Jamo FRB6 Fire Rated Back Box Fire rated back box Hole Size: 11.5" x 7.8" Product dimensions (HxWxD): 18" x 17.75" x 3.5" Designed to work with Jamo 661K4, 660A2, 660CS, I/O 660A Every Jamo back box exceeds the one-hour fire rating specifications and meet UL2043 requirements for installation into plenum spaces.