Which condition permits the installation of equipment and devices in ducts?

The installation of equipment and devices in ducts is permitted primarily for environmental air management. This aligns with the NEC guidelines that specify conditions under which electrical equipment can be installed in spaces designed for environmental air handling, such as ducts. Such installations are allowed because they can facilitate the distribution of air and promote efficient HVAC systems. Equipment installed in this manner must be suitable for the environment, ensuring that it maintains safety and functionality within systems handling environmental air.

Aesthetic purposes, decorative lighting, or private wiring systems do not provide a justified or code-compliant reason for placing equipment within ducts, as these reasons do not pertain to the operational integrity or safety concerns relevant to environmental air management. Therefore, the focus on environmental air management ensures that installations are safe, effective, and adhere to the NEC requirements regarding the conditions under which electrical equipment can be housed within air-handling spaces.