200V to 230V Buck-Boost Transformers
At 200 volts, a 230 volt motor is running 13 percent below nameplate, outside the plus or minus 10 percent NEMA MG-1 permits. 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
- 230V
- Correction
- Boost (raise voltage) · 15.0%
- Size from
- System phase and equipment nameplate amps
Technical details
200V to 230V technical overview
At 200 volts, a 230 volt motor is running 13 percent below nameplate, outside the plus or minus 10 percent NEMA MG-1 permits. A 200V to 230V buck-boost transformer applies a 15 percent boost and puts that motor back inside its design window. The pair comes up constantly on HVAC, pumping and compressed air equipment, because 230V is the standard nameplate for motors intended for a 240 volt system, and plenty of those motors end up on services that measure 200. Single-phase and three-phase, 10 through 60 amps.
Where 200V to 230V correction is used
Almost every load on this correction is a motor. The 230 volt nameplate is what motor manufacturers stamp on machines built for a 240 volt system, and the equipment carrying it is the equipment that keeps a building running.
- Condensing units, heat pumps and rooftop package units with 230V compressors
- Reciprocating and rotary screw air compressors in shops and plants
- Submersible well pumps, booster pumps and irrigation pumps
- Hydraulic power units on presses, balers and dock lifts
- Refrigeration racks, walk-in condensers and ice machines
- Gearmotors on conveyors, augers and mixers
The supply side is usually one of three things: an older 200 volt industrial distribution system, a 208 volt service that sags to 200 under load, or a rural and end-of-line service that never reaches nominal. Contractors also meet this pair on replacement work, when equipment specified for a 240 volt residential-style supply is installed in a light commercial building fed at 208.
Why this 200V to 230V voltage pair matters
200 volts is 13 percent below a 230 volt nameplate, and NEMA MG-1 stops at 10 percent. Outside that band the manufacturer makes no performance guarantee at all.
The mechanics are simple. A motor driving a fixed mechanical load must deliver the same shaft power regardless of the supply, so when voltage falls the current climbs to compensate. Winding heating rises with the square of that current, and the rule of thumb used across the industry is that every additional 10 degrees C of winding temperature halves insulation life. Torque moves the other way: starting and breakdown torque fall with the square of voltage, so at 200 volts a 230V motor develops about 76 percent of rated starting torque.
In the field that looks like compressors stalling against head pressure on a hot afternoon, overload relays tripping an hour into a run, contactor coils chattering, and motors failing years early with no obvious cause. The 15 percent boost from 200 to 230 removes the cause rather than the symptom.
Installation notes
Sizing guidance
Size from the motor nameplate, not the breaker. A 30 amp breaker ahead of an 18 amp compressor does not mean you need the 30 amp correction; it means the branch circuit was sized for motor starting under NEC Article 430. Use the full-load amps printed on the equipment, total everything running through the transformer at once, and pick the next standard rating up from 10, 15, 20, 30, 40, 50 or 60 amps.
Phase follows the load. Three-phase compressors and pumps take the three-phase correction; a single-phase condensing unit takes the single-phase unit.
Inrush does not drive the selection, because a buck-boost transformer handles only the difference between input and output, roughly 15 percent of the load's power on this pair, and is normally selected on continuous full-load current. Do revisit overload settings afterward. Once the motor sees 230 volts its running current drops, and thermal heaters or electronic overloads set around the old current may need to be reset.
Installation notes
Verify the voltage before you buy. Measure line to line at the equipment disconnect while the building is loaded, not at the main at 7 a.m. A buck-boost transformer is a fixed-ratio device, so it will boost 208 volts to about 239 just as readily as it boosts 200 to 230.
Three-phase corrections normally use two transformers in an open delta connection on a three-wire supply. That connection derives no neutral and creates no separately derived system, so if the equipment needs 120 volts line to neutral for its controls, that has to come from somewhere else.
Because the unit is an autotransformer, the 230 volt output is not isolated from the 200 volt supply. Size conductors and overcurrent devices for the full load current on both sides, follow NEC Article 450 for transformer protection and 210.9 or 215.11 for autotransformers supplying branch circuits and feeders, and confirm the requirements with the AHJ before rough-in.
Common questions
- How much starting torque does a 230 volt motor lose at 200 volts?
A 230 volt motor operated at 200 volts develops roughly 76 percent of its rated starting torque, a loss of about a quarter. Induction motor starting and breakdown torque vary with the square of applied voltage, so a 13 percent voltage shortfall becomes a 24 percent torque shortfall. On loads that start under pressure or against inertia, such as compressors, augers and loaded conveyors, that deficit lengthens the start, drives current higher for longer, and is a common cause of nuisance overload trips.
- My motor is marked 208-230V. Do I still need a buck-boost transformer at 200 volts?
Possibly not. A motor nameplated 208-230V is rated for both systems, and the plus or minus 10 percent NEMA MG-1 tolerance applies to each rating, so a 200 volt supply sits about 3.8 percent below the 208 volt rating and is within tolerance. The 200 to 230 correction is aimed at equipment nameplated 230 volts only, or at supplies that fall meaningfully below 208. Read the nameplate before assuming a correction is needed.
- Will boosting from 200 to 230 volts reduce my motor's running current?
Yes, in most cases. A motor driving a constant mechanical load draws current roughly in inverse proportion to applied voltage, so restoring 230 volts to a motor that has been running at 200 typically lowers running amps and reduces winding temperature. Because the current changes, check the overload relay setting or heater selection after the correction so the motor remains properly protected at its new operating current.
- Do I need two or three transformers for a three-phase 200 to 230 correction?
Three-phase buck-boost corrections on a three-wire supply are most often made with two transformers connected open delta, which is the standard arrangement for this type of voltage correction. Three units are used where the connection or the correction ratio requires it. The open delta connection corrects line-to-line voltage only; it does not derive a neutral and does not create a separately derived system.
- Can a buck-boost transformer stop the voltage dip when a large motor starts?
No. A buck-boost transformer applies a fixed ratio and does not regulate, so a starting dip on the supply appears on the output scaled by that same ratio. What the correction does is raise the entire operating band, so the bottom of the dip starts from a higher point and is less likely to drop equipment below its undervoltage threshold. Persistent deep dips point to conductor, service or transformer capacity issues that need their own fix.