200V to 220V Buck-Boost Transformers
A 200V to 220V buck-boost transformer raises a 200 volt supply by 10 percent so equipment built to a 220 volt nameplate sees what it was designed for. Choose the system phase and equipment nameplate amps below to see an available, buyable unit.
Load current (amps)
Quick answer
- Measured input
- 200V
- Required output
- 220V
- Correction
- Boost (raise voltage) · 10.0%
- Size from
- System phase and equipment nameplate amps
Technical details
200V to 220V technical overview
A 200V to 220V buck-boost transformer raises a 200 volt supply by 10 percent so equipment built to a 220 volt nameplate sees what it was designed for. Two situations produce this pair. The first is an older industrial building still distributing a legacy 200 volt system. The second is a machine specified at 220V landing on a 208V service that measures closer to 200 under load. Both single-phase and three-phase corrections are offered, in load ratings of 10, 15, 20, 30, 40, 50 and 60 amps.
Where 200V to 220V correction is used
The 200 volt to 220 volt correction shows up in buildings and equipment that predate the 208Y/120 standard, and in imported machinery specified for a 200 volt domestic supply but built around 220 volt components. Older plants still distribute 200 volts to machine tools, and the control transformers, brake coils and resistance heaters inside those machines were frequently ordered for 220.
Typical loads on this correction:
- Machine tools with 220V spindle motors, coolant pumps and brake solenoids
- Imported molding auxiliaries, chillers and dryers marked 220V
- Older welders and resistance heating equipment carrying a 220V input rating
- Commercial laundry extractors and dryers on legacy 200 volt panels
- Packaging and conveyor drives with 220V contactor coils
- Farm and fleet shop equipment fed from a long 200 volt service
The common thread is that the equipment is not adjustable. There is no tap to move, no dual-voltage jumper on the terminal block, and replacing internal components costs more than correcting the supply that feeds them.
Why this 200V to 220V voltage pair matters
At 200 volts, a 220 volt load runs 9.1 percent below nameplate, right at the edge of the plus or minus 10 percent that NEMA MG-1 permits for motors. Being inside the limit is not the same as being fine. MG-1 allows operation there; it does not promise rated efficiency, rated temperature rise or rated torque at the extremes.
A motor holding constant torque at 9 percent low voltage draws proportionally more current, and that current becomes winding heat. Starting and breakdown torque fall with the square of voltage, so a 220V motor on 200 volts develops roughly 83 percent of rated starting torque. On a loaded conveyor or a compressor starting against head pressure, that is the difference between a clean start and a long, hot one that runs the overload out. Non-motor loads suffer differently. A 220V heating element gives up about 17 percent of its output, and a 220V contactor coil sits near its dropout threshold, so the momentary dip when a large motor starts on the same panel is enough to open it.
Installation notes
Sizing guidance
Two decisions: phase and amperage. Phase follows the load, not the building. A three-phase 220V machine on a three-phase 200 volt service needs the three-phase correction even if the same panel also feeds single-phase loads. A single-phase 220V load takes the single-phase unit.
Amperage comes from the load current at the corrected voltage, not from the breaker. Read full-load amps off the equipment nameplate, total everything that will run through the transformer at the same time, and select the next rating up from 10, 15, 20, 30, 40, 50 or 60 amps. Leave headroom if another machine is planned.
The reason a physically small transformer supports a large load is that a buck-boost only processes the difference between input and output. On a 10 percent boost, roughly a tenth of the load's power passes through the windings and the remainder flows straight through the autotransformer connection. That is why this correction costs a fraction of an isolation transformer rated for the same load.
Installation notes
A buck-boost transformer is an insulating transformer reconnected as an autotransformer. Input and output share a winding, so there is no isolation between the 200 volt supply and the 220 volt output. If the application requires galvanic separation, this is the wrong device.
For three-phase work, two transformers wired open delta is the standard connection on a three-wire supply. That connection derives no neutral and creates no separately derived system, so it cannot turn a three-wire delta into a four-wire wye. Size conductors and overcurrent protection for the actual line current, which is the load current rather than a fraction of it. Autotransformer branch-circuit and feeder rules appear in NEC 210.9 and 215.11, and transformer overcurrent protection in Article 450; verify against the code cycle your AHJ enforces. Finally, measure the supply line to line under real load before ordering. The correction is a fixed ratio, so it boosts whatever it is given.
Common questions
- Will a 200V to 220V buck-boost transformer work on a 208 volt service?
A 200V to 220V buck-boost transformer boosts whatever voltage it is fed by about 10 percent, so a 208 volt input produces roughly 229 volts rather than 220. Buck-boost transformers are fixed-ratio devices with no regulation, so the correct unit is chosen from the voltage measured at the panel under real load. If a nominal 208 volt service reads near 200 when the building is working, the 200 to 220 correction fits. If it holds at 208, a smaller boost is the right choice.
- Is 200 volts too low for a 220 volt motor?
200 volts is 9.1 percent below a 220 volt nameplate, just inside the plus or minus 10 percent operating band NEMA MG-1 allows for motors. The motor will run, but at the edge of that band it draws more current for the same mechanical work, runs hotter, and develops about 83 percent of rated starting torque because torque falls with the square of applied voltage. Sustained operation there shortens winding life and produces nuisance overload trips on hard-starting loads.
- Can one buck-boost transformer feed several 220 volt machines?
Yes. A buck-boost transformer can correct an entire panel or subpanel provided its amperage rating covers the total current of everything running through it at the same time. Add the full-load amps from each nameplate, apply the demand you realistically expect, and select the next rating up. Correcting once at the panel is normally cheaper and simpler than installing a separate unit at every machine.
- Does a 200V to 220V buck-boost transformer isolate the load from the supply?
No. Buck-boost transformers are insulating transformers reconnected as autotransformers, which means input and output share a common winding and remain electrically connected. There is no galvanic isolation, no derived neutral and no separately derived system. Applications that need true isolation, a new neutral, or a ground reference independent of the source require an isolation transformer instead.