Simulation of an Automatic Phase Changer (APC) circuit was performed in this project. The simulation was carried out using Proteus 8. Automatic Phase Changer (APC) automatically changes the phase as the name suggests. In three phase power system 3 inputs of APC circuit are connected to three phases of the system and its three outputs are connected to three different loads. These three loads always need their normal rated voltage for proper operation. Results observed that if voltage of any phase goes below the nominal rating the loads may malfunction. Here, Automatic Phase Changer comes into action. When a phase voltage goes below its nominal rating, APC provides correct level of voltage to the load connected to that phase.