Abstract: The invention provides an expandable modular enclosure system for enclosing circuit boards connected by a stackable bus. The enclosure system includes one or more carriers and spacers, a base unit, and a top unit. The spacers and carriers are alternately stacked between the base unit and top unit, and each circuit board is mounted to a carrier. A side portion with recessed vents extends from one side of each carrier, and each carrier and spacer define an opening. Air flows through the vents, over each circuit board, and through the openings in the carrier to cool the circuit boards. Circuit boards with varying functionality can be included in the enclosure, and as many circuit boards as are necessary can be enclosed without increasing the size of the footprint of the enclosure.