7.2 Flexible Raceways: FMC, LFMC, and LFNC

7.2 Flexible Raceways: FMC, LFMC, and LFNC

Quick Answer: Flexible raceways provide vibration isolation and routing versatility in tight spaces. Flexible Metal Conduit (FMC, Article 348 / "Greenfield") is a spiral-wound metallic raceway approved for dry locations only and prohibited in wet environments or underground. Liquidtight Flexible Metal Conduit (LFMC, Article 350) features an outer moisture-proof thermoplastic jacket approved for wet and outdoor locations like HVAC units. Liquidtight Flexible Nonmetallic Conduit (LFNC, Article 356) offers chemical and corrosion resistance across Types A, B, and C. Under NEC 250.118(5) and (6), FMC (20A limit) and LFMC (60A limit for 3/4"+) can serve as an equipment grounding conductor only if length does not exceed 6 feet, fittings are listed for grounding, and flexibility is not required after installation. When installed for motor vibration, an equipment grounding conductor is always mandatory. Flexible conduits must be supported within 12 inches of terminations and every 4.5 feet, with exceptions permitting 6 feet for fixture whips and 3 feet for terminal flexibility.

Flexible conduit systems bridge the gap between rigid structural raceway runs and moving or vibrating machinery. For the Alabama Journeyman examination, candidates must understand the precise operational limitations, grounding permissions, and support rules governing FMC, LFMC, and LFNC.

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Flexible Metal Conduit (FMC — Article 348 / "Greenfield")

Flexible Metal Conduit (FMC), historically known as Greenfield, is fabricated from a continuous, spirally wound, interlocked strip of coated zinc-galvanized steel or aluminum alloy.

1. Uses Permitted (NEC 348.10)

FMC is permitted in exposed and concealed installations; in dry locations; for luminaire tap connections (fixture whips) above accessible ceilings; and for connections to dry-type transformers, motors, and industrial machinery where flexibility is required to mitigate mechanical vibration.

2. Uses Prohibited (NEC 348.12)

FMC is strictly prohibited:

• In wet locations (FMC possesses an unsealed spiral spiral seam that permits moisture infiltration).
• In hoistways, other than permitted in Section 620.21.
• In storage battery rooms.
• In hazardous (classified) locations, except where specifically permitted in Articles 501, 502, and 504.
• Underground or embedded in poured concrete.
• Where subject to physical damage.

3. Trade Size Limitations (NEC 348.20)

The minimum size for FMC is trade size 1/2 (metric designator 16). However, trade size 3/8 (metric designator 12) is permitted under specific exceptions:

1. In lengths not exceeding 6 feet (1.8 m) for luminaire tap connections (fixture whips per NEC 410.117(C)).
2. In lengths not exceeding 6 feet (1.8 m) for equipment connections (such as motor lead drops).
3. As part of a listed assembly or manufactured wiring system.

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Liquidtight Flexible Metal Conduit (LFMC — Article 350)

Liquidtight Flexible Metal Conduit (LFMC) consists of an interlocked flexible metal core surrounded by an extruded, sunlight-resistant, liquidtight thermoplastic outer jacket.

1. Uses Permitted (NEC 350.10)

LFMC is widely specified for exterior and wet locations, including:

• Outdoor rooftop HVAC air-conditioning condensing units, chillers, and exhaust fans.
• Connections to pumps, commercial dishwashers, and food processing equipment subject to wash-down.
• Hazardous (classified) locations where flexible connections are permitted under Chapter 5.
• Direct burial where specifically listed and marked for direct burial on the conduit print legend.

2. Uses Prohibited (NEC 350.12)

LFMC is prohibited:

• Where subject to physical damage.
• Where ambient or operating temperatures exceed the temperature rating stamped on the conduit jacket (standard jackets are rated 60°C wet, with high-temperature variants rated 75°C or 90°C dry).

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Liquidtight Flexible Nonmetallic Conduit (LFNC — Article 356)

Liquidtight Flexible Nonmetallic Conduit (LFNC) is a nonconductive, flame-retardant flexible raceway fabricated from polymeric materials. NEC Article 356 recognizes three distinct manufacturing types:

• Type LFNC-A: Features a smooth seamless inner core and cover bonded together with helical reinforcement between the core and cover.
• Type LFNC-B: Features a smooth inner surface with integral reinforcement embedded directly within the conduit wall. Type LFNC-B is the most prevalent nonmetallic flexible conduit used in modern commercial and residential construction.
• Type LFNC-C: Features a corrugated internal and external surface without integral reinforcement.

Installation Rules & Length Limitations (NEC 356.10 & 356.12)

LFNC is permitted in exposed and concealed work, wet locations, corrosive atmospheres, direct burial where listed, and for swimming pool pump motor connections under Article 680. Where flexibility is required for equipment movement, Type LFNC-B is generally limited to 6 feet (1.8 m) in length, unless a longer length is specifically approved by the authority having jurisdiction or installed with fixed support clamps per 356.30.

