206V to 220V Buck-Boost Transformer
Two shortfalls stack up in this one, and each looks tolerable on its own. Choose the system phase and equipment nameplate amps below to see an available, buyable unit.
Load current (amps)
Quick answer
- Measured input
- 206V
- Required output
- 220V
- Correction
- Boost (raise voltage) · 6.8%
- Size from
- System phase and equipment nameplate amps
Technical details
206V to 220V technical overview
Two shortfalls stack up in this one, and each looks tolerable on its own. The first is structural: 208V and 220V are different systems, so a 220V load on a 208V wye service starts 5.5% low on the day it is installed. The second is load dependent: that service reads 206V by mid-shift, and now the load is 6.4% low. A 206V to 220V buck-boost transformer adds the 14 volts back with a boost of roughly 7%, correcting both at once. Single-phase and three-phase configurations are offered in 10, 15, 20, 30, 40, 50 and 60 amp selections.
Where 206V to 220V correction is used
This pair belongs to process equipment that runs in cycles, where being slightly low is not a fault but a slow tax on throughput. It comes up in commercial bakeries with deck ovens and proofers, laundries running dryers, ironers and extractors back to back, food processing with fryers, kettles and heat sealers, plastics operations where barrel heaters, hot runners and thermoformers hold a setpoint all shift, print and packaging with shrink tunnels, laminators and curing lamps, electronics assembly running reflow and wave solder profiles, and small metal finishing with rectifiers and drying ovens.
The pattern is a plant that meets its numbers on a quiet Monday and misses them on a busy Thursday. Cycle times creep, ovens recover more slowly between loads, the seal that used to hold does not, and product quality drifts rather than failing outright. Because nothing throws a hard fault, the supply voltage is usually the last thing anyone checks, and the machine builder gets the phone call first.
Why this 206V to 220V voltage pair matters
220V is a real specification, not a rounded-off version of 208V or 240V. Machine builders that state 220V plus or minus 5% are describing a window of 209V to 231V, so a service measuring 206V is already outside it before any allowance for drop within the machine. Two things follow. Heating elements deliver power in proportion to the square of applied voltage, so an element fed 206V produces about 12% less heat than it does at 220V, which is why setpoints are reached but recovery between loads takes longer. Motor-driven auxiliaries on the same machine move the other way, drawing more current at reduced voltage and running closer to their overload settings all shift. Correcting 206 to 220 puts the machine back inside the window its builder specified, which also removes the supply from the list of suspects during any service call.
Installation notes
Sizing guidance
Resistive loads size differently from motors here, and getting it backwards leads to an undersized selection. A motor draws more current when voltage is low. A heater draws less, because current follows voltage through a fixed resistance. The clamp reading taken today at 206V therefore understates what the machine will pull once it sees 220V, so size on the nameplate full-load current at 220V rather than on today's measurement.
From there: confirm phase; total the nameplate current for everything fed through the same disconnect, including barrel and band heaters, drive motors, vacuum pumps and controls; add 25% for anything running three hours or more, which covers most process heat; then round up to the next offered amperage among 10, 15, 20, 30, 40, 50 and 60. The transformer handles only the 14 volt difference rather than the machine's full power, so the correct unit is a fraction of the machine's own rating.
Installation notes
On a 208Y/120 service the building's 120V circuits are derived from the wye neutral, and a buck-boost transformer correcting phase-to-phase voltage does not lift that neutral with it. Do not take 120V control power, receptacles or lighting from the corrected side. Control power comes from ahead of the correction or from a separate control transformer.
The unit is an insulating transformer reconnected as an autotransformer, so there is no isolation between input and output and no separately derived system is created. Three-phase corrections are made open delta using two units, correcting line-to-line voltage only.
Size conductors and overcurrent protection on both sides per NEC Article 450 and the branch-circuit rules, allowing for input current that exceeds output current on a boost connection. Verify the incoming voltage before ordering, since 206V and 208V select different units, and confirm the corrected reading at the machine terminals during a normal production cycle.
Common questions
- Can 220V equipment run on a 208V service?
It will usually run, but 5.5% below its nameplate, and less than that once the service sags under load. Equipment specified as 220V plus or minus 5% has a lower limit of 209V, so a 208V system is marginal at nominal and out of specification at any reading below 209V. Resistive heat is the first thing to suffer, since heat output falls with the square of applied voltage.
- Why does my oven or heater take longer to reach temperature on a 208V service?
Because a resistive heating element produces power in proportion to the square of the voltage applied to it. An element rated for 220V and fed 206V produces roughly 12% less heat, so it still reaches setpoint but takes longer to get there and recovers more slowly between loads. The controller compensates by calling for heat more of the time, which is why the symptom looks like a slow process rather than a failure.
- Does boosting from 206V to 220V increase the current drawn from the service?
Yes, on the supply side. Power is conserved through the transformer, so raising the load voltage means the current drawn from the 206V source is higher than the current the load itself draws at 220V. On a resistive load the effect compounds, since the load also draws more current at the higher voltage. This is why the input conductors and overcurrent protection are sized for the input side of a boost connection.
- Is a reading of 206V a utility problem I should report?
Not necessarily. ANSI C84.1 places the Range A service voltage window for a nominal 208V system between 197V and 218V, so 206V is a normal delivered voltage and the utility is under no obligation to raise it. The mismatch is between a 208V system and equipment built to a 220V nameplate, which is a correction to be made on the customer side rather than a service complaint.