4 in 120-volt Firebox.

PROP831-FB 7 1/2 INCH; NEW CONSTRUCTION INSTALLATION; UL CUL APPROVAL; DAMP LOCATION APPLICATION; STEEL HOUSING MATERIAL; NON INSULATED CEILING TYPE; 4 INCH NOMINAL APERTURE SIZE; SQUARE APERTURE SHAPE; 4 1/2 INCH CEILING OPENING SIZE; AIR TIGHT; FIREBOX[R] BRAND PROGRESS