The Power supply option D2 has a range of 90-265 V AC or 100-370 V DC so 277 V AC is about 5% higher than the rated voltage. When used at this higher voltage and in prolonged period of time will have great effect on the power supply that may cause the meter to break down early. It is recommended to seek an alternative 120 V AC regular plug or use a control power transformer to step down the voltage from 480 V to 120 V.