Liquid-Tight Flexible Conduit Guide

Liquid-tight flexible conduit is the wiring method you reach for when a run needs to bend, absorb vibration, or survive weather and washdown, all at once. It's the sealed, flexible “whip” you see connecting an outdoor air-conditioning condenser, a motor, or a pump, where rigid pipe can't flex and ordinary flexible conduit wouldn't keep water out. There are two families, metallic (LFMC) and nonmetallic (LFNC), each with its own code article and its own strengths, and a grounding rule that catches people out. One thing is true of both: the conduit is only as liquid-tight as the fittings on its ends, so the right conduit fittings matter as much as the conduit itself. This guide covers the differences, the code, and how to install it correctly.

What Liquid-Tight Flexible Conduit Is

Liquid-tight flexible conduit is a circular raceway built to do two jobs a single product rarely combines: bend freely and keep liquids out. A flexible core provides the movement, and a continuous, sealed outer jacket keeps water, oil, dust, and vapor away from the conductors inside. That combination makes it the standard choice for the final connection to equipment that vibrates, moves, or sits outdoors, exactly the spots where a rigid raceway would crack under vibration and a standard flexible conduit would let moisture in.

The Two Types: LFMC vs LFNC

Everything starts with choosing between the metallic and nonmetallic versions.

LFMC (Liquidtight Flexible Metal Conduit)

LFMC, covered by NEC Article 350 and often called by the trade name Sealtite, has a spiral-wound metal core wrapped in a liquid-tight thermoplastic jacket. The metal core adds physical strength, provides electromagnetic shielding for sensitive circuits, and, within strict limits explained below, can serve as the equipment grounding path. The jacket protects that metal core from the moisture and oil that would otherwise corrode it. It's the go-to where you want the robustness of metal plus flexibility and a liquid-tight seal.

LFNC (Liquidtight Flexible Nonmetallic Conduit)

LFNC, covered by NEC Article 356, is all nonmetallic, typically PVC, and comes in three listed constructions (types LFNC-A, B, and C, with LFNC-B being the most common). With no metal anywhere, it cannot corrode, which makes it ideal for chemical plants, coastal sites, and washdown areas, and it's lighter and less expensive than LFMC. The trade-off is that it offers no grounding path and no EMI shielding, so a separate equipment grounding conductor is always required.

When to Use Which

• Choose LFMC when you want physical robustness, electromagnetic shielding for sensitive equipment, or a short grounding connection within code limits, and where corrosion isn't severe.
• Choose LFNC when corrosion is the enemy (chemicals, salt air, constant moisture), when weight or cost matters, or when a metal path simply isn't needed. Just plan on running a separate ground.
• Either works for general outdoor, wet, or vibrating connections; the environment and the grounding approach usually make the decision.

Where It's Used

Liquid-tight flexible conduit earns its place at the final connection to equipment, where a bit of flex prevents vibration from loosening terminations and lets gear be serviced or moved. Typical applications include air-conditioning condensers and heat pumps, motors and pumps, rooftop and outdoor equipment, transformers and generators, machine tools on a factory floor, and any connection in a wet, oily, dusty, or corrosive environment. Anywhere equipment shakes, gets hot, or sits in the weather, this is usually how the wiring reaches it.

The Grounding Rule That Trips People Up

This is the detail that causes the most code trouble. LFMC can serve as the equipment grounding conductor, but only inside a narrow set of conditions from NEC 250.118: the conduit must be terminated in listed fittings; in trade sizes 3/8 to 1/2 inch the circuit overcurrent protection must be 20 amperes or less; in trade sizes 3/4 to 1-1/4 inch it must be 60 amperes or less; and the total combined length of flexible conduit in the ground-fault path must not exceed 6 feet. Beyond any of those limits, you must run a separate wire-type equipment grounding conductor. Crucially, there's a further catch: where the conduit is used specifically to allow movement or to minimize the transmission of vibration, a separate grounding conductor is required regardless of length, because a metal path that flexes constantly can't be relied on for safety grounding. Since that vibration-isolation role is one of the main reasons to use liquid-tight conduit in the first place, running a separate EGC is the safe default. LFNC, having no metal, always needs one.

The Fittings Are Everything

A liquid-tight conduit connected with ordinary flexible-conduit fittings is not liquid-tight, and this is one of the most common field mistakes. You must use listed liquid-tight connectors, which include a sealing gland or O-ring that compresses against the jacket to complete the watertight seal at the box or enclosure. Standard flex connectors have no such seal. Choose the correct electrical fittings for the conduit type and trade size, in straight, 45-degree, or 90-degree configurations as the routing needs, and add insulating bushings where required to protect conductors at terminations. For cable entries into equipment rather than raceway, cable glands perform the same sealing job. The seal is only ever as good as the weakest fitting.

Installation Best Practices

• Cut it square and ream the ends so sharp edges don't nick conductors as they're pulled.
• Respect the bend radius. Kinking the conduit damages the jacket and the core; follow the minimum radius in the code tables.
• Secure and support it generally within 12 inches of each box or fitting and at intervals not exceeding about 4.5 feet, with recognized exceptions for the final length at equipment where flexibility is the whole point.
• Leave a drip loop. Route the conduit so any condensation or water collects at a low point and drains away rather than running down into the equipment enclosure.
• Match the environment. Use conduit marked sunlight-resistant for outdoor exposure, direct-burial-rated where buried, and confirm the temperature rating for hot locations.
• Use wet-rated conductors. In wet locations, the conductors inside must be listed for wet use, such as THWN.
• Keep it out of harm's way. Liquid-tight flex is not for areas subject to physical damage; use rigid raceway there.

Common Mistakes

• Using standard flex fittings, which don't seal, so the run isn't actually liquid-tight.
• Relying on LFMC for grounding beyond its limits, past 6 feet, on oversized circuits, or on a vibration connection, instead of pulling a separate EGC.
• Skipping the drip loop, so water tracks straight into the motor or disconnect.
• Over-securing the final whip, which defeats the vibration isolation the flex was there to provide.
• Ignoring UV, running non-sunlight-resistant conduit in direct sun, where the jacket degrades and cracks.
• Exceeding the bend radius, crushing the conduit at a tight corner and compromising the seal.

The Bottom Line

Liquid-tight flexible conduit solves a specific problem: getting power to equipment that needs flexibility and moisture protection at the same time. Choose LFMC when you want metal's strength and shielding, and LFNC when corrosion resistance, weight, or cost leads, remembering that only LFMC can ground at all, and only within tight limits, so a separate equipment grounding conductor is the safe default. Above all, use listed liquid-tight fittings, because the seal lives in the connectors, and follow the bend-radius, support, and drip-loop practices that keep water out and the run reliable. Match the conduit and fittings to the environment, and it will protect that final connection for the life of the equipment.