You have a 30 amp main circuit and a 15 amp branch circuit correct? While this would be small for a house wall panel, you need to expand your idea of bus bars. The power comes in through the 30 amp plug, which is wired to a neutral bus bar and ground bus bar, The input goes to the 30 amp breaker which does have it's own bus bar, the 15 amp circuit picks up that bus bar and distributes it to the GFI plug and then i suspect on the the power supply plug. If you wanted to add an additional 15 amp circuit you could easily? While your build does not have a heavy AC load they are using parts that could be used for larger systems.