When and How to Use a Buck-Boost Transformer

Buck-boost transformers correct modest line voltage errors so equipment sees its rated voltage. They are available as isolating transformers (with separate windings) or autotransformers (single winding with taps). Use them for small percentage adjustments - commonly up to about 25% - and install them at the equipment or at the panel depending on whether you need point-of-use or feederwide correction. Always follow NEC and local codes and have a qualified electrician size and install the unit.

What a buck-boost transformer does

A buck-boost transformer provides small, economical corrections to supply voltage so equipment sees the voltage written on its nameplate. You can use one when incoming line voltage runs a bit high or low, when motor starting voltages need trimming, or when imported equipment expects a slightly different nominal voltage.

These units come as either true isolating transformers (separate primary and secondary windings) or as autotransformers (a tapped single winding). Isolating models provide galvanic isolation; autotransformer connections are more compact and cost-effective but do not isolate the load from the supply.

Typical applications and size choices

Buck-boost transformers are intended for relatively small adjustments - commonly up to about 25% of the nominal voltage . They aren't a replacement for a step-up/step-down isolation transformer or a full voltage regulator when large voltage shifts are required.

Choose a unit by matching the equipment nameplate voltage and the required VA (volt-ampere) rating. For sensitive loads, select an isolating buck-boost with a VA rating at or above the application's steady-state draw. For simple corrections (lighting ballasts, control circuits, small motors), a compact single-phase buck-boost is often sufficient. Three-phase options exist for larger HVAC or industrial equipment.

Installing: at the equipment or at the panel

You can install a buck-boost transformer at the equipment terminal or at the feeder/panel that supplies multiple pieces of equipment. Installing at the panel can correct voltage for several loads at once, but it requires proper sizing for the combined VA load. Installing at the equipment terminal lets you tailor the correction to a single device.

Autotransformer connections alter voltage by changing how primary and secondary windings are tied together. Isolating connections keep windings separate and maintain isolation. Make sure you know which configuration you are buying and that it matches the desired outcome.

Safety, code, and sourcing

Have a qualified electrician size and install the transformer. Follow the National Electrical Code (NEC) and any local regulations for overcurrent protection, grounding, and enclosure ratings. Tighten connections to manufacturer torque specs, and confirm wire sizes for the expected current.

Buy from reputable suppliers and check datasheets for VA ratings, tap options, voltage ranges, and whether the unit is autotransformer or isolating. Many vendors provide wiring diagrams and installation instructions.

When to choose something else

If you need large voltage changes, isolation plus precise regulation, or full phase conversion, consider a larger isolation transformer, an automatic voltage regulator, or a phase converter instead of a buck-boost unit.

1. Confirm the typical maximum adjustment range for common buck-boost transformers (statement: "commonly up to about 25%") against current manufacturer datasheets and standards.