An apparatus and method for powering a lamp connected to a ballast circuit. The ballast circuit is connected to a first alternating current (AC) source having a first phase and to a second AC source having a second phase for powering the lamp. A first switching circuit is connected between the first AC source and a rectifier circuit. A second switching circuit is connected between the second AC source and the rectifier circuit. A control circuit selectively energizes the first and second switching circuits to provide power from one of the first and second AC sources to the lamp load via the rectifier circuit and an inverter circuit. A detection circuit connected between the first and second AC sources generates a detection signal indicating whether power is being supplied by each the first and second AC sources. The detection signal is provided to a dimming regulation circuit to generate a dim level command signal for dimming the lamp.