Wiring methods and cabling systems permitted in plenums shall be installed in ducts specifically fabricated for environmental air-handling purposes ______.

The correct answer reflects that wiring methods and cabling systems permitted in plenums must be installed in ducts specifically made for environmental air-handling purposes, particularly when those installations are necessary for connecting to equipment or devices, or when the total length is limited to a specific distance, such as 4 feet.

This acknowledges that there are safety and regulatory considerations when wiring in air-handling spaces, such as ensuring that any materials used do not pose a hazard in the event of a fire and that they are suitable for the conditions in which they will be installed.

Ducts designated for this purpose are specifically designed to minimize the risk of fire hazards and should be used appropriately. When wiring systems are limited in length, they can be safely accommodated without compromising the environmental integrity of air-handling spaces, allowing for safe and efficient installation practices.