208V to 218V Buck-Boost Transformer
On a 208V panel the question is rarely whether to correct, it is how far. 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
- 218V
- Correction
- Boost (raise voltage) · 4.8%
- Size from
- System phase and equipment nameplate amps
Technical details
208V to 218V technical overview
On a 208V panel the question is rarely whether to correct, it is how far. 208 to 218 is the smallest step available, roughly 4.8%, and it is the right answer when the circuit still carries equipment that is genuinely happy at 208V. A larger boost toward 230 or 240 fixes one machine and overdrives everything else sharing the feeder. This correction lifts a low or sagging 208V supply back to the top of its own range without pushing any 208V-rated load outside its ratings. Single-phase and three-phase, in 10, 15, 20, 30, 40, 50 and 60 amp selections.
Where 208V to 218V correction is used
208Y/120 is the standard commercial building service, so this pair belongs to shared panels rather than to dedicated machine feeds. It comes up in restaurant and commercial kitchen panels feeding combi ovens, dishwashers and refrigeration from the same busbar, in IT and telecom closets where rack power distribution units and cooling share a circuit, in retail and office space with rooftop unit circuits and small VFD panels, in elevator machine rooms, dental and medical office suites, laundromats and self-service equipment, and in older buildings whose service simply reads 202V to 206V most of the working day.
The defining situation is a mixed panel. One or two loads are nameplated 220V or 230V and complain, while everything else on the feeder is rated 208V and behaves. Correcting the whole panel by 10% or more solves the complaint and creates new ones. A 5% lift moves the low equipment into range while leaving the 208V loads inside the window they were built for.
Why this 208V to 218V voltage pair matters
218V is the top of the Range A service voltage window ANSI C84.1 defines for a nominal 208V system, which runs from 197V to 218V. That makes it the highest target you can choose while every 208V-rated device downstream is still operating inside the range its manufacturer designed around. The practical ladder looks like this:
- A 5% correction to 218V suits mixed panels and services that read a few volts low, and keeps 208V equipment in range.
- A 10% correction toward 230V suits circuits where every load is nameplated 220V or 230V.
- Corrections beyond that belong on dedicated feeds to 240V equipment, with nothing 208V-rated downstream.
Without any correction, a panel sitting at 202V to 206V produces the intermittent kind of trouble: drives faulting on bus undervoltage during acceleration, contactors dropping out on inrush, compressors that restart poorly at peak demand, and heaters that hold setpoint but never quite recover between loads.
Installation notes
Sizing guidance
Establish the range of the supply before the size of the correction. A buck-boost transformer applies a fixed percentage rather than holding a setpoint, so a 5% correction turns a source that swings from 202V to 212V into an output that swings from 212V to 222V. Both ends stay inside what 208V and 230V equipment tolerate. The same source given a 10% correction produces 222V to 233V, which is fine for 230V loads and too high for 208V ones. Checking the high end of the supply, not just the low, is what makes the correction size defensible.
From there the selection is straightforward. Match phase to the load, with three-phase corrections using two units in open delta. Take running current at the correction point, which means the sum of everything downstream if the transformer goes at the panel, add 25% for continuous loads, and round up to the next offered amperage among 10, 15, 20, 30, 40, 50 and 60.
Installation notes
The neutral is the thing to watch on a 208Y/120 service. The building's 120V circuits are derived from the wye neutral, and a buck-boost transformer corrects phase-to-phase voltage without lifting that neutral with it. Do not feed a subpanel carrying 120V branch circuits from the corrected output, and take control power from ahead of the correction or from a dedicated control transformer.
The unit is an insulating transformer reconnected as an autotransformer, so input and output are not isolated and no separately derived system is created. Three-phase corrections are open delta with two units, correcting line-to-line voltage only.
Label the panel or enclosure so the boosted voltage is obvious to whoever works on it next, and record which loads sit downstream of the correction. Size conductors and overcurrent protection per NEC Article 450 and the branch-circuit rules, and confirm the measured input voltage before ordering, since a service reading 204V or 206V calls for a different selection than one reading 208V.
Common questions
- Is 218V within the ANSI C84.1 range for a 208V system?
Yes. ANSI C84.1 defines the Range A service voltage window for a nominal 208V system as 197V to 218V, with a utilization minimum of 191V. Boosting a 208V supply to 218V therefore lands at the top of the range the standard describes, which is why it is the largest correction that still leaves every 208V-rated load on the circuit operating inside its intended window.
- Why not boost 208V all the way to 240V?
Because of what else is on the circuit. A 208V to 240V correction is roughly 15% and is correct when every load downstream is nameplated 240V. Applied to a shared 208V panel, it pushes 208V-rated equipment about 15% above nameplate, beyond the plus or minus 10% NEMA MG-1 allows for motors, which increases core loss and shortens the life of coils and electronics. Choose the correction that suits the whole feeder, not just the one machine that complains.
- Does the output stay at 218V if the incoming voltage changes?
No. A buck-boost transformer applies a fixed ratio rather than regulating to a setpoint, so the output tracks the input. With a roughly 5% correction, an input of 202V produces about 212V and an input of 212V produces about 222V. Any load fed through it should tolerate the full range the supply actually produces, which is why the supply should be logged across a working day before the correction size is chosen.
- Can one buck-boost transformer correct a panel with both 208V and 220V equipment?
Yes, and a 5% correction is usually the reason it works. Boosting to 218V puts 220V nameplate equipment within about 1% of its rating while keeping 208V equipment roughly 5% above nominal, inside the plus or minus 10% band NEMA MG-1 allows. Larger corrections cannot serve both groups at once, and in that situation the 220V or 230V loads should be moved to a separately corrected circuit.