503.7.7 Single-wall penetrations of combustible walls.
FGC § 503.7.7
A single-wall metal pipe shall not pass through a combustible exterior wall unless guarded at the point of passage by a ventilated metal thimble not smaller than the following: 1. For listed appliances with draft hoods and appliances listed for use with Type B gas vents, the thimble shall be not less than 4 inches (101.6 mm) larger in diameter than the metal pipe. Where there is a run of not less than 6 feet (1828.8 mm) of metal pipe in the open between the draft hood outlet and the thimble, the thimble shall be permitted to be not less than 2 inches (50.8 mm) larger in diameter than the metal pipe.
2.For unlisted appliances having draft hoods, the thimble shall be not less than 6 inches (152.4 mm) larger in diameter than the metal pipe.
3.For residential and low-heat appliances, the thimble shall be not less than 12 inches (304.8 mm) larger in diameter than the metal pipe. Exception: In lieu of thimble protection, all combustible material in the wall shall be removed a sufficient distance from the metal pipe to provide the specified clearance from such metal pipe to combustible material. Any material used to close up such opening shall be noncombustible.
