Flexible Conduit (FMC, LFMC, LFNC)

What it is

Flexible conduit bends by hand and absorbs vibration. There are three common types:

• Flexible metal conduit (FMC): a spiral-wound metal strip with no jacket. NEC Article 348. Often called "Greenfield".
• Liquidtight flexible metal conduit (LFMC): FMC with a PVC jacket that keeps out liquids. NEC Article 350.
• Liquidtight flexible nonmetallic conduit (LFNC): plastic conduit with no metal core. NEC Article 356. It comes in types LFNC-A, LFNC-B and LFNC-C.

How to identify

FMC shows bare interlocked metal. LFMC looks like FMC covered in a smooth or ribbed jacket, usually gray or black, and you can feel the metal core. LFNC is lighter and has no metal core; it may be smooth or corrugated. The jacket print shows the type, trade size and listing.

Types compared

Grounding limitations

FMC and LFMC can count as an equipment grounding conductor only within strict conditions in NEC 250.118. These include a maximum total length in the ground return path, the trade size, the overcurrent device rating and whether flexibility is needed after installation. When any condition is not met, or flexibility is needed for vibration or movement, a separate equipment grounding conductor must be installed. LFNC is never a grounding path.

Key points

• Use fittings listed for the conduit type. Liquidtight fittings are needed for LFMC and LFNC to stay liquidtight.
• Securing, support and length limits for each type are in its NEC article.
• Flexible conduit is usually limited to short runs where flexibility is needed, not long feeders.
• Sunlight resistance and oil resistance are separate markings. Check the jacket print.

Common mistakes

• Relying on flex as the only ground on a vibrating motor connection.
• Using FMC in a wet location.
• Using indoor straight connectors on liquidtight conduit.
• Treating LFNC like metal flex for grounding.

Governing standards: NEC Articles 348, 350 and 356; NEC 250.118; UL 1 (FMC); UL 360 (LFMC); UL 1660 (LFNC); UL 514B (fittings).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.