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.

On this page: Selection Steps, Fusing Buck-Boost Transformers, Applications, Accessories 

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.

Applications

Ideal for low voltage lighting control applications

Sola/Hevi-Duty buck-boost transformers are designed to supply power to low voltage lighting circuits, control panels or other systems requiring 12, 16, 24, 32, or 48 Volts. When connected as an insulating transformer (by following the wiring diagram on the inside of the transformer case), the transformer's capacity matches the nameplate KVA rating.

Sola/Hevi-Duty buck-boost transformers are also suited for low voltage landscape lighting. They are UL listed for outdoor service and their compact size makes them the perfect solution for providing power to accent lighting applications. Electrical Connection diagrams are shown in the attached pdf.

When using dimmers for low voltage applications, use only dimmers designed and rated for use with magnetic loads. We strongly recommend contacting the dimmer manufacturer for advice on your specific lighting application.

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. Refer to the Selection Tables.

Three phase, buck-boost applications require two or three transformers. Check the "Quantity Required" column of the Three Phase Selection Tables for the exact quantity.

Fusing Buck-Boost Transformers

For determining the correct size of breaker or fuse for a given range of input or output ampere ratings, refer to Section 450-4, of the National Electric Code (NEC).

Accessories

• Surge Suppression devices provide additional protection and longevity to any electronic equipment. 

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