277V to 240V Buck-Boost Transformers
277 volts is the line-to-neutral leg of a 480Y/277 system, the voltage that feeds most commercial lighting in 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
- 277V
- Required output
- 240V
- Correction
- Buck (lower voltage) · 13.4%
- Size from
- System phase and equipment nameplate amps
Technical details
277V to 240V technical overview
277 volts is the line-to-neutral leg of a 480Y/277 system, the voltage that feeds most commercial lighting in the United States. A 277V to 240V buck-boost transformer brings that leg down about 13 percent so a 240 volt load can run from a circuit that already exists, instead of waiting on a new feeder from a panel several floors away. This is the largest correction in this group and one of the most practical, because on a rooftop or above a ceiling the 277 volt circuit is often the only power within reach. Single-phase and three-phase, 10 through 60 amps.
Where 277V to 240V correction is used
The 277 volt circuit is almost always already there. That is the entire reason this correction gets specified.
Rooftops are the common case. A 480Y/277 building gives the roof 277 volt lighting and 480 volt three-phase for the packaged units, and then something needs 240 volts: a condensate pump, a small make-up air fan, a heat-trace or snow-melt circuit, a damper or gate operator, a sign, a booster pump, or a piece of imported equipment that only came in a 240 volt configuration.
The same situation appears in parking structures and site work, where 277 volt pole and canopy lighting is the nearest source; in ceiling spaces during tenant fit-outs, where the lighting home run is available and the nearest 208 volt panel is not; and in retrofits that add a 240 volt appliance, heater or motor to a building distributed entirely at 480Y/277.
Warehouse and industrial buildings hit it too, usually with unit heaters, door operators, air curtains and small compressors specified at 240 volts. XFMR Direct sells buck-boost transformers from multiple manufacturers, and the selector on this page returns the units matching your phase and current.
Why this 277V to 240V voltage pair matters
The alternative to correcting 277 volts is running new copper, and that comparison is usually what decides it. Pulling a 240 volt circuit from a distant panel means conduit, conductors, spare panel capacity and, on a roof or in an occupied ceiling, access and downtime. A buck-boost transformer mounted at the load uses the circuit already serving the area.
What happens without the correction is not subtle. 277 volts applied to a 240 volt load is 15 percent over nameplate, outside the plus or minus 10 percent NEMA MG-1 allows for motors and outside the tolerance of essentially any 240 volt control or electronic device. Resistive loads take it worst, because power follows the square of voltage: a 240 volt heating element on 277 volts dissipates roughly a third more watts than it was built for. Motors overheat, elements burn out and electronics fail quickly rather than gradually.
Installation notes
Sizing guidance
Most 277 volt applications are single-phase, taken line to neutral from a 480Y/277 panel, and that is the option to select unless your supply is genuinely three-phase at 277 volts. Confirm the source before choosing phase. The panel schedule and the meter should agree.
Size on load current at 240 volts, not at 277. Take full-load amps from the equipment nameplate, add anything else that will be fed through the same transformer, and select the next step up from 10, 15, 20, 30, 40, 50 or 60 amps. Account for motor starting behavior in your judgment rather than sizing exactly at nameplate.
The economics come from the fact that a buck-boost transformer processes only the voltage difference. Correcting 37 volts means the unit handles roughly 13 to 15 percent of the load apparent power, which is why it is far smaller and cheaper than a 277 to 240 volt isolation transformer covering the same load.
Installation notes
One point governs this correction. A buck-boost transformer is an autotransformer, so the 240 volt output is not isolated from the 277 volt circuit feeding it. Connected line to neutral on a 480Y/277 system, the result is 240 volts measured between the corrected conductor and the system neutral, with the full 240 volts appearing between that conductor and ground. It is not the center-tapped 120/240 volt arrangement most North American 240 volt equipment expects, and no 120 volt line-to-ground leg exists. Equipment needing 120 volts for controls, or expecting each leg to sit near ground potential, cannot be fed this way.
Nothing here derives a neutral or creates a separately derived system, and three-phase versions wired open delta correct line-to-line voltage only. Size conductors and overcurrent protection per the NEC for the actual current on each side. Confirm with a meter that the circuit is genuinely 277 volts to neutral, and remember that it originates in a 480Y/277 panel with the qualified-person and arc-flash requirements that go with it.
Common questions
- Can I run 240 volt equipment from a 277 volt lighting circuit?
Yes, with a buck-boost transformer wired to reduce 277 volts to 240 volts. 277 volts is the line-to-neutral voltage of a 480Y/277 system and is about 15 percent above a 240 volt nameplate, which is more than almost any 240 volt equipment tolerates. A buck-boost transformer at the load makes the existing circuit usable without pulling a new feeder from a 208 or 240 volt panel.
- Does a 277 to 240 volt buck-boost transformer also give me 120 volts?
No. It produces a single 240 volt output referenced to the existing system neutral, not the center-tapped 120/240 volt supply common in North American work. There is no 120 volt line-to-ground leg, and a buck-boost transformer cannot derive a neutral because it is an autotransformer rather than a separately derived source. Equipment needing 120 volts for controls or accessories must have it supplied separately.
- What happens if I connect 240 volt equipment directly to 277 volts?
It runs about 15 percent over nameplate, outside the plus or minus 10 percent NEMA MG-1 allows for motors and outside the rating of most 240 volt electronics and control components. Resistive loads are affected more, since power varies with the square of voltage: a 240 volt heating element on 277 volts produces roughly a third more heat than its design. Expect overheating and short life rather than instant failure.
- Is 277 to 240 too large a correction for a buck-boost transformer?
No. It sits at the larger end of the normal buck-boost range, which typically runs from about 5 to 20 percent. The practical consequence of a larger correction is that the transformer processes a larger share of the load power than a small trim does, so the unit needed for a given load current is bigger than it would be for a 5 percent change.