An electrical back box (10, figure 1), for mounting in a hole of a mounting surface, comprising: a body having a side wall of substantially circular outer profile, a flange member (18) adapted to contact the mounting surface when the back box is inserted into the hole, thereby preventing further insertion, and where the back box includes one or more existing or formable cutout portions 19 adapted to increase the internal space of the back box. Preferably the cut out portions are provided by removable tabs. A second embodiment discloses an electrical back box with a circular body, a flange member, and contact means (22) which are movable to contact the rear of the mounting surface. The contact means may comprise two lugs. A third embodiment discloses an electrical back box comprising; a rear collar 14, a front collar 12, and a depth adjustment mechanism (28, 30, figure 4) adapted to allow the collars to be adjusted to alter the depth of the back box. The depth adjustment mechanism may comprise a friction fit or spring biased mechanism, and pawl and ratchet members.
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