A thermal management system for a electronics cabinets having an electronics heat source therein. The thermal management system includes a first passive thermal device having an evaporator portion and a condenser portion. The thermal management system can also include a heat sink in contact with air inside the cabinet and in thermal contact with the evaporator portion of the first passive thermal device, wherein the heat sink is contained within the sealed cabinet. The condenser portion of the first passive thermal device can be in contact with a liquid to liquid heat exchanger.