A load bank for testing power sources includes a load resistor assembly having a plurality of resistors which can be switched between wye and delta configurations for testing a variety of voltages and power sources requiring various load applications. First and second power connections are connected with primary and redundant power sources and a relay alternately connects the first and second power connections with the load resistor assembly for alternately testing the primary and redundant power sources while maintaining the sources electrically isolated. A damper is arranged adjacent the load resistor assembly and is operable between an open position which permits cold air to be delivered to the load resistor assembly to cool the assembly when the load bank is operating and a closed position which prevents hot air from being recirculated when the load bank is not operating.