An electrical floor box assembly includes a box having a bottom wall, a plurality of side walls and a top portion having an opening at a top end thereof for providing access to the electrical box. An adapter has an open top end, an open bottom end and a passage extending between the top end and bottom end. The adapter has an outer dimension complementing a dimension of the opening in the electrical box and is received in the opening for movement between a retracted position and an extended position within the opening. The electrical box and adapter are configured for coupling and fixing the axial movement of the adapter with respect to the electrical box by rotation of the adapter to a locking position. In one embodiment, the electrical box and the adapter have interlocking teeth that mate with each other to prevent axial movement of the adapter. A locking member is provided to fix the position of the adapter relative to the electrical box to prevent rotation of the adapter.