CMC · California Mechanical Code
What are the general installation, slope and underground duct requirements?
Underground air ducts in California must be approved for burial, sloped at least 1/8 inch per foot back to the main riser, and metal ducts under slabs must be encased in at least 2 inches of concrete. All ductwork must meet Chapter 6 installation rules (pressure rating, Seal Class A sealing, protection from earth, supports and SMACNA‑compliant joints).
Last reviewed: July 6, 2026
What the code requires — 2-4 sentences
The California Mechanical Code requires that air ducts be installed per Chapter 6 rules (pressure rating, protection, supports and sealing) and that underground ducts be approved for underground installation and sloped back to the main riser at not less than 1/8 inch per foot. Key material and installation limits (e.g., separation from earth, encasement in concrete, and listing for air dispersion systems) are specified in § 603.0 family of provisions. § 603.11 controls underground installations and § 603.12 controls air dispersion systems.
The single most important practical rule: underground ducts must be approved for burial and must slope at least 1/8 inch per foot back to the main riser so water drains to the riser, and metal ducts under slabs must be encased in concrete per the code. § 603.11.
Requirements in detail
General installation (chapter-level requirements)
- Ducts must be installed in accordance with Chapter 6 installation rules and the manufacturer’s instructions; pressure classification must be at least the design operating pressure and all ducts shall be sealed to Seal Class A. § 603.1 / § 603.1.1.
- Protection from earth: air ducts other than plastic ducts must be installed with not less than 4 inches (102 mm) separation from earth unless installed as a liner inside concrete, tile or metal pipe, and they must be protected from physical damage. § 603.1.3.
- Under-floor / crawl space: ducts shall not block required access and where you must move under ducts for access provide a vertical clearance of not less than 18 inches (457 mm). § 603.2.
Support, joints and sealing
- Ducts and fittings must be supported per SMACNA standards and per the manufacturer. § 603.3 / § 603.7.
- Joints and seams: must comply with SMACNA construction standards, be made substantially airtight (tape, mastic, gasketing, etc.), and all ducts shall be sealed to Seal Class A. Crimp joints for round ducts require a contact lap of not less than 1 1/2 inches (38 mm) and be mechanically fastened by not less than three sheet-metal screws (or equivalent). § 603.9.
Underground installation (slope, materials, encasement)
- Approval and slope: Ducts installed underground shall be approved for the installation and shall have a slope of not less than 1/8 inch per foot (10.4 mm/m) back to the main riser. § 603.11.
- Materials in ground/slab: Ducts, plenums, and fittings installed in the ground or in a concrete slab may be constructed of concrete, clay, or ceramics provided joints are sealed and the duct is secured in accordance with SMACNA. § 603.11.
- Metal ducts in/under slabs: Metal ducts installed in or under a concrete slab shall be encased in not less than 2 inches (51 mm) of concrete, secured per SMACNA. § 603.11.
Air dispersion systems
- Air dispersion systems (those that intentionally introduce or diffuse air into spaces via distributed emitters) must be completely in exposed locations in duct systems under positive pressure, must not pass through or penetrate fire‑resistance‑rated construction, and must be listed and labeled in accordance with UL 2518. § 603.12.
Decision‑relevant dimensions / values (quick reference)
| Topic | Required value | Code Reference |
|---|---|---|
| Underground duct slope | Not less than 1/8 inch per foot (10.4 mm/m) back to main riser | § 603.11 |
| Metal ducts in/under slab — concrete encasement | Not less than 2 inches (51 mm) of concrete | § 603.11 |
| Separation from earth (non‑plastic ducts) | Not less than 4 inches (102 mm) | § 603.1.3 |
| Crawlspace clearance under ducts | Not less than 18 inches (457 mm) vertical clearance where access requires moving under ducts | § 603.2 |
| Round duct crimp joint lap | Not less than 1 1/2 inches (38 mm) contact lap and at least 3 screws | § 603.9 |
| Seal level for all ducts | Seal Class A required | § 603.1.1 |
Exceptions & special cases
- Materials: buried ductwork may use concrete, clay, or ceramics when installed in the ground or slab if joints are sealed and secured per SMACNA; this is an allowed alternative to metallic ducts. § 603.11.
- Plastic ducts: plastic ducts have their own requirements — see § 603.5 for permitted use and limits (plastic ducts are an exception to the 4‑inch separation requirement when installed as permitted). § 603.1.3 / § 603.5.
- Air dispersion systems: must be listed per UL 2518 and be installed exposed in positive‑pressure ducts; they must not penetrate fire‑rated construction — installation is limited and requires listing. § 603.12.
If a particular application or material is not explicitly covered by § 603.11 or § 603.12, the code requires compliance with the general installation rules in Chapter 6 and with the referenced SMACNA standards; consult the manufacturer’s instructions and the AHJ for approval.
Common mistakes
- Failing to slope underground ducts: some designers forget to provide the 1/8 inch per foot slope back to the riser — this causes water accumulation and code noncompliance. § 603.11.
- Burying metal ducts without concrete encasement: metal ducts in or under slabs must be encased in >= 2 in. concrete. Omitting this is a frequent violation. § 603.11.
