Flush-mounted box with reduced dimensions and kit and method for installing an electrical basic element in a hollow wall

Flush-mounted box for installation of an electrical base element in a hollow wall, comprising a hollow body with an open base surface and a side wall, a flange around the open base surface, at least one clamp for clamping the flush-mounted box in the hollow wall and a clamp screw through the at least one clamp, wherein the flange extends in a direction parallel to the open base surface and away from the hollow body, wherein the hollow body can be placed in a cylindrical space up to the flange, wherein the at least one clamp can be moved by tightening the clamp screw along the side wall to a plane formed by the open base surface, characterized in that the cylindrical space in which the hollow body can be placed has an inner diameter of 37 mm±4 mm, wherein the clamp screw of the at least one clamp is placed on a circular outline with a diameter of no more than 28.5 mm and wherein the circular outline and the cylindrical space are concentric.