506.10 Duct Clearances — 2025 California Mechanical Code
California Mechanical Code (Title 24, Part 4) · 2025 edition · updated 2026-09-25 · California
CBSC: official editions, supplements and errata
Italicized text is a California amendment to the model code, as printed in the official publication.
Cite as: 2025 California Mechanical Code (Title 24, Part 4) § 506.10 · Effective January 1, 2026 · Official source: Building Standards Commission
Unless the conditions stipulated in Section 506.10.1 or Section 506.10.2 exist, all 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]
506.10.2 Systems Conveying Combustible Mate- rials. Unless the conditions stipulated in Section 506.10.3 exist, all duct systems and system components handling combustible materials shall have a clearance of not less than 18 inches (457 mm) from stored combustible materials or combustible construction. [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]
| 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 |
For SI units: 1 inch = 25.4 mm, °C=(°F-32)/1.8
EXHAUST SYSTEMS
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]
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