506.10 Duct Clearances. Unless the conditions in Section

506.10.1 or Section 506.10.2 exist, duct systems and system components shall have a clearance of at least 6 inches (152 mm) from stored combustible materials, and not less than ¹ ⁄2 of an inch (13 mm) clearance from combustible construction.

[NFPA 91:4.7.1]

506.10.1 Protection Provided. Where stored combustible material or combustible construction is protected from ductwork by the use of materials or products listed for protection purposes, clearance shall be maintained in accordance with those listings. [NFPA 91:4.7.1.1] rials. 506.10.2 Systems Conveying Combustible Mate- Unless the conditions in Section 506.10.3 exist, duct systems and system components handling combustible material shall have a clearance of not less than 18 inches (457 mm) from combustible construction or a combustible material. [NFPA 91:4.7.2] 506.10.3 Reduced Clearance Permitted. When the ductwork system is operating at 140°F (60°C) or below and is equipped with an approved automatic extinguishing system designed for the specific hazard, the clearance shall be permitted to be reduced to 6 inches (152 mm) from combustible materials and ¹ ⁄2 of an inch (12.7 mm) from combustible construction. [NFPA 91:4.7.2.1] 506.10.4 Clearance Increases. All duct systems and system components operating at temperatures above 140°F (60°C) shall have clearances from stored combustible materials or combustible construction not less than those listed in Table 506.10.4. [NFPA 91:4.7.3]

TABLE 506.10.4 BASIC MINIMUM CLEARANCES TO UNPROTECTED SURFACES

[NFPA 91: TABLE 4.7.3]

(1) Have a thermal conductivity of 1 British thermal

unit inch per hour square foot degree Fahrenheit

[Btu•in/(h•ft ² •°F)] [0.14 W/(m•K)] or less. (2) Be formed of noncombustible material. [NFPA

91:4.7.4.4]

DUCT GAS TEMPERATURE	LARGEST DUCT DIMENSION (inches)	CLEARANCE (inches)
140°F – 600°F incl.	8	8
140°F – 600°F incl.	>8	12
>600°F – 900°F incl.	8	18
>600°F – 900°F incl.	>8	24
~~>900°F~~	~~All ducts lined with~~ refractory	24

506.10.4.1 Temperatures Over 900°F. Ducts handling materials at temperatures in excess of 900°F (482°C) shall be lined with refractory material or the equivalent. [NFPA 91:4.7.3.1] 506.10.4.2 Clearance Reduction. When stored combustible materials or combustible construction are protected from ductwork in accordance with Section 506.11, the clearance established in Table 506.10.4 shall be permitted to be reduced in accordance with Table 506.11, but not to less than specified in Section 506.10. [NFPA 91:4.7.3.2]

with refractory material or the equivalent. [NFPA 91:4.7.3.1] 506.10.4.2 Clearance Reduction. When stored combustible materials or combustible construction are protected from ductwork in accordance with Section 506.11, the clearance established in Table 506.10.4 shall be permitted to be reduced in accordance with Table 506.11, but not to less than specified in Section 506.10. [NFPA 91:4.7.3.2]

506.11.6.1 Air Circulation. Air circulation shall be permitted to be provided by leaving all edges of the wall protecting system open with at least a 1 inch (25.4 mm) air gap. [NFPA 91:4.7.4.7.1] 506.11.6.2 Single Flat Wall. If the means for protection is mounted on a single flat wall away from corners, air circulation shall be permitted to be provided by one of the following: (1) Leaving only the top and bottom edges open to

circulation by maintaining the 1 inch (25.4 mm)

(1) Leaving only the top and bottom edges open to

circulation by maintaining the 1 inch (25.4 mm) air gap. (2) Leaving the top and both side edges open to cir culation by maintaining the 1 inch (25.4 mm)

(2) Leaving the top and both side edges open to cir culation by maintaining the 1 inch (25.4 mm) air gap. [NFPA 91:4.7.4.7.2] 506.11.6.3 Thermal Shielding. Thermal shielding that covers two walls in a corner shall be permitted to be open at the top and bottom edges with not less than 1 inch (25.4 mm) air gap. [NFPA 91:4.7.4.7.3]

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2019 CALIFORNIA MECHANICAL CODE 93

EXHAUST SYSTEMS any other purpose or distributed to any other persons or parti es.

TABLE 506.11 REDUCTION OF DUCT CLEARANCE WITH SPECIFIED FORMS OF PROTECTION

*	[NFPA 91: TABLE 4.7.4]
FORM OF PROTECTION*	MAXIMUM ALLOWABLE REDUCTION IN CLEARANCE (percent)	MAXIMUM ALLOWABLE REDUCTION IN CLEARANCE (percent)
FORM OF PROTECTION*	AS WALL PROTECTOR OR VERTICAL SURFACE	AS CEILING PROTECTOR OR HORIZONTAL SURFACE
~~31⁄2 inch thick masonry wall without ventilated air~~ space	33	~~None~~
~~1⁄2 inch thick noncombustible insulation board over~~ 1 inch glass fiber or mineral wool batts without ventilated air space	50	33
~~0.024 inch (24 gauge) sheet metal over 1 inch~~ glass fiber or mineral wool batts reinforced with wire, or equivalent on rear face with at least a 1 inch air gap	66	66
~~31⁄2 inch (90 mm) thick masonry wall with at least~~ a 1 inch air gap	66	~~None~~
~~0.024 inch (24 gauge) sheet metal with at least a 1~~ inch air gap	66	50
~~1⁄2 inch thick noncombustible insulation board with~~ at least a 1 inch air gap	66	50
~~0.024 inch (24 gauge) sheet metal with ventilated~~ air space over at least 0.024 inch (24 gauge) sheet metal with at least a 1 inch air gap	66	50
~~1 inch glass fiber or mineral wool batts sandwiched~~ between two sheets of 0.024 inch (24 guage) sheet metal with at least a 1 inch air gap	66	50

Clearance reduction applied to and covering all combustible surfaces within the distance specified as required clearance with no protection in Table 506.10.4.

94 2019 CALIFORNIA MECHANICAL CODE

EXHAUST SYSTEM

Part II - Commercial Hoods and Kitchen

Ventilation.

penetration fittings and fasteners, shall be listed in accordance with UL 710 or UL 1978. Seams, joints, and penetrations of the hood enclosure shall comply with Section 508.3.2. Seams, joints, and penetrations of the ductwork shall comply with Section 510.5.3.