Attributes . Welded steel construction with "FM" bracket that mounts on flat side of stud; 3-3/4 in. square x 3-1.2 in. deep; 3-3/4 in. opening in wall allows for margin of error when using a 4 in. square cover; Accepts 4 in. square covers and mud rings; Painted red for dedicated life safety alarm systems; Large capacity design accommodates oversized devices; Supplied with fourteen 1/2-3/4 in. Concentric KO's and one 1/2 in. KO; 40.5 cu. in. capacity; Acceptable for use in 2-hour fire rated walls . Names