- Not sealing joints to Seal Class A: all ducts must be sealed to Seal Class A; using cloth-backed duct tape alone is not compliant. § 603.1.1 / § 603.9.
- Installing air dispersion systems inside inaccessible or fire-rated penetrations: these must be installed in exposed locations in positive‑pressure ducts and be listed to UL 2518. § 603.12.
- Not protecting non‑plastic ducts from direct contact with earth (less than 4 in. separation) unless properly sleeved or lined per code. § 603.1.3.
Worked example — concrete scenario
Scenario: 50 ft of buried return duct from an equipment pit to the riser. The duct is metallic and will pass under a concrete slab.
- Slope: the required slope is 1/8 in/ft back to the main riser per § 603.11. Over 50 ft the required fall = 50 × 1/8 in = 6.25 inches. Ensure the riser connection is at the low point (or slopes to the riser) so condensate/drainage flows to the riser. § 603.11.
- Slab encasement: because this is a metal duct in or under a concrete slab, the duct must be encased in not less than 2 inches (51 mm) of concrete and secured per SMACNA. § 603.11.
- Protection from earth: if any section of that metal duct is outside a concrete sleeve or encasement, ensure at least 4 in. separation from earth or use an approved liner method per § 603.1.3.
- Joints and sealing: all joints must be made substantially airtight and meet Seal Class A—use SMACNA sealing methods and mechanical fastening where required. § 603.9 / § 603.1.1.
Related provisions
- § 603.1 — General installation, pressure classification, Seal Class A.
- § 603.1.3 — Separation from earth (4 in.) for non-plastic ducts.
- § 603.2 — Under-floor / crawl space clearance (18 in.).
- § 603.4 — Requirements for flexible air ducts (support spacing, sag limits).
- § 603.8 — Protection against flood damage for ducts.
- § 603.9 — Joints, seams and closure systems; Table 603.9.1.
- § 603.11 — Underground installation (slope, materials, encasement).
- § 603.12 — Air dispersion systems (location and listing).
Code references
Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:
CMC § 603.9.2.1 High relevance — show source text
603.9.2.1 Duct Leakage Tests for Buildings that Meet Air Distribution System Duct Leak- age Sealing Criteria in Title 24, Part 6. For duct leakage testing, see California Energy Code Sec- tions 150.0(m)(11) for single family buildings, Sec- tion 160.3(b) for multifamily buildings, and Section 120.4(g) for nonresidential and Hotel Motel build- ings. 603.10 Cross Contamination. Exhaust ducts that convey Class 4 air shall be negatively pressurized relative to ducts, plenums, or occupiable spaces through which the ducts pass. Exhaust ducts under positive pressure that convey Class 2 or Class 3 air shall not extend into or pass through ducts, plenums, or occupiable spaces other than the space from which the exhaust air is drawn.
603.11 Underground Installation. Ducts installed underground shall be approved for the installation and shall have a slope of not less than [1] ⁄ 8 inch per foot (10.4 mm/m) back to the main riser. Ducts, plenums, and fittings shall be permitted to be constructed of concrete, clay, or ceramics where installed in the ground or in a concrete slab, provided the joints are sealed and duct is secured in accordance with SMACNA
HVAC Duct Construction Standards – Metal and Flexible.
Metal ducts where installed in or under a concrete slab shall be encased in not less than 2 inches (51 mm) of concrete, secured in accordance with SMACNA HVAC Duct Con struction Standards – Metal and Flexible.
603.12 Air Dispersion Systems. Where installed, air dispersion systems shall be completely in exposed locations in duct systems under positive pressure, and not pass through or penetrate fire-resistant-rated construction. Air dispersion systems shall be listed and labeled in accordance with UL 2518.
603.13 Clearances. Duct clearances shall be in accordance with Section 603.13.1 through Section 603.13.6.
603.13.1 General. Duct clearances to combustible construction shall be in accordance with the listing and the manufacturer’s installation instructions.
603.13.2 Supply Air Ducts to Listed Furnaces. Supply air ducts connecting to listed central heating furnaces shall have the same minimum clearance to combustibles as required for the furnace supply plenum for a distance of not less than 3 feet (914 mm) from the supply plenum. Clearance shall not be required beyond the 3 feet (914 mm) distance. [NFPA 54:10.3.3.7] 603.13.3 Supply Air Ducts to Unlisted Furnaces. Supply air ducts connecting to unlisted central heating furnaces equipped with temperature limit controls with a maximum setting of 250°F (121°C) shall have a minimum clearance to combustibles of 6 inches (152 mm) for a distance of not less than 6 feet (1829 mm) from the furnace supply plenum. Clearance shall not be required beyond the 6 feet (1829 mm) distance. [NFPA 54:10.3.3.8]
140 2025 CALIFORNIA MECHANICAL CODE
), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.
