A power distribution system, modular data center (MDC), and method provide a permanent electrical enclosure sized for an eventual maximum load allowing for flexible ampacity and concurrent upgradeability. A busway is mountable in a MDC to carry three-phase AC electrical power. A three-phase electrical cable is rated to carry a maximum power value to connect to a power distribution unit (PDU) of a first rack-based information handling system (IHS) of the MDC. A bus plug is engaged to the first busway and has at least three conduction paths that electrically connect to the three-phase electrical cable. Interchangeable fused switch modules of different power ratings are selectable to be received in a fused switch module receptacle in the bus plug. A reduced power fused switch module for initial power capacity of the MDC can later be upgraded to a maximum power rating without changing anything but the fused switch module.