What is the maximum allowable length for wiring methods or cabling systems installed in environmental air ducts?

The correct answer is four feet, which aligns with the National Electrical Code's requirements for wiring methods or cabling systems installed in environmental air ducts. This limitation is put in place due to safety considerations related to fire hazards and the potential for the materials in the ducts to contribute to the spread of fire or the generation of hazardous smoke.

The NEC specifies that conductors and cables within ducts that carry air for heating, cooling, or ventilation should not exceed this length to minimize risks and ensure compliance with safety protocols. This rule is part of the code's overall intent to maintain safety and reliability in electrical installations and to prevent the exacerbation of hazardous conditions in environments where there is potential for combustible materials or fire spread.