Wiring in plenums

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mollydodger

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I think I have one of those infamous "gray areas". We are wiring rooftop AC/heat units on a "big box" warehouse type building. Each unit has a "curb" that connects it to the rooftop and all the return/supply air for each unit passes through it. The units have no other ducting and the ceiling inside the building is "open" from the concrete floor to the metal roof sitting on bar joists. The wiring methods throughout the building interior are all "non plenum" types. Questions: Is the area within each "curb" considered a plenum? Does 300.22 (B) apply since all the air is being circulated ( and recirculated) through all the buildings spaces not requiring the wiring methods in the section? Thanks for all replies!!
 
If the duct work is installed throughout the building then IMO the entire building could be a plenum in the sense that the return is pulling thru the areas of those wires so the wiring should be plenum rated. The area where the curb is would not be unless you run thru the duct itself. Maybe I am not visualizing it correctly but that is what it seems like.
 
You likely have duct for environmental air handling through the "curb" at the very least, and at least the conditioned supply duct extending away from the return duct opening(s) a certain distance otherwise you will not get very effective flow to condition the air in the building. As long as you are not in an enclosed space with wiring methods that is dedicated to handling environmental air you are it does not need to be plenum rated. The entire inside of the building is not a plenum. A suspended ceiling that is allowed to let environmental air freely pass through it by design is a plenum, and sometimes other passageways will qualify. But the open area of the store is the space being conditioned and not the plenum for handling the environmental air.
 
plenum wiring

plenum wiring

Dennis, Thanks for the reply. I believe the area inside the building is considered "other space used for environmental air" and is covered by 300.22 (C). The "curb" I am talking about is the sheet metal structure that every rooftop unit sits on and through which all the make up (return air) and supply air passes. The sheet metal curbs (as they are called by the AC folks) are typically about 18 inches high by whatever dimension the AC unit is in lenght and width. Once the rooftop unit is set on the curb there is additional ducting installed inside the building and attached to the bottom side of the curb. It is the space within this ducted and curb area that I believe must conform to 300.22 (B). Am I correct?
 
The rtu's have a space between the inside of the curb and the ductwork where the wiring passes through. That's not a plenum space, it's outside of the ductwork and inside, underneath, the curb.

They don't run totally with no ductwork. There may be 10 ft or so of duct stub into the building and sometimes a D box at the end of that. A plenum is unoccupied mechanical space (that has ductwork attached) that distributes air by fan power. The occupied space building air is moved by convection and sometimes ceiling paddle fans, so, imo, the open building inside is not a plenum. 300.22 is concerned about limiting the spread of products of combustion, by limiting combustible materials in the air handling space, and spreading from the source to other parts of the building unaware of a fire. The code distinguishes the difference between the occupied space and the mechanical space used to handle the air.

300.22 (c) covers unoccupied mechanical spaces or spaces not intended to be occupied, such as mechanical rooms and under raised IT floors. It does not cover occupied building spaces.
 
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The idea is if return area can pull thru the space then it is in the space designated as other space used for environmental air.
 
mollydodger said:
I think I have one of those infamous "gray areas". We are wiring rooftop AC/heat units on a "big box" warehouse type building. Each unit has a "curb" that connects it to the rooftop and all the return/supply air for each unit passes through it. The units have no other ducting and the ceiling inside the building is "open" from the concrete floor to the metal roof sitting on bar joists. The wiring methods throughout the building interior are all "non plenum" types. Questions: Is the area within each "curb" considered a plenum? Does 300.22 (B) apply since all the air is being circulated ( and recirculated) through all the buildings spaces not requiring the wiring methods in the section? Thanks for all replies!!
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If I understand correctly, there is probably 6-18" of duct sticking down below the roof deck that leads straight into the unit. This would be a non-ducted return. IMHO, the curb is not a plenum space or even "other environmental air". As I see it, the concern regarding plenum spaces is the possibility of having a concealed fire or smoke condition that winds up all over the building, as it might in a drop ceiling. I can't see that as being an issue in your typical "big box" store.
 
