208V to 240V Buck-Boost Transformer
208V is not a low 240V. 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
- 240V
- Correction
- Boost (raise voltage) · 15.4%
- Size from
- System phase and equipment nameplate amps
Technical details
208V to 240V technical overview
208V is not a low 240V. It is the line-to-line voltage of a 208Y/120 wye service, a system built around a different design point, and it will not be fixed by tightening lugs or calling the utility. When a building fed 208Y/120 has to run equipment nameplated 240V, the gap gets closed on the customer side with a 208V to 240V buck-boost transformer: an insulating transformer reconnected as an autotransformer, wired to add the missing 32 volts, a boost of about 15 percent. Because it processes only that difference rather than the full load power, it stays small and costs a fraction of an isolation transformer serving the same equipment. Single-phase and three-phase configurations are available in 10, 15, 20, 30, 40, 50 and 60 amp ratings.
Where 208V to 240V correction is used
Nearly every commercial tenant space built in the last fifty years is fed 208Y/120, because one service gives the landlord 120V for receptacles and lighting and 208V for larger loads without a second transformer. The equipment those tenants buy comes out of a catalog, and catalogs are full of 240V nameplates. The two decisions meet at the disconnect.
- Rooftop package units and split-system condensers set on an existing 208V roof feed during a fit-out, then expected to hold capacity on the first hot day.
- Commercial kitchen equipment: fryers, combi and convection ovens, griddles, booster heaters and dish machine tank heaters, every one resistive and short on output at 208V.
- Refrigeration: walk-in condensing units, ice machines, remote compressor racks and reach-in cases nameplated 230/240V.
- Elevators and lift equipment, where the machine room was designed around a 240V hydraulic pump package.
- Air compressors, vacuum pumps and hydraulic power units in multi-tenant industrial condos.
- Medical and dental suites: sterilizers, autoclaves, imaging equipment and process water heaters specified at 240V.
- Schools and municipal buildings, where kilns, shop equipment and kitchen packages arrive on a state contract regardless of the building's service.
The complaint is rarely phrased as a voltage problem. The oven is slow. The unit short cycles. The compressor trips on a hot afternoon. A part gets replaced, the symptom returns, and only then does anyone put a meter on the panel.
Why this 208V to 240V voltage pair matters
Start with the arithmetic. 208V is 13.3 percent below 240V, and that one number drives two separate failure modes.
Resistive loads lose output as the square of the voltage ratio. A 240V element energized at 208V dissipates about 75 percent of its rated wattage, so a fryer that should recover in three minutes takes four and a booster heater falls behind the dish machine at the dinner rush. Nothing is broken. The element is doing exactly what physics says it should at 208V.
Motors fail differently. NEMA MG-1 permits continuous operation at plus or minus 10 percent of nameplate voltage, and a 240V motor on 208V is outside that allowance before the branch circuit is counted. Torque also follows the square of the voltage ratio, so starting and breakdown torque drop roughly 25 percent, and a compressor that starts fine in mild weather stalls against high head pressure in August. Running current rises to hold shaft power, windings run hotter, overloads trip for no visible reason, and insulation life shortens quietly.
Branch-circuit voltage drop compounds it. The informational notes to NEC 210.19(A) recommend holding branch-circuit drop near 3 percent and total drop near 5 percent, a design target rather than an enforceable rule. A rooftop unit at the end of a long run can sit under 200V at its terminals. Boosting 208 to 240 near the equipment restores the margin the run consumed.
Installation notes
Sizing guidance
Buck-boost transformers are selected by current, not by horsepower and not by connected watts. The ratings available for this correction are 10, 15, 20, 30, 40, 50 and 60 amps, in single-phase and three-phase.
Work from the nameplate. For a motor, use full-load amps. For a packaged unit containing a compressor, fans and controls, use the minimum circuit ampacity on the data plate rather than adding the internal parts yourself. Where several loads sit downstream of one transformer, total their currents and treat anything running three hours or more without interruption as continuous, which conventionally means sizing at 125 percent. Then take the next rating above that total, so motor starting and a future addition do not leave the unit at its limit.
