Modular distribution back support for circuit breakers, with the latter of the type intended for high power applications, characterized in that it comprises: - a box-like structure made of insulating material with a parallelepiped shape, in which is defined a front face adapted to receive and support the back side of a circuit breaker; - fixing members able to make a circuit breaker integral with said box-like structure; - at least three conductive rails (41, 42, 43) made of conductive material, housed in said box-like structure so as to be electrically isolated from one another, intended to be electrically connected to corresponding phases of an external three phase power supply line; - at least three holes (21, 22, 23), opening at said front face and extending towards the inside of the box-like structure, provided to receive the corresponding plug connectors (101, 102, 103), protruding from the back face of a circuit breaker and adapted to provide the electric connection between the latter and the above three conductive rails (41, 42, 43); - at least three openings (24, 25, 26), made starting from each of the four side faces (2B, 2C, 2D, 2E) of said box-like structure which are perpendicular to said front face and extended towards the inside of the box-like structure itself, up to make accessible from the outside corresponding portions of said conductive rails (41, 42, 43), with each set of three openings (24, 25, 26) of a side face aligned with the set of openings made in the opposite side face and with said modular distribution back support adapted to be placed side by side with one or more identical back supports, to form rows both in horizontal and vertical directions, in which the respective front face are coplanar, said openings being adapted to receive connecting means, extended through said openings (24, 25, 26) of two adjacent modular back supports and adapted to define electric bridge connections between the respective conductive rails (41, 42, 43), so that each distribution back support can be electrically connected in parallel to adjacent ones placed immediately above, below and at both sides. - a box-like structure made of insulating material with a parallelepiped shape, in which is defined a front face adapted to receive and support the back side of a circuit breaker; - fixing members able to make a circuit breaker integral with said box-like structure; - at least three conductive rails (41, 42, 43) made of conductive material, housed in said box-like structure so as to be electrically isolated from one another, intended to be electrically connected to corresponding phases of an external three phase power supply line; - at least three holes (21, 22, 23), opening at said front face and extending towards the inside of the box-like structure, provided to receive the corresponding plug connectors (101, 102, 103), protruding from the back face of a circuit breaker and adapted to provide the electric connection between the latter and the above three conductive rails (41, 42, 43); - at least three openings (24, 25, 26), made starting from each of the four side faces (2B, 2C, 2D, 2E) of said box-like structure which are perpendicular to said front face and extended towards the inside of the box-like structure itself, up to make accessible from the outside corresponding portions of said conductive rails (41, 42, 43), with each set of three openings (24, 25, 26) of a side face aligned with the set of openings made in the opposite side face and with said modular distribution back support adapted to be placed side by side with one or more identical back supports, to form rows both in horizontal and vertical directions, in which the respective front face are coplanar, said openings being adapted to receive connecting means, extended through said openings (24, 25, 26) of two adjacent modular back supports and adapted to define electric bridge connections between the respective conductive rails (41, 42, 43), so that each distribution back support can be electrically connected in parallel to adjacent ones placed immediately above, below and at both sides.