mollydodger said:
I think I have one of those infamous "gray areas". We are wiring rooftop AC/heat units on a "big box" warehouse type building. Each unit has a "curb" that connects it to the rooftop and all the return/supply air for each unit passes through it. The units have no other ducting and the ceiling inside the building is "open" from the concrete floor to the metal roof sitting on bar joists. The wiring methods throughout the building interior are all "non plenum" types. Questions: Is the area within each "curb" considered a plenum? Does 300.22 (B) apply since all the air is being circulated ( and recirculated) through all the buildings spaces not requiring the wiring methods in the section? Thanks for all replies!!
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I believe the answer that you are looking for is the first sentence in (B) Ducts Specifically Fabricated for Environmental Air. The curb and any duct work attached to it would be "specifically fabricated" the rest of the space in the store would be a "habitable space" and standard wiring practices would apply.

300.22 Wiring in Ducts Not Used for Air Handling, Fabricated Ducts for Environmental Air, and Other Spaces for Environmental Air (Plenums). The provisions of this section shall apply to the installation and uses of electrical wiring and equipment in ducts used for dust, loose stock, or vapor removal: ducts specifically fabricated for environmental air; and other spaces used for environmental air (plenums).
Informational Note: See Article 424, Part VI, for duct heaters.
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(A) Ducts for Dust, Loose Stock, or Vapor Removal. No wiring systems of any type shall be installed in ducts used to transport dust, loose stock, or flammable vapors. No wiring system of any type shall be installed in any duct, or shaft containing only such ducts, used for vapor removal or for ventilation of commercial-type cooking equipment.


(B) Ducts Specifically Fabricated for Environmental Air. Only wiring methods consisting of Type MI cable, Type MC cable employing a smooth or corrugated impervious metal sheath without an overall nonmetallic covering, electrical metallic tubing, flexible metallic tubing, intermediate metal conduit, or rigid metal conduit without an overall nonmetallic covering shall be installed in ducts specifically fabricated to transport environmental air. Flexible metal conduit shall be permitted, in lengths not to exceed 1.2 m (4 ft), to connect physically adjustable equipment and devices permitted to be in these fabricated ducts. The connectors used with flexible metal conduit shall effectively close any openings in the connection. Equipment and devices shall be permitted within such ducts only if necessary for the direct action upon, or sensing of, the contained air. Where equipment or devices are installed and illumination is necessary to facilitate maintenance and repair, enclosed gasketed-type luminaires shall be permitted.
(C) Other Spaces Used for Environmental Air (Plenums). This section shall apply to spaces not specifically fabricated for environmental air-handling purposes but used for air-handling purposes as a plenum. This section shall not apply to habitable rooms or areas of buildings, the prime purpose of which is not air handling.
Informational Note No. 1: The space over a hung ceiling used for environmental air-handling purposes is an example of the type of other space to which this section applies.
Informational Note No. 2: The phrase “Other Spaces Used for Environmental Air (Plenum)” as used in this section correlates with the use of the term “plenum” in NFPA 90A-2009, Standard for the Installation of Air-Conditioning and Ventilating Systems, and other mechanical codes where the plenum is used for return air purposes, as well as some other air-handling spaces.
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Exception: This section shall not apply to the joist or stud spaces of dwelling units where the wiring passes through such spaces perpendicular to the long dimension of such spaces.
(1) Wiring Methods. The wiring methods for such other space shall be limited to totally enclosed, nonventilated, insulated busway having no provisions for plug-in connections, Type MI cable, Type MC cable without an overall nonmetallic covering, Type AC cable, or other factory-assembled multiconductor control or power cable that is specifically listed for use within an air-handling space, or listed prefabricated cable assemblies of metallic manufactured wiring systems without nonmetallic sheath. Other types of cables, conductors, and raceways shall be permitted to be installed in electrical metallic tubing, flexible metallic tubing, intermediate metal conduit, rigid metal conduit without an overall nonmetallic covering, flexible metal conduit, or, where accessible, surface metal raceway or metal wireway with metal covers.
 
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All depends if the "curb" is also the "duct" for environmental air. If the actual "duct(s)" handling the environmental air pass thru the "curb" then the space within the "curb" is not for handling environmental air, it is just a chase that happens to contain the ducts.
 
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