DUCT SYSTEMS
CMC § 510.1.2 High relevance — show source text
510.1.2 Interconnection. Duct systems shall not be interconnected with any other building ventilation or exhaust system. [NFPA 96:7.1.3]
510.1.3 Duct Installation. All ducts shall be installed with a minimum slope of ¼ inch per linear foot (20.8 mm/m) on horizontal runs up to 75 feet (22 860 mm) and a minimum slope of 1 inch per linear foot (83.2 mm/m) on horizontal runs greater than 75 feet (22 860 mm). Factory-built grease ducts shall be permitted to be installed at a lesser slope in accordance with the listing and the manufacturer’s instructions. All horizontal ducts shall be provided with access in accordance with Section 510.3.3.
Drains shall be provided at low points in horizontal ducts. Where provided, drains shall be continuously welded to the exhaust duct or in accordance with the terms of the listing and the manufacturer’s installation manual.
All ducts shall be installed without forming dips or traps. In manifold (common duct) systems, the lowest end of the main duct shall be connected flush on the bottom with the branch duct. [NFPA 96:7.1.5 – 7.1.5.5]
510.1.4 Accessibility. Openings required for accessibility shall comply with Section 510.3 through Section 510.3.2. [NFPA 96:7.1.6]
510.1.5 Sign. A sign stating the following shall be placed on all access panels:
ACCESS PANEL – DO NOT OBSTRUCT [NFPA 96:7.1.7]
510.1.6 Bracing and Supports. Duct bracing and supports shall be of noncombustible material, securely attached to the structure and designed to carry gravity and lateral loads within the stress limitations of the building code. Bolts, screws, rivets, and other mechanical fasteners shall not penetrate duct walls.
510.1.7 Type I Exhaust Duct Systems. Listed grease ducts shall be installed in accordance with the terms of their listing and the manufacturer’s instructions. [NFPA 96:7.1.8]
510.1.8 Independent Grease Duct System. Single or combined Type I exhaust systems shall be independent of other exhaust systems.
510.2 Clearance. Clearance between ducts and combustible materials shall be provided in accordance with the requirements of Section 507.4 through Section 507.4.3.3. [NFPA 96:7.2] 510.3 Openings. Openings shall be provided at the sides or at the top of the duct, whichever is more accessible, and at changes of direction. Openings shall be protected by approved access constructed and installed in accordance with the requirements of Section 510.3.7. [NFPA 96:7.3.1, 7.3.2]
Exception: Openings shall not be required in portions of the duct that are accessible from the duct entry or discharge.
[NFPA 96:7.3.3]
510.3.1 Access Panel. For hoods with dampers in the exhaust or supply collar, an access panel for cleaning and inspection shall be provided in the duct or the hood within 18 inches (457 mm) of the damper. [NFPA 96:7.3.4]
Exception: Dampers that are accessible from under the hood.
CMC § 0.65 High relevance — show source text
Duct leakage tests shall be performed by a technician certified by the Associated Air Balance Council (AABC), the National Environmental Balancing Bureau (NEBB), the Testing, Adjusting and Balancing Bureau (TABB), or other equivalent approved agencies. Representative sections totaling not less than 10 percent of the total installed duct area shall be tested. Where the tested 10 percent fail to comply with the requirements of this section, then 40 percent of the total installed duct area shall be tested. Where the tested 40 percent fail to comply with the requirements of this section, then 100 percent of the total installed duct area shall be tested. Sections shall be selected by the building owner or designated representative of the building owner. Positive pressure leakage testing shall be permitted for negative pressure ductwork. The permitted duct leakage shall be not more than the following:
Lmax = CLP [0.65 ] (Equation 603.9.2)
Where:
Lmax = maximum permitted leakage, (ft [3] /min)/100 square feet [0.0001 (m [3] /s)/m [2] ] duct surface
area.
CL = six, duct leakage class, (ft [3] /min)/100 square feet [0.0001 (m [3] /s)/m [2] ] duct surface area at 1 inch water column (0.2 kPa).
P = test pressure, which shall be equal to the design duct pressure class rating, inch water column (kPa).
Exception: Transfer air duct operating at less than 1 inch of water column (0.25 kPa). 603.9.2.1 Duct Leakage Tests for Buildings that Meet Air Distribution System Duct Leak- age Sealing Criteria in Title 24, Part 6. For duct leakage testing, see California Energy Code Sec- tions 150.0(m)(11) for single family buildings, Sec- tion 160.3(b) for multifamily buildings, and Section 120.4(g) for nonresidential and Hotel Motel build- ings. 603.10 Cross Contamination. Exhaust ducts that convey Class 4 air shall be negatively pressurized relative to ducts, plenums, or occupiable spaces through which the ducts pass. Exhaust ducts under positive pressure that convey Class 2 or Class 3 air shall not extend into or pass through ducts, plenums, or occupiable spaces other than the space from which the exhaust air is drawn.
603.11 Underground Installation. Ducts installed underground shall be approved for the installation and shall have a slope of not less than [1] ⁄ 8 inch per foot (10.4 mm/m) back to the main riser. Ducts, plenums, and fittings shall be permitted to be constructed of concrete, clay, or ceramics where installed in the ground or in a concrete slab, provided the joints are sealed and duct is secured in accordance with SMACNA
HVAC Duct Construction Standards – Metal and Flexible.
Metal ducts where installed in or under a concrete slab shall be encased in not less than 2 inches (51 mm) of concrete, secured in accordance with SMACNA HVAC Duct Con struction Standards – Metal and Flexible.
