This document describes an automatic phase selection circuit that can select the optimal power supply phase among three phases (R, Y, B) based on voltage levels. The circuit uses transformers, comparators, transistors and relays for each phase. It monitors the voltage of each phase and automatically switches the load to a different phase if the current phase voltage drops below 200V, in order to provide uninterrupted power supply. The circuit has applications in small offices and homes to ensure stable power supply. It offers advantages like reliability, constant voltage, high stability and reduced maintenance compared to manual phase changing.