Which wiring method is permitted in ducts or plenums designed to transport environmental air?

The correct answer is that electrical metallic tubing is permitted in ducts or plenums designed to transport environmental air because of its characteristics and the regulations outlined in the National Electrical Code (NEC). Electrical metallic tubing (EMT) is a type of conduit that provides a robust and secure pathway for electrical wiring. Its metal construction offers excellent protection against physical damage, as well as grounding capabilities, which is essential in air transport environments.

In ducts and plenums, where environmental air is transported, the wiring must adhere to specific safety and fire resistance standards. EMT is non-combustible and does not support combustion, making it suitable in these applications. Additionally, it ensures that any potential electrical failures do not introduce hazards to the air being circulated.

The other methods mentioned do not offer the same level of safety or compliance with regulations regarding environmental air transport. Understanding these aspects helps to reinforce the importance of selecting appropriate wiring methods based on their intended use and the environment in which they are installed.