CMC § 603.1 High relevance — show source text
603.1 General. Air ducts shall be installed in accordance with this chapter and the installation instructions.
603.1.1 Pressure Classification. The pressure classification of ducts shall be not less than the design operating pressure of the air distribution in which the duct is utilized. All ducts regardless of pressure classification(s) shall be sealed to Seal Class A.
603.1.2 Air Temperature. The temperature of the air to be conveyed in a duct shall not exceed 250°F (121°C).
603.1.3 Protection. Air ducts, other than plastic ducts, shall be installed with not less than 4 inches (102 mm) of
separation from earth, except where installed as a liner inside of concrete, tile, or metal pipe and shall be protected from physical damage.
603.1.4 Vertical Risers. Ducts listed and labeled to UL 181 shall not be used for vertical risers in air-duct systems serving more than two stories.
603.1.5 Penetrations. Ducts listed and labeled to UL 181 shall not penetrate a fire-resistance-rated assembly or construction.
603.1.6 Earthquake Loads. Ducts located in structures that are installed in areas classified as seismic design categories shall be restrained to resist displacement due to earthquake motion.
603.2 Under-Floor or Crawl Space. Air ducts installed under a floor in a crawl space shall be installed in accordance with the following:
(1) Shall not prevent access to an area of the crawl space.
(2) Where it is required to move under ducts for access to areas of the crawl space, a vertical clearance of not less than 18 inches (457 mm) shall be provided.
603.3 Metal Ducts. Ducts shall be supported at each change of direction and in accordance with SMACNA HVAC Duct
Construction Standards – Metal and Flexible. Riser ducts shall be held in place by means of metal straps or angles and channels to secure the riser to the structure.
Ducts shall be installed in a building with clearances that will retain the full thickness of fireproofing on structural members.
603.4 Flexible Air Ducts. Flexible air ducts shall comply with UL 181, and shall be installed in accordance with the manufacturer’s installation instructions and SMACNA
HVAC Duct Construction Standards – Metal and Flexible.
Flexible air duct installations shall comply with the following:
(1) Ducts shall be installed using the minimum required length to make the connection.
(2) Horizontal duct runs shall be supported at not more than 4 feet (1219 mm) intervals.
(3) Vertical risers shall be supported at not more than 6 feet (1829 mm) intervals.
(4) Sag between support hangers shall not exceed [1] ⁄ 2 inch (12.7 mm) per foot (305 mm) of support spacing.
(5) Supports shall be rigid and shall be not less than 1 [1] ⁄ 2 inches (38 mm) wide at point of contact with the duct surface.
(6) Duct bends shall be not less than one duct diameter bend radius.
(7) Screws shall not penetrate the inner liner of non-metallic flexible ducts unless permitted in accordance with the manufacturer’s installation instructions.
(8) Fittings for attaching non-metallic ducts shall be beaded and have a collar length of not less than 2 inches (51 mm) for attaching the duct. Metal worm-gear clamps shall be used.
CMC § 509.2.4.1 High relevance — show source text
509.2.4.1 Size and Pitch. Grease drip trays shall be kept to the minimum size needed to collect grease. Grease drip trays shall be pitched to drain into an enclosed metal container having a capacity not exceeding 1 gallon (4 L). [NFPA 96:6.2.4.2, 6.2.4.3]
114 2025 CALIFORNIA MECHANICAL CODE
), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.
EXHAUST SYSTEMS
509.2.5 Grease Filter Orientation. Grease filters that require a specific orientation to drain grease shall be clearly so designated on the face of the filter as to be visible with the filter installed, or the hood or filter shall be constructed so that filters cannot be installed in the wrong orientation. [NFPA 96:6.2.5]
509.3 Solid-Fuel Grease Removal Devices. Where solid-fuel cooking equipment is provided with grease removal devices, these devices shall be in accordance with Section 517.0.
510.0 Exhaust Duct Systems.
510.1 General. Ducts shall not pass through fire walls.
[NFPA 96:7.1.1]
510.1.1 Fire Hazards. All ducts shall lead directly to the exterior of the building, so as not to unduly increase any fire hazard. [NFPA 96:7.1.2]
510.1.2 Interconnection. Duct systems shall not be interconnected with any other building ventilation or exhaust system. [NFPA 96:7.1.3]
510.1.3 Duct Installation. All ducts shall be installed with a minimum slope of ¼ inch per linear foot (20.8 mm/m) on horizontal runs up to 75 feet (22 860 mm) and a minimum slope of 1 inch per linear foot (83.2 mm/m) on horizontal runs greater than 75 feet (22 860 mm). Factory-built grease ducts shall be permitted to be installed at a lesser slope in accordance with the listing and the manufacturer’s instructions. All horizontal ducts shall be provided with access in accordance with Section 510.3.3.
Drains shall be provided at low points in horizontal ducts. Where provided, drains shall be continuously welded to the exhaust duct or in accordance with the terms of the listing and the manufacturer’s installation manual.
