245V to 220V Buck-Boost Transformer

245V to 220V technical overview

A service that measures 245V is delivering roughly 11% more voltage than a 220V nameplate expects, and a 245V to 220V buck-boost transformer is the standard way to close that gap. The unit is an insulating transformer reconnected as an autotransformer, wired to subtract about 25 volts from the incoming supply so the equipment receives the voltage it was designed for. Because it processes only the difference between 245 and 220 rather than the full load power, it stays physically small and costs far less than an isolation transformer sized for the same connected load.

Where 245V to 220V correction is used

The 245 to 220 correction shows up wherever imported or older machinery lands on a modern US 240V service. Equipment built to IEC conventions is commonly nameplated 220V, and it does not care that the US nominal is 240V. It sees whatever the building actually delivers, which on a well-regulated or lightly loaded feeder is often 243 to 248V.

• European and Asian machine tools, CNC routers, lathes and mills brought in through equipment dealers or auctions
• Imported packaging, printing, textile and woodworking machinery specified at 220V single-phase or three-phase
• Laboratory, dental and imaging equipment built for 220V markets
• Older domestic equipment from the era when 220V was the common US nameplate, still earning its keep in shops and plants
• Commercial laundry, food service and refrigeration units bought overseas or supplied by an OEM using a 220V electrical package

Machine shops, contract manufacturers, universities and small plants hit this most often, usually right after an install, when a new machine trips or runs hot on a service that tests perfectly fine by utility standards.

Why this 245V to 220V voltage pair matters

Correcting 245 to 220 matters because 245V sits outside the tolerance band the equipment was designed around. NEMA MG-1 allows a motor to operate at plus or minus 10% of nameplate, which puts the ceiling for a 220V motor at 242V. A 245V supply is past that line before any load-side variation is counted, and MG-1 is explicit that operation at the limits is permitted rather than optimal: torque, efficiency and winding temperature all move the wrong way.

What follows is predictable. Motor core loss climbs faster than the voltage causing it, so laminations and windings run hotter with no additional work performed. Contactor and relay coils, control transformers and switch-mode power supplies age faster under sustained overvoltage. Resistive elements dissipate about 24% more power at 245V than at 220V, which cooks their controllers and shortens element life. None of this fails on day one. It surfaces as premature coil failures, nuisance overload trips and a machine that never quite reaches its rated service life.

Sizing guidance

Size a 245 to 220 buck-boost by load current, not by the horsepower of the largest motor alone. Two decisions drive the selection: phase and amperage. Single-phase and three-phase versions are both offered, with amperage choices of 10, 15, 20, 30, 40, 50 and 60 amps.

Measure before you order. Record the actual line-to-line voltage with a true-RMS meter at the point where the transformer will land, ideally during a light-load period when the reading runs highest, and collect the nameplate full-load amps of everything the transformer will feed. Add those currents, then choose the next amperage step above the total so the unit is not sitting at its limit during motor starting or peak production.

The counterintuitive part is that the transformer's own rating is far smaller than the load it supports, because it handles only the 25-volt difference. That is the entire reason the buck-boost approach exists instead of a full isolation transformer.

Installation notes

A buck-boost transformer becomes an autotransformer once it is reconnected, so the 220V output shares a conductive path with the 245V input. There is no galvanic isolation and no separately derived system. If the application requires isolation for noise, grounding or safety reasons, a buck-boost is the wrong device and an isolation transformer is the right one.

Three-phase corrections are normally made open delta with two units. That arrangement corrects all three line-to-line voltages but does not create a neutral, and it cannot turn a 3-wire delta source into a 4-wire wye system. If the machine needs a neutral for 220V single-phase control power, that neutral has to come from a separate control transformer.

Size overcurrent protection and conductors for the line current on each side rather than for the transformer's own small rating, and follow the supplied connection diagram exactly. Re-measure the output under real load after energizing. A wiring choice that looks right on paper takes one minute to confirm with a meter.