Section 348.10/Uses Permitted for Flexible Metal Conduit (FMC)
NEC 348.10 and 348.12 define where flexible metal conduit (FMC) is permitted and prohibited, including limitations on length and grounding path.
Flexible metal conduit (FMC) is permitted in exposed and concealed locations where flexibility is needed or where installation conditions make rigid conduit impractical. Common uses include final connections to motors, transformers, vibrating equipment, recessed luminaires, and HVAC equipment. Section 348.12 prohibits FMC in wet locations (unless the conductors are approved for wet locations and the installation prevents water from entering the enclosure), in hoistways, in storage battery rooms, in hazardous locations (except as permitted in Articles 501 through 504), and where subject to physical damage. FMC is permitted as an equipment grounding conductor only when the total length of FMC in the ground-fault return path does not exceed 6 feet and the circuit overcurrent device does not exceed 20 amperes per Section 250.118(5). For circuits over 20 amperes or FMC runs over 6 feet, a separate equipment grounding conductor must be installed per Section 250.122.
Why this section exists
Flexible metal conduit provides mechanical protection for conductors while allowing the flexibility needed for connections to vibrating equipment, adjustable fixtures, and equipment that must be moved for servicing. The flexibility comes from the interlocking spiral metal strip construction, but this same construction creates higher impedance in the ground-fault return path compared to rigid conduit. The 6-foot and 20-ampere limits for using FMC as a grounding path reflect the point at which the impedance becomes too high for reliable overcurrent device operation during a ground fault. Beyond these limits, the fault current through the FMC may not be high enough to trip the breaker quickly, creating a shock hazard.
What plan reviewers look for
Plan reviewers check the electrical drawings for FMC connections and verify each use complies with the permitted locations. They measure the FMC length shown on equipment connection details. For FMC runs over 6 feet or circuits over 20 amperes, they verify a separate equipment grounding conductor is specified in the conduit. They check that FMC is not specified in wet locations without wet-rated conductors and moisture-tight fittings. They verify connector temperature ratings match the installation. For motor connections, they verify the FMC is specified with the correct trade size for the number and size of conductors.