All ducts shall be installed without forming dips or traps. In manifold (common duct) systems, the lowest end of the main duct shall be connected flush on the bottom with the branch duct. [NFPA 96:7.1.5 – 7.1.5.5]
510.1.4 Accessibility. Openings required for accessibility shall comply with Section 510.3 through Section 510.3.2. [NFPA 96:7.1.6]
510.1.5 Sign. A sign stating the following shall be placed on all access panels:
ACCESS PANEL – DO NOT OBSTRUCT [NFPA 96:7.1.7]
CMC § 120.4 High relevance — show source text
Nonresidential and hotel/motel buildings shall comply with the applicable requirements of Sections 120.4(a) through 120.4(g).
Exception to Section 120.4: Systems serving healthcare facilities shall comply with the applicable requirements of the California Mechanical Code.
(a) CMC compliance. All air distribution system ducts and plenums, including but not limited to building cavities, mechanical closets, air-handler boxes and support platforms used as ducts or plenums, shall meet the requirements of the CMC Sections 601.0, 602.0, 603.0, 604.0, and 605.0, and ANSI/SMACNA-006-2006 HVAC Duct Construction Standards Metal and Flexible, 3rd Edition incorporated herein by reference. Connections of metal ducts and the inner core of flexible ducts shall be mechanically fastened. Openings shall be sealed with mastic, tape, aerosol sealant or other duct-closure system that meets the applicable requirements of UL 181, UL 181A, or UL 181B. If mastic or tape is used to seal openings greater than [1] / 4 inch, the combination of mastic and either mesh or tape shall be used.
Portions of supply-air and return-air ducts conveying heated or cooled air located in one or more of the following spaces shall be insulated to a minimum installed level of R-8:
Outdoors; or
In a space between the roof and an insulated ceiling; or
In a space directly under a roof with fixed vents or openings to the outside or unconditioned spaces; or
In an unconditioned crawlspace; or
In other unconditioned spaces.
2025 CALIFORNIA ENERGY CODE 77
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
NONRESIDENTIAL, HOTEL/MOTEL OCCUPANCIES, AND COVERED PROCESSES—MANDATORY REQUIREMENTS
Portions of supply-air ducts that are not in one of these spaces, including ducts buried in concrete slab, shall be insulated to a minimum installed level of R-4.2 or be enclosed in directly conditioned space.
(b) Duct and plenum materials.
- Factory-fabricated duct systems. A. All factory-fabricated duct systems shall comply with UL 181 for ducts and closure systems, including collars, connections and splices, and be labeled as complying with UL 181. UL 181 testing may be performed by UL laboratories or a laboratory approved by the Executive Director. B. All pressure-sensitive tapes, heat-activated tapes, and mastics used in the manufacture of rigid fiberglass ducts shall comply with UL 181 and UL 181A. C. All pressure-sensitive tapes and mastics used with flexible ducts shall comply with UL 181 and UL 181B. D. All ductwork and plenums with pressure class ratings shall be constructed to Seal Class A. Joints and seams of duct systems and their components shall not be sealed with cloth-back rubber adhesive duct tapes unless such tape is used in combination with mastic and drawbands.
Exception to Section 120.4(b)1D: Ductwork located in occupied space and exposed to view is not required to meet Seal Class A.
CMC § 602.5.2 High relevance — show source text
602.5.2 Gypsum. Where gypsum products are exposed in ducts or plenums, the air temperature shall be restricted to a range from 50°F (10°C) to 125°F (52°C), and moisture content shall be controlled so that the material is not adversely affected. All gypsum products shall have a mold or mildew resistant surface. For the purpose of this section, gypsum products shall not be exposed in supply ducts.
602.5.3 Air Dispersion Systems. Air dispersion systems shall be listed and labeled in accordance with UL
602.5.4 Fibrous Glass Duct. Fibrous glass ducts, plenums, or fittings shall be constructed in accordance with SMACNA Fibrous Glass Duct Construction Stan dards or NAIMA Fibrous Glass Duct Construction Stan dards.
602.5.5 Other Materials. Flexible and rigid ducts, plenums, or fittings for use in heating, ventilation, and air conditioning systems of other nonmetallic materials listed and labeled to UL 181 shall be permitted.
Exception: Plastic ducts shall comply with Section 603.5.
602.6 Vibration Isolators. Vibration isolation connectors installed between mechanical equipment and metal ducts (or casings) shall be made of an approved material and shall not exceed 10 inches (254 mm) in length.
602.7 Corridors. Corridors shall not be used to convey air to or from rooms where the corridor is required to be of fire-resistive construction in accordance with the building code except where permitted by the building code. Corridors shall not serve as supply, return, exhaust, relief, or ventilation air ducts.
603.0 Installation of Ducts.
603.1 General. Air ducts shall be installed in accordance with this chapter and the installation instructions.
603.1.1 Pressure Classification. The pressure classification of ducts shall be not less than the design operating pressure of the air distribution in which the duct is utilized. All ducts regardless of pressure classification(s) shall be sealed to Seal Class A.
603.1.2 Air Temperature. The temperature of the air to be conveyed in a duct shall not exceed 250°F (121°C).
603.1.3 Protection. Air ducts, other than plastic ducts, shall be installed with not less than 4 inches (102 mm) of
separation from earth, except where installed as a liner inside of concrete, tile, or metal pipe and shall be protected from physical damage.
