9.3 Flexible Metal & Liquidtight Flexible Metal Conduit

9.3 Flexible Metal & Liquidtight Flexible Metal Conduit

Quick Answer: Flexible Metal Conduit (FMC) is Article 348; Liquidtight Flexible Metal Conduit (LFMC) is Article 350. Fasten both within 12 inches of boxes/cabinets and support at intervals not exceeding 4½ ft—much tighter than EMT/RMC’s 3 ft / 10 ft pattern. Use FMC in dry locations for flexibility; use LFMC where wet, damp, or liquid exposure needs a jacketed flexible raceway. Watch 250.118 length limits when the flex is asked to serve as the equipment grounding conductor.

Prov Raceways items mix “flex whip at the motor,” “liquidtight outdoors,” and support spacing that candidates accidentally answer with EMT numbers. This section locks the flexible-metal rules so you never strap FMC every 10 ft or put ordinary greenfield in a washdown area.

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FMC vs LFMC at a glance

Trade slang: “greenfield” often means FMC; “sealtite” / “liquidtight” means LFMC. On the exam, trust the Code names and article numbers.

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Uses permitted

FMC — 348.10

FMC is permitted in exposed or concealed locations where flexibility is useful and the environment is suitable—commonly dry interior spaces, connections to motors or transformers that vibrate, and routing around obstacles where rigid bends would be impractical. Minimum length and listing rules still apply; very short nipples and listed assemblies have their own Exception paths.

LFMC — 350.10

LFMC is permitted where flexibility is required and protection from liquids, vapors, or solid contaminants is needed—wet locations, outdoor equipment connections, and many industrial washdown-adjacent runs when the product is listed for the exposure. Direct burial and concrete embedment are allowed only where the article and listing expressly permit those uses (often with restrictions)—do not bury ordinary LFMC just because it looks “waterproof.”

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Uses not permitted — high-yield traps

FMC — 348.12 highlights

FMC shall not be used:

• In wet locations.
• Where subject to physical damage.
• In hoistways (except as expressly permitted).
• In storage-battery rooms (with limited exceptions in the Code text).
• Underground or embedded in poured concrete (unless a specific Exception/listing path applies—ordinary FMC is not a burial raceway).
• Where flexibility is not needed and another wiring method is required by the occupancy or equipment article.

LFMC — 350.12 highlights

LFMC shall not be used where subject to physical damage. It is also limited by trade-size rules, temperature ratings of the jacket, and any Chapter 5 restrictions. LFMC fixes the wet-location problem of FMC; it does not turn flexible conduit into armor against forklifts.

Exam contrast: Stem says “flexible connection to a rooftop A/C unit in the rain” → LFMC, not FMC. Stem says “dry mechanical room motor whip” → FMC or LFMC may both appear, but FMC is the classic dry answer if liquidtight is not required.

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Securing and supporting — 348.30 / 350.30

The numbers to tattoo next to your EMT tabs

Horizontal runs through framing holes or notches spaced not more than 4½ ft apart may be considered supported, but you still secure within 12 in of terminations (348.30(B) / 350.30(B) style rules).

Flexibility Exceptions (whips)

Both articles include Exceptions that relax intermediate support for short lengths where flexibility is necessary after the last point of support—classic motor, luminaire, or equipment connections. Lengths in the Exception language are limited (commonly discussed around 3 ft for flexibility at equipment and other stated limits for luminaires or accessible ceilings—read the Exact Exception in your 2023 book for the stem in front of you). “Unsupported for 20 ft because it’s flex” is never Code.

Finished walls / fished installations

Where FMC or LFMC is fished in existing walls or similar finished construction and support is impracticable, Exceptions may waive intermediate supports. The raceway must still be secured at accessible ends per the article. Prov loves to test whether you know the Exception exists and that it does not delete the 12 in rule at the box you can reach.

Worked examples

Example A — Dry AHU connection: 4 ft of FMC from a junction box to a vibrating air handler indoors. Secure within 12 in of the box; if the entire flexible length qualifies under a flexibility Exception, intermediate straps may not be required—but confirm length against the Exception text.

Example B — Outdoor condensing unit: LFMC from a disconnect to the unit. Use listed liquidtight connectors; secure within 12 in; keep the unsupported flex within Exception limits; do not substitute FMC.

Example C — Long flexible run across a ceiling: If the run exceeds Exception whip lengths, install supports every 4½ ft maximum and fasten within 12 in of each termination. At that point, ask whether rigid or EMT would have been the better design—Prov may still test the flex numbers.

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Fittings and continuity

Use listed connectors designed for FMC or LFMC as applicable. LFMC connectors must engage the jacket and core so the connection is liquidtight where that listing is claimed. Angle connectors, straight connectors, and combination couplings must be made up per instructions—loose flex connectors are a common bonding failure.

Cut ends should not leave sharp edges that slice conductor insulation. Where the flex enters a box, the connector provides the securing means; additional clamps may be required by the box listing.

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Equipment grounding — 250.118 limits

250.118 permits FMC and LFMC to serve as an equipment grounding conductor only under stated conditions—typically involving listed fittings, minimum trade sizes, and maximum length (commonly 6 ft for the flexible raceway used as the EGC, with additional qualification details in the section). Outside those limits, install a wire-type EGC sized from Table 250.122 and bond the equipment.

Prov strategy: If the stem asks whether a 15 ft FMC run can be the sole EGC, open 250.118—expect no. If the stem asks for support spacing on that same run, answer 12 in / 4½ ft from Article 348, then still pull a green/bare EGC if grounding continuity is in doubt.

Pair this with Chapter 5 bonding material: flexible connections are exactly where painters, vibration, and loose connectors break the metal path.

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Fill, bends, and ampacity notes

Conductor fill still follows Chapter 9 / Annex C based on the raceway’s internal area. Flexible conduit is harder to pull than EMT; designers often upsize trade size for fill and pull ease. Ambient and jacket temperature ratings matter for LFMC in rooftop sun. Article 430 motor connection practices often interact with these whips—size conductors and OCPD from the motor article, then apply flex wiring-method rules.

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Common Prov traps

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Timed open-book tips

Tab 348.12, 348.30, 350.10, 350.12, 350.30, and 250.118(5)/(6) (FMC/LFMC EGC paragraphs as numbered in your printing). Decision tree: (1) Wet or dry? (2) Physical damage? (3) Support 12 in / 4½ ft or Exception whip? (4) Is the flex being asked to be the EGC? Run that tree and you will bank flexible-raceway points inside Arkansas Prov’s eight Raceways and Enclosures items.