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Equipment Grounding Conductor (EGC) Rules & Exam Traps

One of the most heavily tested topics on the Prov electrical examination is determining when a flexible conduit may serve as the equipment grounding path, and when a separate wire-type equipment grounding conductor is legally mandatory.

+-------------------------------------------------------------------------+
|          EQUIPMENT GROUNDING PATH PERMISSIONS IN FLEXIBLE CONDUIT       |
+-------------------+--------------------+------------------+-------------+
| Raceway Type      | Max Circuit OCPD   | Max Length       | Flexibility |
|                   |                    |                  | Required?   |
+-------------------+--------------------+------------------+-------------+
| FMC (Art. 348)    | 20 Amperes         | 6 feet (1.8 m)   | NO (Fixed)  |
| LFMC 3/8" - 1/2"  | 20 Amperes         | 6 feet (1.8 m)   | NO (Fixed)  |
| LFMC 3/4" - 1-1/4"| 60 Amperes         | 6 feet (1.8 m)   | NO (Fixed)  |
| LFNC (Art. 356)   | NEVER PERMITTED    | N/A              | N/A         |
+-------------------+--------------------+------------------+-------------+

1. Flexible Metal Conduit as an EGC (NEC 250.118(5))

FMC is permitted to serve as the sole equipment grounding conductor only if ALL four of the following conditions are simultaneously met:

1. The conduit is terminated in fittings listed for grounding.
2. The circuit conductors contained in the conduit are protected by overcurrent devices rated at 20 amperes or less.
3. The total combined length of FMC and LFMC in the same ground return path does not exceed 6 feet (1.8 m).
4. The conduit is NOT installed to provide flexibility after installation.

2. Liquidtight Flexible Metal Conduit as an EGC (NEC 250.118(6))

LFMC is permitted to serve as the sole equipment grounding conductor only if ALL four of the following conditions are met:

1. The conduit is terminated in fittings listed for grounding.
2. For trade sizes 3/8 and 1/2, overcurrent protection does not exceed 20 amperes; for trade sizes 3/4 through 1-1/4, overcurrent protection does not exceed 60 amperes (sizes 1-1/2 and larger can never serve as an EGC).
3. The total combined length in the ground return path does not exceed 6 feet (1.8 m).
4. The conduit is NOT installed to provide flexibility after installation.

3. The "Flexibility After Installation" Trap (NEC 250.118(5) & (6))

Notice condition 4 for both FMC and LFMC: "The conduit is not installed to provide flexibility after installation." When flexible conduit is installed to connect a motor, vibrating pump, transformer, or machinery subject to physical adjustment, flexibility is required after installation to isolate vibration. Under continuous mechanical vibration, the interlocking spiral metallic convolutions flex, oxidize, and loosen, creating a high-resistance or open electrical path. Therefore:

The Critical Rule: Whenever FMC or LFMC is installed to accommodate vibration or movement after installation, a separate wire-type equipment grounding conductor MUST be installed, regardless of circuit length or overcurrent rating!

4. Grounding in Nonmetallic LFNC (NEC 250.118 & 356.60)

Because LFNC is fabricated entirely from nonconductive thermoplastic, it has zero conductivity. An equipment grounding conductor (sized according to Table 250.122) must always be routed inside or outside the conduit.

5. External Equipment Bonding Jumpers (NEC 250.102(E)(2))

Where an equipment bonding jumper is routed on the outside of flexible conduit, it must not exceed 6 feet (1.8 m) in length, must closely follow the outer contour of the raceway, and must be bonded at both terminations.

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Securing and Supporting Mandates (NEC 348.30, 350.30, 356.30)

Under standard installation conditions, FMC, LFMC, and LFNC must be:

• Securely fastened in place within 12 inches (300 mm) of each box, cabinet, conduit body, or other conduit termination.
• Supported at intervals not exceeding 4.5 feet (1.4 m) along the conduit run.

The Three Critical Code Exceptions:

1. Fixture Whip Exception (NEC 348.30(A) Ex. 1, 350.30(A) Ex. 1, 410.117(C)): Lengths not exceeding 6 feet (1.8 m) from the luminaire terminal connection for tap conductors to luminaires in accessible ceiling cavities do not require securing or intermediate support.
2. Terminal Flexibility Exception (NEC 348.30(A) Ex. 2, 350.30(A) Ex. 2): Lengths not exceeding 3 feet (900 mm) from a terminal connection where flexibility is necessary to isolate motor vibration or allow terminal movement are exempt from the 12-inch securing rule.
3. Concealed Fishing Exception (NEC 348.30(A) Ex. 3): Lengths fished between access points in finished building construction where fastening is impractical are exempt from securing rules.

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Cumulative Bending Rules (NEC 348.26, 350.26, 356.26)

Just like rigid conduit systems, flexible raceways are strictly governed by the universal 360-degree bend rule. Between any pull box, junction box, or termination fitting, the cumulative total of all bends (including offsets and s-curves) cannot exceed 360 degrees (the equivalent of four 90-degree bends). Pulling conductors through over-bent flexible conduit can unravel the inner metal core, stripping insulation and creating internal ground faults.

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Master Comparison: Flexible Raceways