603.1.4 Vertical Risers. Ducts listed and labeled to UL 181 shall not be used for vertical risers in air-duct systems serving more than two stories.
603.1.5 Penetrations. Ducts listed and labeled to UL 181 shall not penetrate a fire-resistance-rated assembly or construction.
603.1.6 Earthquake Loads. Ducts located in structures that are installed in areas classified as seismic design categories shall be restrained to resist displacement due to earthquake motion.
603.2 Under-Floor or Crawl Space. Air ducts installed under a floor in a crawl space shall be installed in accordance with the following:
(1) Shall not prevent access to an area of the crawl space.
(2) Where it is required to move under ducts for access to areas of the crawl space, a vertical clearance of not less than 18 inches (457 mm) shall be provided.
CMC § 160.3 High relevance — show source text
Exception 2 to Section 160.3(c)1: Piping that conveys fluids with a design operating temperature range between 60°F and 105°F.
Exception 3 to Section 160.3(c)1: Where the heat gain or heat loss to or from piping without insulation will not increase building source energy use. Exception 4 to Section 160.3(c)1: Piping that penetrates framing members shall not be required to have pipe insulation for the distance of the framing penetration. Metal piping that penetrates metal framing shall use grommets, plugs, wrapping or other insulating material to ensure that no contact is made with the metal framing. 2. Requirements for air distribution system, ducts and plenum. Multifamily common areas shall comply with the applicable requirements of Sections 160.3(c)2A through 160.3(c)2F. A. CMC compliance. All air distribution system ducts and plenums, including, but not limited to, building cavities, mechanical closets, air-handler boxes and support platforms used as ducts or plenums shall meet the requirements of CMC Sections 601.0, 602.0, 603.0, 604.0 and 605.0, and ANSI/SMACNA-006-2006 HVAC Duct Construction Standards Metal and Flexible 3rd Edition, incorporated herein by reference. Connections of metal ducts and the inner core of flexible ducts shall be mechanically fastened. Openings shall be sealed with mastic, tape, aerosol sealant or other duct-closure system that meets the applicable requirements of UL 181, UL 181A or UL 181B. If mastic or tape is used to seal openings greater than [1] / 4 inch, the combination of mastic and either mesh or tape shall be used. B. Portions of supply-air and return-air ducts conveying heated or cooled air located in one or more of the following spaces shall be insulated to a minimum installed level of R-8:
i. Outdoors; or ii. In a space between the roof and an insulated ceiling; or iii. In a space directly under a roof with fixed vents or openings to the outside or unconditioned spaces; or iv. In an unconditioned crawl space; or v. In other unconditioned spaces. Portions of supply-air ducts that are not in one of these spaces, including ducts buried in concrete slab, shall be insulated to a minimum installed level of R-4.2 or be enclosed in directly conditioned space. C. Duct and plenum materials. i. Factory-fabricated duct systems. a. All factory-fabricated duct systems shall comply with UL 181 for ducts and closure systems, including collars, connections and splices, and be labeled as complying with UL 181. UL 181 testing may be performed by UL laboratories or a laboratory approved by the Executive Director. b. All pressure-sensitive tapes, heat-activated tapes and mastics used in the manufacture of rigid fiberglass ducts shall comply with UL 181 and UL 181A. c. All pressure-sensitive tapes and mastics used with flexible ducts shall comply with UL 181 and UL 181B. d. Ductwork and plenums with pressure class ratings shall be constructed to Seal Class A. Joints and seams of duct systems and their components shall not be sealed with cloth back rubber adhesive duct tapes unless such tape is used in combination with mastic and drawbands.
CMC § 5.70 High relevance — show source text
A. Use appropriate size Shurlock/Key-Lock � coupler listed in Table 22.
B. Use a straight section of at least 18 inches of appropriate size PVC conduit listed in Table 6 on Page 6.
C. Use appropriate size end bell for PVC conduit listed in Table 13 on Page 8.
Table 24 Data and Material Codes for Shur-Lock II Coupler
Figure 31 Shur-Lock II Coupler
Conduit Size
(inches)Dimension
(inches)Material Code Conduit Size
(inches)Length Length 2 5.70 027241 3 10.75 027242 4 12.00 027243 5 14.25 027244 6 14.25 027256 062288 Page 14 of 20 Rev. #20: 03−25−22
UG-1: Conduits Greenbook Underground Conduits
Conduit Construction, Built-up Method
3”
2”
2”
3”
Plastic Conduit
3” 3” 2” 3” 2” 3” (see Table 6 on Page 6)
Separation between conduits when using spacer. See Figure 16 on Page 8.
Concrete
Figure 32 Typical Straight Run Installation (6” conduit, 6-way shown)
Spacer
Figure 33 Typical Perspective View of Installation Around Obstruction
Figure 34 Typical Perspective View of Installation Under Obstruction
Rev. #20: 03−25−22 062288 Page 15 of 20
UG-1: Conduits Greenbook Underground Conduits
Construction and Termination of Conduit Line into a Manhole or Vault
Notes
Slope the conduit sufficiently to provide adequate drainage. On level ground, slope the duct line from the center to each manhole.
