What Is 3/4 Liquid Tight Flexible Metal Conduit and When to Use It?

Three-quarter-inch liquidtight flexible metal conduit is a flexible raceway with a metal core and a liquid-resistant outer jacket, used where wiring needs protection from moisture and oils but must still bend and move.

A motor terminal box filled with rainwater, a pump housing coated in machine oil, an air handler that vibrates all day — the wiring feeding each one has to survive conditions that would destroy standard conduit. Three-quarter-inch liquid tight flexible metal conduit answers that problem by pairing a bendable metal core with a sealed outer jacket, so conductors stay dry while the raceway absorbs movement. The sections below cover what the spec actually means, where the code allows it, and the installation limits that trip up first-timers.

What Does The 3/4-Inch Label Actually Measure?

UL 360, the standard governing liquid-tight flexible metal conduit, designates this trade size as metric designator 21.

Current Southwire Titan2 Type UL dimensional data shows a minimum internal diameter around 0.820 inch and a maximum near 0.840 inch. The outside diameter runs roughly 1.030 to 1.050 inches. The product weighs about 38 pounds per 1,000 feet, and the listed minimum bend radius is 5 inches.

That gap between the label and reality matters at the supply counter. A fitting sized for “3/4 inch” matches the trade designation, and pulling conductor fill calculations from the outside diameter will give you the wrong answer.

Where Does The NEC Allow Liquidtight Flexible Metal Conduit?

NEC Article 350 permits liquidtight flexible metal conduit wherever flexibility or protection from machine oils, liquids, vapors, or solids is required. It may be installed exposed or concealed, and it is allowed in certain hazardous classified locations where the code specifically says so.

The jacket is what separates it from ordinary flexible metal conduit. FMC has no thermoplastic outer layer, so it is generally the wrong raceway for wet locations. LFMC is built for that job — but only when paired with listed liquidtight fittings and installed per the code.

Direct burial is a separate question. LFMC can go underground only when the product is specifically listed and marked for direct burial, so check the jacket printing before you dig.

UL 360 states the conduit is not intended for locations subject to physical damage. Where the raceway might get struck, crushed, or driven over, the wiring method needs added protection approved by the local authority having jurisdiction.

How Do You Install It Correctly?

Installation follows eight steps, and the order matters because each one constrains the next. Skipping a support or bend check means pulling wire through a raceway that will fight you the whole way.

1. Select 3/4-inch LFMC and fittings listed for the conduit type, environment, conductor size, and any required temperature or hazardous-location rating.
2. Cut and trim the conduit without nicking the jacket, metal core, or bonding strips.
3. Install a listed liquidtight connector at every termination. The connector must preserve both the liquidtight seal and the grounding path.
4. Pull conductors within the NEC Chapter 9, Table 1 percentage-fill limits.
5. Make field bends without flattening the conduit or shrinking its internal diameter. Follow the bend radius in NEC Chapter 9, Table 2.
6. Keep total bends between pull points at or below 360 degrees.
7. Secure the conduit within 1 foot of each termination and at intervals no greater than 4.5 feet. Listed fittings can serve as securement.
8. Bond and ground the raceway and fittings per NEC Article 250 and the product instructions.

Where the equipment must keep moving after installation — a vibrating compressor, a pump on isolation pads — the unsecured length for trade sizes 1/2 through 1-1/4 inches generally may not exceed 3 feet from the last securement point, subject to NEC exceptions.

Buying for a job? Our tested picks for 3/4 liquid tight conduit compare the options worth pricing.

What Causes Most Installation Failures?

• Confusing trade size with actual size. A “3/4-inch” conduit has an internal diameter near 0.82 inch — fill math done from the label will be wrong.
• Using unlisted fittings. A sealed jacket means nothing if the connector at the end does not maintain the liquidtight and grounding bond.
• Exceeding the temperature marking. Many LFMC products carry a 60 °C wet-location limit even when the dry rating is higher, so read the jacket print before specifying it near heat.
• Assuming direct burial is allowed. It requires a product specifically listed and marked for burial.
• Overbending. Excess bending narrows the internal diameter and can damage conductors during the pull.
• Ignoring support, fill, and 360-degree bend limits. These are code requirements, not suggestions, and inspectors check them.

The specific product listing, jacket markings, fittings, and the local authority having jurisdiction control what is acceptable on any given job. Pricing varies too widely by material, jacket, length, color, and supplier to quote a single figure — get a dated seller quote before budgeting.