208V to 236V Buck-Boost Transformer
A 208V to 236V buck-boost transformer raises a nominal 208V supply by roughly 13.5 percent so equipment nameplated for 240V runs inside its design range. 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
- 236V
- Correction
- Boost (raise voltage) · 13.5%
- Size from
- System phase and equipment nameplate amps
Technical details
208V to 236V technical overview
A 208V to 236V buck-boost transformer raises a nominal 208V supply by roughly 13.5 percent so equipment nameplated for 240V runs inside its design range. The 236V target sits just under 240V on purpose: a 240V heating element or contactor coil sees close to its rated voltage, and there is still headroom if the building service drifts high during light-load hours. On a 208Y/120 service this is the standard correction for 240V equipment that was specified, ordered and installed before anyone measured what the panel actually delivers. Single-phase and three-phase configurations are available in 10 through 60 amp ratings.
Where 208V to 236V correction is used
The 208 to 240 mismatch appears wherever the equipment is chosen from a catalog and the building is chosen by a landlord. Restaurants, strip malls, schools, medical offices and multi-tenant industrial condos are nearly always fed 208Y/120, while the equipment those tenants buy is built for 240V.
- Commercial kitchen equipment: fryers, convection and combi ovens, griddles, booster heaters and dishmachine tank heaters, all resistive and all of which lose output on 208V.
- Refrigeration: walk-in cooler and freezer condensing units, ice machines and remote compressor racks whose nameplates read 230/240V.
- Rooftop units and split-system condensers dropped onto an existing 208V roof feed during a tenant fit-out.
- Storage and point-of-use water heaters, duct heaters and process heat where recovery time is written into the spec.
- Imported and packaged machinery built around a 240V power and control scheme.
The pattern repeats in every case. The load runs, more or less, but never performs the way the submittal promised, and the fault gets pinned on a bad appliance rather than a supply that is 13 percent low.
Why this 208V to 236V voltage pair matters
Resistive loads make the clearest case for this correction. Power in a resistive element varies with the square of the applied voltage, so a 240V element energized at 208V produces about 75 percent of its rated wattage. At 236V it produces about 97 percent. That is the difference between a fryer that recovers between baskets and one that falls behind, or a water heater that never quite catches up through a lunch rush.
Motors suffer differently. NEMA MG-1 permits operation at plus or minus 10 percent of nameplate voltage, and a 240V motor on a 208V feed sits about 13 percent low, outside that allowance. Undervoltage raises running current, adds winding heat, cuts available starting torque and produces the nuisance overload trips usually blamed on the starter. Boosting 208 to 236 returns the load to the band its manufacturer designed around without pushing a 240V component over its rating.
Installation notes
Sizing guidance
Size the transformer by the current the load draws, not by horsepower or connected watts. Buck-boost units are rated by the line current they pass at the corrected voltage, and the choices here are 10, 15, 20, 30, 40, 50 and 60 amps in both single-phase and three-phase.
Start at the equipment nameplate. Use full-load amps for a motor or rated amps at 240V for a heater, then take the next size up. Where several pieces of equipment share one circuit, add their currents and treat anything running three hours or more at a stretch as a continuous load, which conventionally means sizing to 125 percent of that current. Match the phase configuration to the load: a single-phase load fed from two legs of the 208Y/120 service takes the single-phase unit, a three-phase load takes the three-phase unit.
Because a buck-boost transformer only processes the 28-volt difference between input and output rather than the whole load, it stays far smaller and cheaper than an isolation transformer feeding the same equipment.
Installation notes
Measure before ordering. Take a phase-to-phase reading at the panel that will feed the transformer, with the building under normal load rather than on a quiet Sunday morning. If the service reads well below 208, a different input voltage is the right selection.
A buck-boost transformer is an insulating transformer reconnected as an autotransformer. The output is not isolated from the input and the source grounding reference carries through, so it does not replace an isolation transformer where isolation is the actual requirement. Three-phase corrections are wired open-delta with two coils, which raises the three phase-to-phase voltages and nothing else. No neutral is derived, so a 3-wire feed cannot become a 4-wire 240/120V system and 120V loads stay on the original 208Y/120 panel.
Conductor sizing and overcurrent protection follow the branch-circuit rules plus NEC Article 450 for the transformer. Have a licensed electrician confirm the connection diagram and local inspection requirements.
Common questions
- Will a 208V to 236V buck-boost transformer fix a 240V oven that heats slowly on a 208V service?
In most cases, yes. A 240V resistive heating element supplied with 208V produces roughly 75 percent of its rated wattage, because power varies with the square of the applied voltage. Boosting the supply to 236V brings that element to about 97 percent of rated output, which restores normal preheat and recovery times. The transformer must be rated for at least the appliance current at the corrected voltage.
- Is 236V close enough for equipment nameplated 240V?
Yes. 236V is about 1.7 percent below 240V, well inside the plus or minus 10 percent tolerance NEMA MG-1 allows for motors and inside the range published by most equipment manufacturers. Targeting 236V rather than 240V also leaves margin for the hours when a building service floats high under light load, so the equipment is never pushed above its rated voltage.
- Does a 208V to 236V buck-boost transformer supply 120V for controls?
No. A buck-boost transformer is wired as an autotransformer and corrects phase-to-phase voltage only. It does not derive a neutral, so it cannot supply 120V. Control circuits, receptacles and other 120V loads must remain connected to the original 208Y/120 panel, which already has a neutral, or be fed from a separate control transformer.
- Can one buck-boost transformer serve several 240V appliances?
Yes, provided it is rated for the total current of everything downstream. Add the full-load amps of each appliance at the corrected voltage, apply the 125 percent factor conventionally used for continuous loads, and select an amperage rating at or above that total. A single unit feeding a subpanel of 240V equipment is a common arrangement in commercial kitchens.