What is the maximum length of flexible metal conduit allowed in ducts or plenums used for environmental air?

The correct maximum length of flexible metal conduit (FMC) allowed in ducts or plenums used for environmental air is indeed 4 feet. The National Electrical Code (NEC) sets specific requirements regarding the use of flexible metal conduit in such applications primarily to enhance safety and ensure efficient airflow. This 4-foot limitation is in place because longer lengths could potentially create additional points of resistance or obstruction to airflow, which may lead to overheating or impede the overall functionality of the duct system. By restricting the length to 4 feet, the code aims to minimize risks associated with using flexible conduit in environments where air is circulated for ventilation purposes. Understanding these regulations helps ensure compliance and promotes safe electrical installations in sensitive areas.