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

The maximum length of flexible metal conduit (FMC) allowed in ducts or plenums for environmental air is 4 feet. This restriction is in place due to safety considerations and the need to minimize the risk of contaminants entering the air distribution system. Flexible metal conduit is typically used to provide mechanical protection for conductors, but in situations where it can be exposed to air that will be distributed throughout occupied spaces, there are stricter limitations on its length to ensure safe and effective ventilation.

Limiting the length to 4 feet helps maintain air quality and prevents potential issues associated with dust and other particles that may accumulate in longer sections of conduit. This standard is set in the National Electrical Code (NEC), which governs electrical installations, ensuring they meet safety and performance requirements.