240V to 264V Buck-Boost Transformer
This one is a step-up, not a sag correction. Choose the system phase and equipment nameplate amps below to see an available, buyable unit.
Load current (amps)
Quick answer
- Measured input
- 240V
- Required output
- 264V
- Correction
- Boost (raise voltage) · 10.0%
- Size from
- System phase and equipment nameplate amps
Technical details
240V to 264V technical overview
This one is a step-up, not a sag correction. A 240V to 264V buck-boost transformer takes a healthy 240V supply and raises it 10% because a specific piece of equipment requires an input the building does not provide. The source is not the problem. The gap is between what the utility delivers and what a nameplate asks for, and 264V is above the normal system voltage range for a nominal 240V service, so this correction belongs on a dedicated circuit feeding the equipment that needs it rather than on a panel serving general loads. Single-phase and three-phase, in 10, 15, 20, 30, 40, 50 and 60 amp selections.
Where 240V to 264V correction is used
Nothing in a US building is nominally 264V, so this pair is always driven by an equipment specification. The recurring cases:
- Imported or purpose-built machinery specified for a 250V to 277V input window, delivered to a site that has 240V delta and nothing higher.
- Induction, dielectric and infrared heating power supplies, and plating or anodizing rectifiers, where delivered process power scales with input voltage.
- Laser and plasma cutting supplies, UV curing systems and similar loads whose control electronics enforce a minimum input before they will fire.
- OEM skid packages and process modules engineered around a service the receiving site does not have.
- Test and QA benches performing high-line verification, where equipment has to be run at the top of its rated input range rather than at nominal.
The other honest case is a long downstream run to a load with a high minimum input, where the correction is made at the source end and the drop along the run is part of the design.
Why this 240V to 264V voltage pair matters
264V is exactly 10% above 240V, which is why it matches so many equipment input windows: a nameplate rated 240V with a plus or minus 10% tolerance tops out there, and equipment specified anywhere in the 250V to 270V region sits comfortably inside it. What goes wrong without the correction is different from an ordinary undervoltage complaint. Equipment specified above the available supply frequently does not run at reduced output, it refuses to start, holds on an undervoltage lockout, or delivers process power well below rating because rectifier-fed heat falls with the square of applied voltage.
The limit is worth stating plainly. ANSI C84.1 puts the Range A service voltage ceiling for a nominal 240V system at 252V and the wider Range B ceiling at 254V, so 264V is not a system voltage. It is a dedicated feed for a load whose nameplate asks for it, and it should not be applied to general 240V equipment.
Installation notes
Sizing guidance
Read the equipment nameplate input range first, then log the supply for a full day before selecting. On this pair the number that matters is the high end of the source, not the average. A buck-boost transformer applies a fixed ratio, so a 240V source that drifts to 250V overnight produces about 275V at the output, and if the equipment's ceiling is 264V there is no headroom left to absorb that. Verify what the service actually does at 3am before committing.
Select amperage from the equipment's rated input current at 264V rather than from the disconnect or breaker size, add 25% for continuous operation, and round up to the next offered value among 10, 15, 20, 30, 40, 50 and 60 amps. Phase follows the equipment; three-phase corrections use two units in open delta. Because the transformer handles only the 24 volt difference, it is a small fraction of the size of the load it supports.
Installation notes
Treat this as a dedicated circuit and label it. The next person opening that enclosure will not expect to find 264V on a nominal 240V system, and nothing else on the premises should be connected to it.
A common misconception is worth clearing up: boosting 240 to 264 does not produce a 277V lighting supply. 264V here is a phase-to-phase value on a 3-wire system, and a buck-boost transformer connected as an autotransformer derives no neutral, so there is no line-to-neutral voltage to take. It also provides no isolation, since the output shares a winding with the input.
Three-phase corrections are open delta with two units. Size conductors and overcurrent protection per NEC Article 450 and the branch-circuit rules, noting that on a step-up the input current is higher than the output current, so the supply-side conductors carry more than the load's own rating suggests. Confirm the output at the equipment terminals before commissioning.
Common questions
- Why would equipment need 264V when the building supplies 240V?
Because the equipment was designed around a higher supply voltage than a US 240V service provides. This is common with imported machinery specified for a 250V to 277V window, with induction and dielectric heating supplies and process rectifiers whose output scales with input voltage, and with equipment that enforces a minimum input before it will operate. In these cases the supply is healthy and the requirement is simply higher than nominal.
- Is 264V safe for standard 240V equipment?
It is not the right way to run general 240V loads. 264V is exactly 10% above 240V, which is the outer edge of the tolerance NEMA MG-1 allows for a 240V nameplate motor and above the 252V Range A ceiling ANSI C84.1 sets for a nominal 240V system. Sustained operation there increases core loss, heats contactor and relay coils, and shortens the life of electronics. Boost to 264V only for equipment whose nameplate calls for it, on a dedicated circuit.
- Is a buck-boost transformer the same as a step-up transformer?
It performs a step-up, but only a small one and without isolation. A buck-boost transformer is an insulating transformer reconnected as an autotransformer, which limits it to corrections of roughly 5% to 20% and leaves the output electrically connected to the input. Large ratio changes such as 240V to 480V require a conventional two-winding transformer, which is both isolated and sized for the full load rather than for the difference.
- Can I get 277V lighting from a 240V to 264V boost?
No. A buck-boost transformer corrects the voltage between the conductors it is connected to and derives no neutral, so a 3-wire 240V source remains a 3-wire system after the boost. 277V lighting requires a line-to-neutral connection on a 480Y/277V system, which only a properly derived 4-wire wye source can supply.