Installation in Plenum vs. Air-Handling Space
When mounting electrical equipment in the ceiling of an office space, such as a power pack, installers must consider the question of what constitutes a “plenum,” as opposed to an “environmental space.” Industry organizations such as the National Electric Code (NEC), the National Fire Protection Association (NFPA) and Underwriters Laboratories (UL) have clear definitions regarding what constitutes a plenum, as well as specific standards that electrical products must meet in order to be approved for installation in a plenum, rather than any other spaces within a building.Plenum Defined
A plenum is widely accepted as the area where air return for an air conditioning system takes place. The NEC and the NFPA offer the same definition of a plenum. According to Article 100 of the NEC, a plenum is a “compartment or chamber to which one or more air ducts are connected, and that forms part of the air distribution system.” This definition is similar to what most people call “air ducts” or “central air systems” designed to carry environmental air, and would be a metal or fiberglass duct made at the job site, or brought in pre-fabricated and assembled at the site. With reference to the NEC and NFPA definition of a plenum, installation of electrical parts or systems are greatly restricted in plenums.However, the NEC distinguishes between a plenum and “other space(s) used for environmental air,” or “air-handling spaces.” In fact, an air-handling space is the area of interest for The Watt Stopper and contractors installing products from The Watt Stopper. In Article 300.22(C), the NEC specifies that the “space over a hung ceiling used for environmental air-handling purposes is an example of the type of other space.” Some people incorrectly refer to this “other” space as a plenum, leading to confusion among electrical engineers, contractors and mechanical engineers who have varying opinions about plenums and air-handling space.
Plenum Safety Issue
At the heart of the plenum issue is fire safety, especially since air-handling spaces have become more common with the emergence of integrated heating ventilation and air conditioning. Because the space above a ceiling is the area where wiring and cable are installed (out of sight above the drop ceiling), toxic fumes released from any electrical equipment that is on fire (such as wiring, plastic cases, PCBs) would endanger occupants of the space where air is distributed, even if the occupants were located far away from the fire. Industry organizations such as the NEC, the NFPA and UL emphasize plenum fire safety because the plenum represents the path through which both flame and smoke will travel, subsequently permeating throughout the entire building.Plenum Rating
NEC Article 300.22(C)(2) states in part that the only equipment permitted in air-handling areas are “electrical equipment with a metal enclosure or with a nonmetallic enclosure listed for the use and having adequate fire-resistant and low smoke producing characteristics . . .” The result is the “Plenum Rated” listing covered in UL 2043. This UL standard lays out strict fire and smoke producing test requirements, which include subjecting product samples to an open flame and measuring the amount of smoke they emit, determining whether the product catches fire, etc. In short, the need for plenum rated equipment is to ensure that even if a product were to be subjected to fire, it would not contribute to or fuel the fire.After passing this UL 2043 test, a product receives the “Plenum Rated” stamp of approval and can then be used anywhere in an air-handling space.