Local city ordinances may require a minimum cover greater than PG&E’s requirements of 24” for conduits containing circuits energized at 750 volts or less, and 36” for conduits containing circuits energized in excess of 750 volts. Depth may be reduced in either case if adequate mechanical protection − as defined in Section 3.3.6 of the Greenbook− is provided. where i_chronicle_id ='09131aad80e064e4' and any r_version_label='LIVE')
Where required, the depth of the conduit window may be increased. Where this is necessary, tie the conduit envelope concrete into the manhole reinforcing steel.
Vertical staggering of conduits at entrance shown in Detail A will assist in arranging for cable crossover if required.
Col1 B Col3 Col4 Col5 Col6 Col7 B B Manhole Cover Plan View Manhole Cover
CMC § 1.0 High relevance — show source text
D. The outdoor design temperatures for cooling shall be no greater than the 1.0 percent Cooling Dry Bulb and Mean Coincident Wet Bulb values.
214 2025 CALIFORNIA ENERGY CODE
on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.
MULTIFAMILY BUILDINGS—MANDATORY REQUIREMENTS
- Outdoor condensing units . A. Clearances. Installed air conditioner and heat pump outdoor condensing units shall have a clearance of at least 5 feet (1.5 meters) from the outlet of any dryer vent. B. Liquid line drier. Installed air conditioner and heat pump systems shall be equipped with liquid line filter driers if required, as specified by manufacturer’s instructions.
- Central forced-air heating furnaces . A. Temperature rise . Central forced-air heating furnace installations shall be configured to operate in conformance with the furnace manufacturer's maximum inlet-to-outlet temperature rise specifications.
- Air-distribution and ventilation system ducts, plenums and fans. A. CMC compliance.
i. All air-distribution system ducts and plenums, including, but not limited to, mechanical closets and air-handler boxes, shall meet the requirements of the CMC Sections 601.0, 602.0, 603.0, 604.0 and 605.0 and ANSI/SMACNA006-2006 HVAC Duct Construction Standards Metal and Flexible 3rd Edition, incorporated herein by reference. ii. Portions of supply-air and return-air ducts and plenums of a space heating or cooling system shall be insulated in accordance with either Subsection a or b below:
a. Ducts shall have a minimum installed level of R-6.0, or
Exception to Section 160.3(b)5Aiia : Portions of the duct system located in conditioned space below the ceiling separating the occupiable space from the attic are not required to be insulated if all of the following conditions are met: i. The noninsulated portion of the duct system is located entirely inside the building’s thermal envelope as confirmed by visual inspection. ii. At all locations where noninsulated portions of the duct system penetrate into unconditioned space, the penetration shall be draft stopped compliant with CFC Sections 703.1 and 704.1 and air-sealed to the construction materials that are penetrated, using materials compliant with CMC Section E502.4.2 to prevent air infiltration into the cavity. All connections in unconditioned space are insulated to a minimum of R-6.0 as confirmed by visual inspection. b. Ducts do not require insulation when the duct system is located entirely in conditioned space. For buildings with three or fewer habitable stories, duct systems located entirely in conditioned space shall be confirmed through field verification and diagnostic testing in accordance with the requirements of Reference Residential Appendix RA3.1.4.3.8. iii. Connections of metal ducts and the inner core of flexible ducts shall be mechanically fastened. iv. Openings shall be sealed with mastic, tape or other duct-closure system that meets the applicable requirements of UL 181, UL 181A or UL 181B or aerosol sealant that meets the requirements of UL 723. If mastic or tape is used to seal openings greater than [1] / 4 inch, the combination of mastic and either mesh or tape shall be used. v. Building cavities, support platforms for air handlers, and plenums designed or constructed with materials other than sealed sheet metal, duct board or flexible duct shall not be used for conveying conditioned air.
