Which of the following is NOT allowed in spaces used for environmental air?

In spaces used for environmental air, such as shallow spaces above ceilings or below floors where air circulation is part of the building's heating, ventilating, and air conditioning (HVAC) system, specific wiring methods must adhere to safety standards to prevent potential hazards like fire or toxic fumes. Conductors without a suitable covering are not permitted in these areas because they lack a protective insulation layer, which exposes them to environmental factors and increases the risk of electrical shorts or insulation failures. This could, in turn, lead to serious safety issues, including the release of harmful substances into the air.

Nonmetallic raceways, flexible metal conduits, and metallic raceways are typically accepted for use in these spaces provided they meet the necessary fire and safety codes, as they can provide adequate protection for conductors and are designed to minimize risks in the designated environment.