An electrical box assembly for mounting an electrical device or fixture on a wall. The electrical box assembly includes a box having a back wall and a sidewall defining an enclosure, and flanges and bosses extending outward from the sidewall. Teeth are provided along the outer sidewall of the box in alignment with and extending from each boss to the back wall. An elongated bracket with resilient legs is adjustably connected to the back wall of the box. After a hole is made in the drywall, the teeth rip a path for the bosses as the electrical box assembly is pushed into the hole. An adjustment arrangement including fasteners is provided for adjusting the distance of the bracket with respect to the box and securing the box to the wall. The clamping action of the bracket legs and flange on the drywall hold the electrical box assembly secure to the wall.