is there any clause in NFPA that describes the procedure of accepting certain (minimum) thickness of Galvanized Steel Ducts of how to be fire rated (without any type of coating or insulation) ?
the story is that the contractor is claiming that a 1.2mm thickness ducts is sufficient to consider it fire rated for specified duration and temp. (400oC) and as per the specification tendered is 2mm thickness. and I don't have a reference for the 2 mm
is his claim correct?