Liquid tight flexible metal conduit, commonly abbreviated as LFMC, acts as a protective raceway for electrical cables exposed to moisture, oil, chemicals, and physical stress. Choosing the correct conduit configuration prevents premature wiring failure, short circuits, and costly downtime in commercial and industrial settings.
An LFMC assembly consists of a hot-dipped zinc-galvanized steel core covered by a waterproof and sunlight-resistant polyvinyl chloride jacket. This dual-layer construction shields internal copper conductors from mechanical damage while maintaining the physical flexibility required to route wiring around vibrating machinery, tight angles, and harsh outdoor environments.
Core Construction and Materials
Understanding the physical build of LFMC helps installers select the proper grade for specific field conditions. The inner core uses a galvanized steel strip shaped into either a square-lock or a fully interlocked profile. Smaller trade sizes, typically ranging from three-eighths of an inch to one and one-quarter inches, rely on a square-lock design containing a continuous bonding strip. Larger diameters use an interlocked structure that handles heavy mechanical strain.
The outer plastic jacket protects the metal core against corrosive elements. Standard jackets consist of gray thermoplastic compounds formulated to resist ultraviolet rays, oil, and moisture. Specialized variants utilize high-temperature fluoropolymers or halogen-free compounds for chemical processing plants, marine docks, and confined spaces where toxic smoke emission is a safety concern. As discussed in detailed reports by Reuters, the effects are notable.
Applications and Code Compliance
National electrical standards dictate where and how you can install LFMC safely. According to National Electrical Code (NEC) Article 350, this conduit type is approved for wet locations, direct burial in earth or concrete, and outdoor exposure to sunlight.
Common field deployments include:
- Motor connections on pumps, HVAC units, and industrial machinery where equipment vibration demands physical flexibility.
- Outdoor lighting installations, illuminated signage, and perimeter security systems.
- Marinas, boatyards, and wastewater treatment facilities exposed to constant moisture and washdowns.
- Solar photovoltaic arrays and generator hookups requiring weather-resistant wiring paths.
Installers must secure LFMC at regular intervals. NEC guidelines require fastening the raceway within 12 inches of every box, cabinet, or fitting, and at intervals not exceeding 4.5 feet along the run to prevent sagging and strain on connectors.
Step-by-Step Installation Process
Proper installation technique ensures a permanent waterproof seal and reliable circuit grounding.
- Measure and Cut: Determine the required length using a tape measure. Make a clean, square cut across the conduit using a fine-tooth hacksaw or electric band saw. A jagged cut prevents the compression fitting from seating correctly.
- Clean the Cut Edge: Remove any metal burrs from the interior core and wipe down the outer PVC jacket to ensure smooth contact with the gland ring.
- Attach the Connector: Push the conduit into the liquid-tight connector until the inner core seats firmly against the ferrule. Thread the gland nut onto the body by hand.
- Tighten the Assembly: Use channel locks or wrench grips to tighten the gland nut to the manufacturer's specified torque. This compresses the internal sealing ring around the jacket, locking out water and dust.
- Secure Grounding: Verify that your installation meets equipment grounding requirements. Sizes up to one and one-quarter inches often use an internal bonding strip, while larger diameters or specific codes require a separate external or internal equipment grounding conductor.
Common Mistakes to Avoid
Avoiding standard installation errors extends the lifespan of the wiring system and prevents code violations.
Don't stretch the conduit during installation. Cutting LFMC too short and pulling it tight places constant tension on the outer jacket, which degrades the plastic over time and breaks the waterproof seal at the fittings. Always leave a slight sweep or expansion bend in the run.
Never route LFMC where it faces severe physical crushing forces or temperatures exceeding its rated threshold. Standard runs are rated for 80 degrees Celsius in dry conditions and 60 degrees Celsius in wet environments. Exceeding these limits degrades the PVC jacket and exposes the metal core to corrosion.
Finally, avoid mixing mismatched fittings and conduit brands. Using connectors from different manufacturers can compromise the ingress protection rating, allowing moisture to seep into control panels and junction boxes.
Frequently Asked Questions
Can liquid tight flexible metal conduit be buried underground?
Yes, LFMC is approved by the NEC and CSA for direct burial in earth or encasement in concrete, provided the specific product is marked and rated for direct burial applications.
Is a separate grounding wire always required inside LFMC?
It depends on the trade size and local code jurisdiction. Sizes from three-eighths to one and one-quarter inches often include an integral bonding strip that complies with grounding rules under specific conditions, but larger sizes and Canadian Electrical Code rules generally mandate a separate dedicated bonding conductor.
How do I cut LFMC without ruining the inner core?
Use a fine-tooth hacksaw or a dedicated band saw with a sharp blade. Support both sides of the cut to prevent the metal spiral from unspooling or flattening out of round.
