A motor control center (MCC) unit for installation in a compartment of an MCC includes a frame, a bus contact assembly supported by the frame and comprising at least one moveable bus contact configured to releasably engage at least one power bus of the MCC, an auxiliary power input supported by the frame and configured to be connected to an external power source, and a control circuit supported by the frame. The MCC unit further includes an interlock circuit configured to selectively couple the control circuit to the at least one bus contact and the auxiliary power input responsive to positioning of the at least one bus contact. The interlock circuit may be configured to prevent power flow between the control circuit and the auxiliary power input when the at least one bus contact is engaged with the power bus.