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Buck-Boost Transformers

Buck-Boost transformers are small, single phase, dry type distribution transformers designed and shipped as insulating/isolating transformers. They have a dual voltage primary and a dual voltage secondary. These transformers can be connected for a wide range of voltage combinations. The most common use is to buck (lower) or boost (raise) the supply voltage a small amount, usually 5 to 27%. Buck-Boost transformers are in compliance with NEC Article 210-9, Exception 1 when field connected as an autotransformer.

The major advantages of Buck-boost transformers are their low cost, compact size and light weight. They are also more efficient and cost less than equivalent isolation transformers. When connected as an autotransformer, they can handle loads up to 20 times the nameplate rating. A buck-boost transformer is the ideal solution for changing line voltage by small amounts.

When a buck-boost has the primary and secondary windings connected, per recommended instructions, it becomes an autotransformer. Now only the secondary windings are transforming voltage and current. The majority of the KVA load passes directly from the supply to the load. This is why buck-boost transformers can supply a load with a much larger KVA rating than the nameplate indicates.

Selection Steps

  1. Input Line Voltage
    Measure the supply voltage with a voltmeter.

  2. Voltage Required for the Load
    Check the load equipment to determine the voltage requirement.

  3. KVA or Ampere Rating of the Load
    Find either the load KVA or the load amperage requirements. This information is listed on the nameplate of the load equipment.

  4. Frequency
    Either 50 or 60 Hz. The frequency of the transformer must match the frequency of the load.

  5. Number of Phases
    Single or three phase line and load must match. (A transformer cannot convert single to three phase.) A common application is to make a single phase connection from a three phase supply by using one phase of the three phase supply circuit. Be careful not to overload that phase of the three phase supply. For buck-boost applications the supply must provide load KVA - not just the nameplate rating of the buck-boost.

  6. Refer to Selection Table
    - Single Phase Selection Tables
    - Three Phase Selection Tables

PDF Files

Selection Steps
Specifications/Low Voltage Electrical Connections
Single Phase Selection Tables
Three Phase Selection Tables
Buck-Boost Electrical Connections
Application Note
Operation & Maintenance Recommendations


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