A tap-off base assembly of the present invention provides protection against the ingress of dust and liquids into an electrical busway distribution system of the type having access openings for receiving power tap-off devices. The tap-off base assembly includes a base molded from an electrically insulating material and a pivotally attached door. A tap-off opening seal is placed on the housing around the tap-off opening. The base is slidably received in the busway tap-off opening such that the seal is compressed thereby sealing the tap-off opening. The base has a planar outside surface which includes a number of access openings. Each access opening provides access to one the electrical conductors contained within the busway housing. The door may be selectively moved between an open position which provides access to the electrical conductors and a closed position which prohibits access to the conductors. The door also has a flange around its perimeter and a sealing rib which extend outward from the inside surface of the door. When the door is in the closed position the flange overlaps an edge portion of the base and the sealing ribs are pressed against a gasket in the planar outside surface of the base which surrounds the access openings thereby sealing the access openings and preventing the ingress of dust and liquids into the busway housing. A latch is provided for securing the door in its closed position.