246V to 220V Buck-Boost Transformer
Equipment built to a 220V specification does not become 240V equipment when it is shipped to the United States. Choose the system phase and equipment nameplate amps below to see an available, buyable unit.
Load current (amps)
Quick answer
- Measured input
- 246V
- Required output
- 220V
- Correction
- Buck (lower voltage) · 10.6%
- Size from
- System phase and equipment nameplate amps
Technical details
246V to 220V technical overview
Equipment built to a 220V specification does not become 240V equipment when it is shipped to the United States. A 246V to 220V buck-boost transformer resolves that mismatch by subtracting 26 volts, about 10.5% of the incoming supply, so the machine sees its design voltage on a US 240V-class service. Machine builders, systems integrators and equipment importers use this correction constantly, because respecifying a machine's entire electrical package for the US market costs far more than correcting the voltage in front of it. The device is an insulating transformer reconnected as an autotransformer, processing only the difference between 246 and 220.
Where 246V to 220V correction is used
This correction is an integration problem more than a maintenance problem. It appears when equipment designed for a 220V market is installed on a US service, and the people who buy it in volume are the people who install that equipment for a living.
Common sources include packaging and filling lines, converting and labeling machinery, laser cutters and markers, plastics auxiliaries, CNC machining centers and press brakes, textile and printing equipment, semiconductor and laboratory instruments, and process skids assembled overseas. Contract manufacturers, research labs, breweries, food plants and print shops end up owning the problem after delivery.
It also arrives through a second route. OEMs building for both domestic and export markets often standardize on a 220V electrical package to keep a single bill of materials, then specify a buck-boost transformer ahead of the machine on US installations. That is a deliberate design decision rather than a field repair, and it is why integrators order this pair by phase and amperage rather than by machine model.
Why this 246V to 220V voltage pair matters
The gap between 246V and 220V is roughly 12% at the equipment terminals, which places the supply well outside what a 220V design tolerates. NEMA MG-1 permits plus or minus 10% on a motor, capping a 220V motor at 242V, and machine electronics are usually specified more tightly than the motors are.
On imported machinery the weak points are predictable. Internal control transformers step down from the machine's rated input, so a 12% overvoltage on the incoming line becomes a 12% overvoltage on the 24V and 110V control buses. Servo and spindle drives run with a higher DC bus voltage than intended. Contactor coils, cabinet cooling fans and any resistive heating inside the machine all run hot. Equipment manufacturers commonly specify a supply voltage range as a condition of correct operation, so an uncorrected installation becomes both a reliability problem and a support problem.
Correcting the voltage ahead of the machine solves all of it at once, without touching the machine's internal wiring.
Installation notes
Sizing guidance
Integrators size this from the machine's electrical drawing, which makes it faster than a field retrofit. Work from the machine's rated input current and phase, then confirm both against the panel that will feed it.
- Phase: single-phase and three-phase versions are both available. Match the machine's requirement and the available service, and note that a buck-boost cannot convert single-phase supply into three-phase power.
- Current: amperage options are 10, 15, 20, 30, 40, 50 and 60 amps. Use the machine nameplate full-load current, or the sum of full-load currents if one transformer will feed several machines, then select the next rating above it.
- Headroom: if the line is planned to expand, size for the future load rather than the present one.
The transformer's own rating falls far below the load it supports, because it carries only the 26-volt difference. Sizing it as though it had to pass full load power is the most expensive mistake available on this pair.
Installation notes
Panel and skid integration is the usual context, so mechanical placement and heat matter as much as the electrical connection. The unit dissipates heat continuously and needs the clearance and airflow its instructions call for.
Electrically, three points govern. First, this is an autotransformer once connected for buck service: the 220V output is not isolated from the 246V source, so any isolation the machine specification requires must come from a separate isolation transformer. Second, three-phase corrections are made open delta, typically with two units, correcting line-to-line voltages without deriving a neutral, which means a machine expecting a 220V neutral will not get one from this device. Third, both sides carry full load current, so conductors and overcurrent protection are sized for load current rather than for the transformer's rating.
Before commissioning, measure the actual voltage at the machine terminals with the machine running. Design assumptions and field readings diverge often enough that the check is always worth the time.
Common questions
- Can a buck-boost transformer be installed inside an equipment control panel?
Yes, panel and skid mounting is a routine application for buck-boost transformers, which is why machine builders specify them for 220V-rated equipment sold into 240V markets. The unit must be mounted in the orientation its instructions permit, with the clearance and ventilation they specify, since it dissipates heat continuously while energized. Enclosure and mounting requirements vary by model, so confirm them against the specific unit selected.
- What is the difference between a buck-boost transformer and an isolation transformer for a 246V to 220V correction?
A buck-boost transformer is connected as an autotransformer and processes only the 26-volt difference between input and output, so it is small and inexpensive relative to the load it serves, but the output is not electrically isolated from the input. An isolation transformer processes the full load power and provides galvanic separation between primary and secondary, making it substantially larger and more costly for the same connected load. Choose a buck-boost when the requirement is voltage correction, and an isolation transformer when the requirement is isolation.
- Do I need one buck-boost transformer per machine, or one for the whole line?
Either works, and the choice is about load current and flexibility. A single transformer sized above the combined full-load current of the line corrects everything downstream of it, while one unit per machine keeps each machine independently serviceable and simplifies future additions or relocations. Machine builders shipping equipment with the transformer included generally use one per machine so the corrected voltage travels with the equipment.
- Will bucking 246V to 220V also correct the control voltage inside the machine?
Yes. A machine's internal control transformer steps down from the incoming line voltage, so its secondary output scales with whatever the machine is fed. Correcting the input from 246V to 220V brings the 24V and 110V control buses back to their intended values as well, which is why input voltage correction resolves control-circuit overvoltage without any modification to the machine's internal wiring.