SBCCI, BOCA, and ICBO accept Series 4G grease duct as a 0-inch clearance to combustibles system, the company states. According to the company, there is no need to fabricate and install an additional fire-rated enclosure. This grease duct is a two-hour rated system designed for continuous operation at 500 degrees F and for intermittent operation at a temperature of 2,000 degrees. The manufacturer states that it is a complete double-wall system, preinsulated with ceramic fiber insulation, and that it is the only factory-built duct system to meet AC121 test standards.