equipment fails to operate at maximum efficiency. A buck and boost transformer is a simple and ECONOMICAL means of correcting this off-standard voltage.
5) What are the most common applications for buck-boost transformers?
Boosting 208V to 230V or 240V and vice versa for commercial and industrial air conditioning systems; boosting 110V to 120V and 240V to 277V for lighting systems; voltage correction for heating systems and induction motors of all types. Many applications exist where supply voltages are constantly above or below normal.
6) Can buck-boost transformers be used to power low voltage circuits?
Yes, low voltage control, lighting circuits, or other low voltage applications requiring either 12V, 16V, 24V, 32V or 48V. The unit is connected as an insulating transformer and the nameplate KVA rating is the transformer's capacity.
Operation and Construction
7) Why do buck-boost transformers have 4 windings?
To make them versatile! A four winding buck-boost transformer (2 primary and 2 secondary windings) can be connected eight different ways to provide a multitude of voltage and KVA outputs. A two winding (1 primary and 1 secondary) buck-boost transformer could be connected only two different ways.
8) Will a buck-boost transformer stabilize voltage?
No. The output voltage is a function of the input voltage. If the input voltage varies, then the output voltage will also vary by the same percentage.
Load Data
9) Are there any restrictions on the type of load that can be operated from a buck-boost transformer?
No, there are no restrictions.
10) Why can a buck-boost transformer operate a KVA load many times larger than the KVA rating on its nameplate?
Since the transformer has been autoconnected in such a fashion that the 22V secondary voltage is added to the 208V primary voltage, it produces 230V output.
The autotransformer KVA is calculated:
Output Volts x Secondary Amps
230 V x 41.67 Amps = 9.58 KVA
11) Can buck-boost transformers be used on motor loads?
Yes, either single or three phase. Refer to the motor data charts in "How to Select a Transformer" for determining KVA and amps required by NEMA standard motors.
12) How are single phase and three phase load amps and load KVA calculated?
Single Phase Amps=
KVA x 1000
Three Phase Amps=
KVA x 1000
Volts x 1.73
Single Phase KVA=
Volts x Amps
Three Phase KVA=
Volts x Amps x 1.73
Three Phase
13) Can buck-boost transformers be used on 3-phase systems as well as single phase systems?
Yes. A single unit is used to buck or boost single phase voltage - two or three units are used to buck or boost three phase voltage. The number of units to be used in a 3-phase installation depends on the number of wires in the supply line. If the 3-phase supply is 4 wire Y, use three buck- boost transformers. If the 3-phase supply is 3 wire Y (neutral not available), use two buck-boost transformers.
14) Should buck-boost transformers be used to develop a 3-phase 4 wire Y circuit from a 3-phase 3 wire delta circuit?
No. A three phase "wye" buck-boost transformer connection should be used only on a 4 wire source of supply. A delta to wye connection does not provide adequate current capacity to accommodate unbalanced currents flowing in the neutral wire of the 4 wire circuit.
Input (Supply System) Desired Output Connection
Delta 3 Wire Wye 3 or 4 Wire DO NOT USE
Open Delta 3 Wire Wye 3 or 4 Wire DO NOT USE
Wye 3 or 4 Wire Closed Delta 3 Wire DO NOT USE
Wye 4 Wire Wye 3 or 4 Wire OK
Wye 3 or 4 Wire Open Delta 3 Wire OK
Closed Delta 3 Wire Open Delta 3 Wire OK
15) Why isn't a closed delta buck-boost connection recommended?
A closed delta buck-boost autotransformer connection requires more transformer KVA than a "wye" or open delta connection and phase shifting occurs on the output. Consequently the closed delta connection is more expensive and electrically inferior to other three-phase connections.
Connection and Frequency
16) How does the installer or user know how to connect a buck-boost transformer?
The connection chart packed with each unit shows how to make the appropriate connections.
17) Can 60 Hertz buck-boost transformers be used on a 50 Hertz service?
No. Acme buck-boost transformers should be operated only at the frequencies recommended. However, units recommended for 50 cycle operation are suitable for 60 cycle operation but not vice versa.
18) Refer to the selection steps in the buck-boost section of the Acme catalog.
Nameplate Data
19) Why are buck-boost transformers shipped from the factory as insulating transformers and not preconnected at the factory as autotransformers?
A four winding buck-boost transformer can be auto connected eight different ways to provide a multitude of voltage and KVA output combinations. The proper transformer connection depends on the user's supply voltage, load voltage and load KVA. Consequently, it is more feasible for the manufacturer to ship the unit as an insulating transformer and allow the user to connect it on the job site in accordance with the available supply voltage and requirements of his load.
20) Why is the isolation transformer KVA rating shown on the nameplate instead of the autotransformer KVA rating?
The KVA rating of a buck-boost transformer when auto connected depends on the amount of voltage buck or boost. Since the amount of voltage buck or boost is different for each connection, it is physically impossible to show all of the various voltage combinations and attainable KVA ratings on the nameplate. A connection chart showing the various attainable 1-phase and 3-phase connections is packed with each unit.
21) Do buck-boost transformers present a safety hazard usually associated with autotransformers?
No. Most autotransformers, if they are not of the buck-boost variety, change voltage from one voltage class to another. (Example 480V to 240V) In a system where one line is grounded, the user thinks he has 240V, yet due to the primary and secondary, being tied together, it is possible to have 480V to ground from the 240V output. A buck-boost transformer only changes the voltage a small amount, such as 208V to 240V. This small increase does not represent a safety hazard, as compared to a buck of 480V to 240V.
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 K-VA - 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 AUTOTRANSFORMER. 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).
An autotransformer changes or transforms only a portion of the electrical energy it transmits. The rest of the electrical energy flows directly thru 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 autotranskormer is smaller, fighter in weight, and less costly than a comparable KVA size insulating transformer.
Please refer to Question 27 for additional information on autotransformers.
Buck-boost transformers are frequently field-connected as autotransformers.
26) Buck-boost transformers are almost always installed as autotransformers. 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.
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 every day application is always a good way to explain the intent of the "Code." Example: A 1 KVA transformer Catalog No. T-1-11683 has a primary of 120 x 240V and a secondary of 12 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
Steps for Selecting The Proper Buck-Boost Transformer
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.
Request a copy of Acme's General catalog ATD-01.
ACME Electric
4815 West 5th Street
Lumberton, NC 28358
Fax: (910)739-0024
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