Abstract: A mounting box for mounting an electrical device into a cavity of a wall has an outer body having an expansion element arranged in a side wall that extends in a depth direction, wherein, in use, the expansion element is pressed against the cavity, the mounting box having a first expansion control element arranged for pressing the expansion element against the cavity from a resting state to a securing state, and wherein the mounting box further includes an inner body, arranged to be mounted into the outer body and arranged for receiving the electrical device, wherein the inner body has adjustment means for moving the inner body in the depth direction inside the outer body regardless of the condition of the expansion element.