Auto transformer (Auto-Transformer) is an electrical transformer in which there is one winding, a portion of which is common to both the primary and the secondary circuits. An auto transformer uses common winding and offer no interference or disturbance isolation. The current in the high-voltage circuit flows through the series and common winding. The current in the low-voltage circuit flows through the common winding and adds vectorially to the current in the high-voltage circuit to give the common winding current. The principle difference between an autotransformer and an isolation transformer is the separation of the secondary windings. Because the auto-transformer uses a single coil winding for both the primary input and the secondary output, any electrical noise, voltage spikes, sags or any other undesirable condition will pass through unchecked. Because the auto-transformer can transmit line disturbances directly, local building codes may prohibit their use in certain areas. Auto-Transformers also should not be used in closed delta connections as they will introduce into the circuit a phase shift which causes higher power use.