An adapter used in conjunction with voltage modules to provide multiple and variable DC output voltages. The adapter may include a circuit for stepping down an AC input voltage to lower AC output voltages. A voltage module is provided having an interface bus, configured to be coupleable to a distribution bus on the voltage adapter. Preferably, the voltage module includes AC-DC conversion circuitry, such that the AC output voltages received from the adapter may be converted to DC voltages. Once the adapter and voltage module are coupled together, the voltage module allows the user to select among the multiple DC output voltages provided. In some embodiments, the adapter includes additional circuitry for converting the stepped-down AC voltages into DC output voltages. Additional separate voltage modules may be coupled to the first voltage module, in series, to form a modular stack of voltage modules. Like the first voltage module, the second voltage module may have a selectable output voltage, as well.