Liquid-tight flexible metal conduit (Type LFMC) in outward appearance resembles its close relative, liquid-tight flexible nonmetallic conduit (LFNC). Both have smooth gray outer jackets that are liquid tight. Type LFMC has an inner steel armor, which makes it stronger and less vulnerable to kinking. Either of these raceways can be formed in gentle curves to change direction and to avoid obstacles. Moreover, the flexibility is useful for terminating at motors where there is likely to be heavy vibration.
Use, installation, and construction specifications are found in NEC Article 350, along with the all-important Uses Permitted and Uses Not Permitted. LFMC is defined as a raceway of a circular cross-section having an outer liquid-tight, nonmetallic, sunlight-resistant jacket over an inner flexible metal sheath with associated couplings, connectors, and fittings for the installation of electric conductors.
LFMC is permitted to be used in exposed or concealed locations where flexibility or protection from liquids, vapors or solids is required. It is permitted in specified hazardous (classified) locations and it may be used for direct burial where listed and marked for the purpose.
LFMC is not to be used where subject to physical damage. (Physical damage in NEC terminology is always to be understood in the context of the wiring under discussion.)
There are some size restrictions. LFMC smaller than metric designator 16 (trade size ½) is not to be used. However, an exception permits metric designator 12 (trade size 3/8) for enclosing motor leads. The maximum size of LFMC permitted is metric designator 103 (trade size 4).
The maximum number of bends is not to be more that the equivalent of four quarter bends between pull points. Subject to exceptions, LFMC is to be securely fastened in place within 12 in. of boxes, and secured at intervals not over 4 ½ ft.
If LFMC is used to minimize vibration to or provide flexibility for moving equipment, an equipment-grounding conductor must be provided. Otherwise, the inner metal sheath will suffice.
Liquid-tight flexible metal conduit is a durable alternative wiring method. Where appropriate, it contributes to an attractive appearance in commercial and industrial work and functions well in residential settings for powering water pumps and stationary appliances where there is ambient moisture along with vibration.