208V to 229V Buck-Boost Transformer
A 230V motor on a 208V service runs about 10 percent below nameplate, right at the edge of what NEMA MG-1 allows and past the point where the machine behaves the way its designer intended. Choose the system phase and equipment nameplate amps below to see an available, buyable unit.
Load current (amps)
Quick answer
- Measured input
- 208V
- Required output
- 229V
- Correction
- Boost (raise voltage) · 10.1%
- Size from
- System phase and equipment nameplate amps
Technical details
208V to 229V technical overview
A 230V motor on a 208V service runs about 10 percent below nameplate, right at the edge of what NEMA MG-1 allows and past the point where the machine behaves the way its designer intended. A 208V to 229V buck-boost transformer closes that gap, lifting the supply about 10 percent so 230V equipment sees essentially nominal voltage. This is the correction to reach for when the nameplate reads 230V rather than 240V, which covers most North American motors, pumps and compressors. The correction is offered in both single-phase and three-phase form, selected by the current the circuit carries.
Where 208V to 229V correction is used
This correction belongs to rotating equipment. Almost every general-purpose induction motor sold in North America carries a 230V or 208-230V nameplate, and the ones marked 230V alone are the ones that struggle on a 208Y/120 building service.
- Air compressors and vacuum pumps in shops, dental and medical suites, and light manufacturing.
- Booster, sump, transfer and circulating pumps in mechanical rooms and pump houses.
- Air handlers, fan coils, exhaust fans and cooling tower fans across commercial HVAC systems.
- Conveyors, mixers, grinders, saws and other machine tools, including imported equipment configured for a 230V supply.
- Elevator and dumbwaiter machine room equipment in older mid-rise buildings.
The complaint is rarely that the motor will not start. It is that a motor runs hot to the touch, an overload relay trips on humid afternoons, a compressor takes longer than it used to in order to build pressure, or a set of identical motors fails years earlier than the ones in a sister building across town that happens to sit on a 240V service.
Why this 208V to 229V voltage pair matters
Choose 229V when the nameplate says 230V. Torque in an induction motor varies with the square of applied voltage, so a 230V motor started at 208V develops only about 82 percent of its rated starting torque. Loaded machines that break away slowly stay in the high-current starting region longer, and that is where windings take their heat.
Running undervoltage costs more than starting torque. To deliver the same shaft power at lower voltage a motor draws more current, and the extra current shows up as winding temperature. As a rule of thumb, insulation life falls by roughly half for every additional 10 degrees C of sustained winding temperature. That is why the visible symptoms are nuisance overload trips, hot frames and a rewind schedule that arrives years early, rather than an outright refusal to run. Boosting 208 to 229 puts the machine back at nameplate voltage instead of merely inside tolerance.
Installation notes
Sizing guidance
Work from measured current, not from horsepower tables. Clamp the running amps on the existing circuit while the machine does its normal work, compare that against the nameplate full-load amps, and size the transformer at or above the higher of the two figures.
Current falls as voltage rises for a constant-power load, so a motor drawing more than its rated amps at 208V will draw less once corrected. Size on the pre-correction reading anyway and keep the headroom. Available ratings are 10, 15, 20, 30, 40, 50 and 60 amps.
Phase selection follows the motor, not the building. A three-phase motor on a 208Y/120 service takes the three-phase configuration, and a single-phase motor fed from two legs takes the single-phase configuration. Because a buck-boost transformer only handles the 21-volt difference between supply and load rather than the entire load, the unit that corrects a large motor circuit is physically small next to an isolation transformer of equivalent load capability.
Installation notes
Three-phase corrections use two coils in an open-delta arrangement. Open delta raises all three phase-to-phase voltages and produces no fourth wire, so a 208Y/120 service feeding a boosted 229V circuit keeps its neutral back at the source panel and the boosted circuit remains 3-wire. Any 120V control power comes from the original panel or from a separate control transformer.
Remember what the device is. A buck-boost transformer is an insulating transformer reconnected as an autotransformer, so input and output share a winding and the load is not isolated from the supply. Starters, disconnects and overload relays downstream still have to be rated for the corrected voltage.
Size conductors and overcurrent protection to the branch-circuit rules and NEC Article 450, and check the connection diagram against the unit before energizing. Verify the actual measured supply voltage first, because a service reading 204V or 205V calls for a different input selection.
Common questions
- Why boost 208V to 229V instead of all the way to 240V?
Because most North American motors are nameplated 230V, not 240V. Correcting 208V to 229V places a 230V motor within half a percent of nominal, while boosting the same circuit to 240V overshoots a 230V nameplate by about 4 percent, and by more whenever the building service floats high. Match the size of the correction to the nameplate on the equipment being fed.
- Will a buck-boost transformer reduce the amp draw of a motor running on 208V?
Usually yes. An induction motor delivering a fixed mechanical load draws current roughly in inverse proportion to applied voltage, so raising the supply from 208V to 229V typically lowers running current by around 8 to 10 percent. Lower current means lower winding temperature, fewer nuisance overload trips and longer insulation life. Size the transformer on the pre-correction measured current.
- Can a buck-boost transformer be installed on the input side of a VFD?
Yes, and it is sometimes the entire point of the installation. A variable frequency drive cannot put out more voltage than it is given, so a 230V motor fed by a drive on a 208V supply is limited to roughly 208V at full speed and loses torque near the top of its range. Boosting the drive input to 229V restores full motor voltage. Confirm the drive's rated input voltage range before boosting.
- Does a buck-boost transformer have to be rated for the full kVA of the load?
No. A buck-boost transformer only processes the difference between input and output voltage, so a unit correcting 208V to 229V handles roughly 10 percent of the load's power. That is why these transformers are far smaller and less expensive than isolation transformers serving the same load, and why they are selected by the line current they must pass rather than by the total kVA of the equipment.
- What actually happens to a 230V motor left running on 208V?
It runs, but it runs hot. At 208V a 230V motor is about 10 percent below nameplate, at the outer limit of the NEMA MG-1 tolerance. It draws higher current for the same shaft load, develops about 82 percent of rated starting torque, runs at a higher winding temperature and trips overloads on hot days. The usual outcome is not immediate failure but shortened winding life and repeat service calls.