308.2 Reduction table.
FGC § 308.2
The allowable clearance reduction shall be based on one of the methods specified in Table 308.2 or shall utilize a reduced clearance protective assembly listed and labeled in accordance with UL 1618. Where required clearances are not listed in Table 308.2, the reduced clearances shall be determined by linear interpolation between the distances listed in the table. Reduced clearances shall not be derived by extrapolation below the range of the table. The reduction of the required clearances to combustibles for listed and labeled appliances and equipment shall be in accordance with the requirements of this section except that such clearances shall not be reduced where reduction is specifically prohibited by the terms of the appliance or equipment listing (see Figures 308.2(1) through 308.2(3)). Table 308.2 a through k Reduction of Clearances With Specified Forms of Protection Type of Protection Applied to and Covering All Surfaces of Combustible Material Within the Distance Specified as the Required Clearance With No ProtectionWhere the Required Clearance With No Protection Appliance, Vent Connector, or Single-Wall Metal Pipe is: (inches)36181296Allowable clearances with specified protection (inches)Use Column 1 for clearances above appliance or horizontal connector. Use Column 2 for clearances from appliance, vertical connector, and single-wall metal pipe.Above Col. 1Sides and rear Col. 2 Above Col. 1Sides and rear Col. 2 Above Col. 1Sides and rear Col. 2 Above Col. 1Sides and rear Col. 2 Above Col. 1Sides and rear Col. 2 Type of Protection Applied to and Covering All Surfaces of Combustible Material Within the Distance Specified as the Required Clearance With No ProtectionWhere the Required Clearance With No Protection Appliance, Vent Connector, or Single-Wall Metal Pipe is: (inches)36181296Allowable clearances with specified protection (inches)Use Column 1 for clearances above appliance or horizontal connector. Use Column 2 for clearances from appliance, vertical connector, and single-wall metal pipe.Above Col. 1Sides and rear Col. 2 Above Col. 1Sides and rear Col. 2 Above Col. 1Sides and rear Col. 2 Above Col. 1Sides and rear Col. 2 Above Col. 1Sides and rear Col. 2 1. 3 1/2-inch-thick masonry wall without ventilated airspace— 24— 12— 9— 6— 52. 1/2-inch insulation board over1-inch glass fiber or mineral wool batts24181299665433. 0.0296 inch (No. 22 gauge) galvanized sheet metal over 1-inch glass fiber or mineral wool batts reinforced with wire on rear face with ventilated airspace1812966453334. 3 1/2-inch-thick masonry wall with ventilated airspace—12—6—6—6—65. 0.024-inch (nominal 24 gage) sheet metal with ventilated airspace1812966453326. 1/2-inch thick insulation board with ventilated airspace1812966453337. 0.024-inch (nominal 24 gage) sheet metal with ventilated airspace over 0.024-inch (nominal 24 gage) sheet metal with ventilated airspace1812966453338. 1-inch glass fiber or mineral wool batts sandwiched between two sheets 0.024-inch (nominal 24 gage) sheet metal with ventilated airspace181296645333 For SI: 1 inch = 25.4 mm, °C = [(°F-32)/1.8], 1 pound per cubic foot = 16.02 kg/m 3 , 1 Btu per inch per square foot per hour per °F = 0.144 W/m 2 K. a. Reduction of clearances from combustible materials shall not interfere with combustion air, draft hood clearance and relief, and accessibility of servicing. b. All clearances shall be measured from the outer surface of the combustible material to the nearest point on the surface of the appliance, disregarding any intervening protection applied to the combustible material. c. Spacers and ties shall be of noncombustible material. No spacer or tie shall be used directly opposite an appliance or connector. d. For all clearance reduction systems using a ventilated airspace, adequate provision for air circulation shall be provided as described. e. There shall be not less than 1 inch between clearance reduction systems and combustible walls and ceilings for reduction systems using ventilated airspace. f. Where a wall protector is mounted on a single flat wall away from corners, it shall have a minimum 1-inch air gap. To provide air circulation, the bottom and top edges, or only the side and top edges, or all edges shall be left open. g. Mineral wool batts (blanket or board) shall have a minimum density of 8 pounds per cubic foot and a minimum melting point of 1,500°F. h. Insulation material used as part of a clearance reduction system shall have a thermal conductivity of 1.0 Btu per inch per square foot per hour per °F or less. i. There shall be not less than 1 inch between the appliance and the protector. In no case shall the clearance between the appliance and the combustible surface be reduced below that allowed in this table. j. All clearances and thicknesses are minimum; larger clearances and thicknesses are acceptable. k. Listed single-wall connectors shall be installed in accordance with the manufacturer's instructions. Notes: "A" equals the clearance with no protection. "B" equals the reduced clearance permitted in accordance with Table 308.2. The protection applied to the construction using combustible materials shall extend far enough in each direction to make "C" equal to "A". Figure 308.2(1)Extent of Protection Necessary to Reduce Clearances from Appliance or Vent Connections For SI: 1 Inch = 25.4 mm. Figure 308.2(2)Wall Protector Clearance Reduction System For SI: 1 Inch = 25.4 mm. Figure 308.2(3)Masonry Clearance Reduction System