One detail specific to a boost: the 208V input side carries roughly 15 percent more current than the 240V output side, because the load's power draw is what stays constant. Size supply-side conductors and overcurrent protection to the input current.
Match phase to the source. A single-phase load taken from two legs of the 208Y/120 service uses the single-phase unit, and a three-phase load uses the three-phase unit. A transformer changes voltage only, so a buck-boost cannot make three-phase power from a single-phase supply.
Installation notes
Measure first, on a weekday, with the building loaded. A panel schedule states the nominal system, not what the conductors are delivering today. Under ANSI C84.1 a 208V system is expected to deliver 197V to 218V at the service, and where the reading lands in that band changes which correction is right.
Check the swing before ordering. A buck-boost applies a fixed ratio, so it multiplies whatever arrives. A service ranging across that full band delivers roughly 227V to 252V at the corrected load, which stays inside the plus or minus 10 percent window for 240V equipment at both ends.
Understand what the device is. Reconnected as an autotransformer, its output is electrically connected to its input. It does not isolate, does not create a separately derived system, and does not derive a neutral. Three-phase corrections are wired open delta with two coils, raising the three line-to-line voltages and nothing else. 120V lighting, receptacles and controls stay on the original 208Y/120 panel, which already has the neutral they need.
Conductors, overcurrent protection and grounding follow the branch-circuit rules and NEC Article 450. Have a licensed electrician confirm the connection diagram, and verify the output with a meter under load after energizing.
Common questions
- Why does 240V equipment underperform on a 208V service?
208V is about 13 percent below 240V, and that gap hits resistive and motor loads differently. A 240V heating element energized at 208V produces roughly 75 percent of its rated wattage, because power in a resistive element varies with the square of the applied voltage. A 240V motor on 208V is outside the plus or minus 10 percent range NEMA MG-1 allows, develops about 25 percent less starting torque, and draws higher current to hold the same shaft load, which raises winding temperature and causes nuisance overload trips. A 208V to 240V buck-boost transformer closes the gap so the equipment runs at its design voltage.
- Is 208V simply a low 240V supply?
No. 208V is the line-to-line voltage of a 208Y/120 wye service, which is a different distribution system rather than a degraded 240V one. In a 208Y/120 system three phase conductors each sit 120V above a common neutral, and the voltage between any two of them is 208V by geometry, not by loss. Nothing on the utility side or in the building will raise it to 240V, so equipment nameplated 240V on such a service needs the voltage corrected with a buck-boost transformer.
- Does a 208V to 240V buck-boost transformer work on a three-phase 208Y/120 service?
Yes. Three-phase corrections from 208V to 240V are normally wired open delta using two coils, which raises all three line-to-line voltages from 208V to about 240V. The arrangement changes line-to-line voltage only. It derives no neutral and cannot convert a 208Y/120 four-wire service into a 240/120V four-wire system, so 120V lighting, receptacles and control circuits stay connected to the original 208Y/120 panel.
- How much energy does a buck-boost transformer add to the load?
Very little relative to the load it serves, because a buck-boost transformer processes only the difference between input and output voltage rather than the full load power. In a 208V to 240V correction the unit handles the 32-volt difference, so its internal losses are proportionate to that small share of the total. This is the same reason a buck-boost transformer is physically smaller and costs far less than an isolation transformer serving the same equipment.
- How is a 208V to 240V buck-boost transformer sized for a rooftop unit or condenser?
Size it on current, using the minimum circuit ampacity printed on the equipment data plate rather than adding up the internal compressor and fan loads by hand. Select the next available amperage rating above that figure so motor starting and any future load do not leave the transformer at its limit. Ratings for this correction are 10, 15, 20, 30, 40, 50 and 60 amps in single-phase and three-phase. Note that the 208V input side carries about 15 percent more current than the 240V output side, so supply conductors and overcurrent protection are sized to the input current.