Wiring methods and cabling systems can be installed in ducts fabricated for environmental air-handling purposes under what condition?

The installation of wiring methods and cabling systems in ducts used for environmental air-handling purposes is allowed under specific conditions to ensure safety and compliance with the National Electrical Code (NEC). The correct condition involves the requirement that the wiring is necessary to connect to equipment and does not exceed a length of 4 feet.

This limitation is based on the need to minimize potential fire hazards and ensure that any conductors installed within air-handling spaces do not contribute to the spread of fire or produce smoke that could compromise air quality. The short length of 4 feet allows for necessary connections without introducing significant risks associated with longer runs of cabling within those ducts.

The other conditions described in the incorrect choices do not align with NEC guidelines for installations in air-handling spaces. For example, a 10-foot length exceeds the safe threshold where potential risks increase, and double insulation or labeling alone does not provide the same level of safety assurance that is needed in such environments. Thus, the emphasis is on maintaining short runs for any wiring necessary to connect equipment while ensuring compliance with safety standards.