Technical Foundation
What Is a Metal Flexible Conduit?
Flexible metal conduit (FMC) is an interlocked helical strip of galvanized steel or aluminum forming a flexible raceway for electrical conductors, used where rigid conduit can't bend around obstructions or where vibration from connected equipment (like motors) would fatigue-crack a rigid connection. Liquidtight flexible metal conduit (LFMC) adds a extruded polymer jacket over the same interlocked metal core, sealing the raceway against moisture, oil and washdown exposure.
Trade size (matching standard rigid conduit sizing, e.g. 1/2 in, 3/4 in, 1 in) determines conductor fill capacity per NEC Chapter 9 tables. FMC alone is not rated for wet locations or as an equipment grounding conductor beyond limited lengths under NEC 250.118 — LFMC with a listed fitting is required for outdoor, washdown or oil-exposed connections, and grounding conductor requirements should be verified against code for the specific run length and location.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Trade Size | 1/2 in through 4 in, matching rigid conduit sizing | Determines conductor fill capacity per NEC Chapter 9 |
| Jacket Type | FMC (bare interlocked metal), LFMC (liquidtight jacketed) | LFMC required for wet, washdown or oil-exposed locations per NEC |
| Core Material | Galvanized steel, aluminum | Aluminum reduces weight for long overhead runs; steel offers greater mechanical protection |
| Grounding Path | Interlocked metal alone has limited length as an equipment ground per NEC 250.118 | Longer or critical runs typically need a separate bonding conductor, not just conduit continuity |
| Connector Type | Straight, 45°, 90° liquidtight or standard fittings | Fitting must match jacket type — a standard FMC fitting won't seal an LFMC run |
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Applications
Where Metal Flexible Conduit Get Used
Flexible metal conduit is specified anywhere rigid conduit can't accommodate vibration, movement or routing obstructions.
Motor & Pump Connections
LFMC final connections to motors and pumps, absorbing vibration and sealing against washdown or oil exposure.
HVAC Equipment Wiring
Flexible conduit routing power to rooftop and mechanical-room HVAC units where rigid conduit can't easily follow the equipment layout.
Food & Beverage Processing
Liquidtight conduit protecting wiring runs in washdown areas subject to daily sanitation cycles.
Machine Tool & Panel Wiring
Short flexible connections between fixed conduit runs and vibrating or moving machine components.
Outdoor Electrical Equipment
LFMC connections to outdoor-mounted disconnects, transformers and equipment exposed to weather.
Commercial & Industrial Retrofit
Flexible conduit used where obstructions make rigid conduit routing impractical during a retrofit.
FAQ
Metal Flexible Conduit Questions, Answered
What's the difference between FMC and LFMC?
Standard FMC (flexible metal conduit) is bare interlocked galvanized steel or aluminum with no liquid seal, suited to dry indoor locations. LFMC (liquidtight flexible metal conduit) adds an extruded polymer jacket over the same metal core, sealing it against moisture, oil and washdown — required by code for wet or outdoor locations.
Can flexible metal conduit be used as the sole equipment ground?
NEC 250.118 permits FMC/LFMC as an equipment grounding conductor only under specific length and fitting conditions — beyond those limits, or for critical circuits, a separate bonding/ground conductor run inside the conduit is required. Always verify against the current NEC edition and local AHJ requirements for the specific run.
What trade size flex conduit do I need?
Trade size matches standard rigid conduit sizing (1/2 in, 3/4 in, 1 in, etc.) and is selected based on conductor fill requirements from NEC Chapter 9 tables for the number and gauge of conductors being routed — not simply matched to the connected rigid conduit's trade size without checking fill.
Do I need special fittings for liquidtight conduit?
Yes — liquidtight fittings have a compression or gland-seal design that seals against the LFMC's outer jacket, while standard FMC fittings only grip the interlocked metal core and won't maintain a liquidtight seal. Always match fitting type to the conduit's jacket construction.
Is aluminum or steel core flex conduit better for my application?
Steel core offers greater mechanical/impact protection and is the standard choice for most industrial runs; aluminum core is lighter, useful for long overhead cable-tray runs where weight matters, but offers less mechanical protection. Choose based on physical exposure risk along the run, not just cost.
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