CMC § 1106.3 High relevance — show source text
1106.3 Normal Operation . . . . . . . . . . . . . 233
1106.4 Natural Ventilation . . . . . . . . . . . . 234
1106.5 Combustion Air . . . . . . . . . . . . . . . 234
1106.6 Ventilation Intake . . . . . . . . . . . . . 234
1106.7 Maximum Temperature . . . . . . . . 234
1106.8 Refrigerant Parts in Air Duct. . . . . 234
1106.9 Dimensions. . . . . . . . . . . . . . . . . . 234
1106.10 Exits . . . . . . . . . . . . . . . . . . . . . . . 234
1106.11 Machinery Room, A2L and B2L . . 234
Table 1106.11.10.2 Refrigerant Detector Set Points, Response Times, Alarms, and Ventilation Levels . . . . . . . . . . . . . 235
Table 1106.11.11.2 Level 1 Ventilation Rate for
Class 2L Refrigerants . . . . . . . . . . . 236
1107.0 Machinery Room, Special Requirements . . . . . . . . . . . . . . . . 236
1107.1 General. . . . . . . . . . . . . . . . . . . . . 236
1108.0 Refrigeration Machinery Room Equipment and Controls . . . . . . . . 237
1108.1 General. . . . . . . . . . . . . . . . . . . . . 237
1108.2 Electrical . . . . . . . . . . . . . . . . . . . . 237
1108.3 Emergency Shutoff . . . . . . . . . . . . 237
2025 CALIFORNIA MECHANICAL CODE
1108.4 Installation, Maintenance, and Testing . . . . . . . . . . . . . . . . . . . . . 237
1108.5 Emergency Pressure Control System . . . . . . . . . . . . . . . . . . . . . 237
1109.0 Refrigeration Piping, Containers, and Valves . . . . . . . . . . . . . . . . . . 237
1109.1 Materials . . . . . . . . . . . . . . . . . . . . 237
Table 1109.1 Materials for Refrigerant Piping, Tubing, and Fittings . . . . . . . . . . . 239
1109.2 Joints . . . . . . . . . . . . . . . . . . . . . . 239
1109.3 Penetration of Piping . . . . . . . . . . 239
1109.4 Location of Refrigeration Piping . . 239
1109.5 Underground Piping . . . . . . . . . . . 239
1109.6 Support . . . . . . . . . . . . . . . . . . . . . 239
1109.7 Pipe Enclosure . . . . . . . . . . . . . . . 239
1109.8 Visual Inspection . . . . . . . . . . . . . 240
CMC § 602.3 Medium relevance — show source text
602.3 Tall Wood (Mass Timber) Buildings . . . . . . . . . . . . . . . . . . . . 137
602.4 Metallic . . . . . . . . . . . . . . . . . . . . . 138
602.5 Nonmetallic Ducts. . . . . . . . . . . . . 138
602.6 Vibration Isolators. . . . . . . . . . . . . 138
602.7 Corridors. . . . . . . . . . . . . . . . . . . . 138
603.0 Installation of Ducts . . . . . . . . . . . 138
603.1 General. . . . . . . . . . . . . . . . . . . . . 138
603.2 Under-Floor or Crawl Space. . . . . 138
603.3 Metal Ducts. . . . . . . . . . . . . . . . . . 138
603.4 Flexible Air Ducts . . . . . . . . . . . . . 138
603.5 Plastic Ducts. . . . . . . . . . . . . . . . . 139
603.6 Protection of Ducts . . . . . . . . . . . . 139
603.7 Support of Ducts. . . . . . . . . . . . . . 139
603.8 Protection Against Flood Damage . 139
603.9 Joints and Seams of Ducts. . . . . . 139
Table 603.9.1 Closure Markings . . . . . . . . . . . . . 140
603.10 Cross Contamination . . . . . . . . . . 140
603.11 Underground Installation . . . . . . . 140
603.12 Air Dispersion Systems . . . . . . . . 140
603.13 Clearances . . . . . . . . . . . . . . . . . . 140
604.0 Furnace Plenums and Air Ducts
Used in Fuel-Gas Appliances . . . . 141
604.1 Furnace Plenums and Air Ducts. . 141
604.2 Supplied as a Part of Furnace . . . 141
604.3 Not Supplied with the Furnace . . . 141
604.4 Return Air . . . . . . . . . . . . . . . . . . . 141
605.0 Insulation of Ducts . . . . . . . . . . . . 141
605.1 General. . . . . . . . . . . . . . . . . . . . . 141
606.0 Smoke Dampers, Fire Dampers, and Ceiling Dampers . . . . . . . . . . 142
606.1 Smoke Dampers. . . . . . . . . . . . . . 142
606.2 Fire Dampers . . . . . . . . . . . . . . . . 142
606.3 Ceiling Radiation Dampers. . . . . . 142
606.4 Combination Fire/Smoke
Dampers . . . . . . . . . . . . . . . . . . . . 142
606.5 Corridor Dampers . . . . . . . . . . . . . 142
606.6 Periodic Testing and Inspection . . 142
Frequently asked questions
Do underground ducts always require concrete encasement?
Only metal ducts installed in or under a concrete slab are required to be encased in not less than 2 inches (51 mm) of concrete per § 603.11. Other permitted materials (concrete, clay, ceramics) may be used in ground or slab if joints are sealed and ducts are secured per SMACNA.
What is the point of the 1/8 in/ft slope for buried ducts?
The 1/8 inch per foot slope in § 603.11 ensures water draining or condensate in buried ducts flows back to the main riser instead of accumulating in low spots.
Can I run a return or supply duct directly in the soil without protection?
Not for non‑plastic ducts — the code requires 4 inches separation from earth for air ducts (except plastic ducts or when installed as an approved liner inside concrete/tile/metal pipe). See § 603.1.3.
Are air dispersion devices allowed inside fire‑rated shafts?
No. Air dispersion systems must be installed completely in exposed locations in ducts under positive pressure and not pass through or penetrate fire‑resistance‑rated construction, and must be listed to UL 2518 per § 603.12.
If a buried duct uses concrete or ceramic materials, do joints still need sealing?
Yes — when ducts, plenums, and fittings of concrete, clay, or ceramics are used in the ground or slab, the joints must be sealed and the duct secured per SMACNA as required by § 603.11.
More in California Mechanical Code
Ask about the CMC
Get cited, plain-English answers on the California Mechanical Code for your project — any code section, any scenario.
Start Free Trial