Indoor metal and nonmetallic conduit can keep dust and moisture out of wiring and protect it from mechanical abrasion. Short sections can protect wiring close to the floor used to power washing machines and clothes dryers in damp basements. These sections must be anchored to masonry walls where there are no wood studs for supporting them. Conduit is also used as vertical ducting between floors to ease the task of pulling power and communications cables through floors. It will also protect all power and communications cables in warehouses and other unoccupied storage buildings where there are no dry internal walls or ceilings.
THHN and THWN wire with thermoplastic insulation are approved for use in conduit. THHN wire, rated for a maximum operating temperature of 90°C (194°F), can be used in both damp and dry locations, while THWN wire, rated for 75°C (167°F), can be used in both wet and dry locations. Service entrance (SE) cable, underground feeder (UF) cable, and nonmetallic (NM) cable can also be installed in conduit.
METALLIC CONDUIT
Metallic tubing, type EMT, is light, thin-wall metal conduit used primarily to protect indoor wiring from physical abuse. It is easy to install, but its thin walls provide only marginal protection from abuse. EMT is available in 10-ft lengths with diameters of 1/2, 3/4, 1.0, 1.1/2, and 2.0 in. EMT 12-in.-diameter conduit can hold up to six No. 14 AWG wires, five No. 10 AWG wires, or two No. 8 AWG wires.
NEC 2002, Article 358, “Electrical Metallic Tubing: Type EMT,” covers the use,installation, and construction specifications for EMT and associated fittings.
Intermediate metallic conduit, type IMC, has thicker walls than EMT and is galvanized so it is approved for exposed outdoor use, but it can also be used indoors where greater protection against abuse is desired. IMC is connected with watertight threaded or compression fittings, and is available in 10-ft lengths with 1/2- and 3/4-in. diameters. NEC 2002, Article 342, “Intermediate Metal Conduit: Type IMC,” covers the use, installation, and construction specifications for IMC and associated fittings.
Rigid metal conduit, type RMC, is actually pipe and provides the greatest protection for wiring, but it is expensive and requires threaded fittings. It is approved for use in industrial, large commercial, and public buildings under all atmospheric conditions and occupancies. Ferrous raceways and fittings protected from corrosion only by enamel paint are approved only for indoor use and in places not subject to corrosive influences.
NEC 2002, Article 344, “Rigid Metal Conduit: Type RMC,” covers the use, installation, and construction specifications for RMC and its fittings.
Flexible metal conduit, type FMC, is approved for use in exposed and concealed locations where rigid conduit is difficult to install. FMC is used to connect permanently wired appliances such as water heaters, electric space heaters, and clothes dryers, because it is easy to bend.
NEC2002, Article 348, “Flexible Metal Conduit: Type FMC,” covers the use, installation,and construction specifications for FMC and its fittings.
Liquidtight flexible metal conduit, type LFMC, is approved for exposed or concealed locations where conditions of installation, operation, or maintenance require flexibility or protection from liquids, vapors, or solids. These conditions are defined in NEC Secs. 501.4(B), 502.4, 503.3, and 504.20. LFMC is also permitted in other hazardous locations where specifically approved,
NEC 2002, Article 350, “Liquidtight Flexible Metal Conduit: Type LFMC,” covers the use, installation, and construction specifications for LFMC and associated fittings.
NONMETALLIC CONDUIT
Electrical nonmetallic tubing, type ENT, is a nonmetallic round raceway that is corrugated.It is made of plastic that is resistant to moisture and chemical atmospheres, and is also flame-retardant. ENT can be bent by hand with reasonable force to fit it in the direction desired for the length of the run. It is available with diameters of 1/2 to 2 in
NEC2002, Article 362, “Electrical Nonmetallic Tubing: Type ENT,” covers the use, installation, and construction specifications for ENT and associated fittings.
Rigid nonmetallic conduit, type RNC, is approved for many different locations that are dry, damp, and wet as well as those subject to severe corrosive influences from chemicals. It can also be used in concealed locations within buildings or underground.
NEC 2002, Article 352, “Rigid Nonmetallic Conduit: Type RNC,” covers the use, installation, and construction specifications for RNC and associated fittings.
Nonmetallic underground conduit with conductors, type NUCC, is approved for direct burial underground and encasement or embedding in concrete or cinder fill.
Under certain conditions it can be buried in underground locations where it will be subject to severe corrosive chemicals.
NEC 2002, Article 354, “Nonmetallic Underground Conduit: Type NUCC,” covers the use, installation, and construction specifications for NUCC with conductors.
Liquidtight flexible nonmetallic conduit, type LFNC, is approved for use where conductors must be protected from vapors, liquids, or solids and where flexibility is required for installation, operation, or maintenance. Users are cautioned that some types of LFNC can become brittle in extreme cold and therefore susceptible to damage from physical contact.
NEC 2002, Article 356, “Liquidtight Flexible Nonmetallic Conduit: Type LFNC,” covers the use, installation, and construction specifications for LFNC and associated fittings.
Polyvinyl chloride (PVC) is a plastic resin used for extruding nonmetallic conduit in diameters up to 11/2in. If the conduit is treated to resist ultraviolet light, it is approved for use where it is continuously exposed to the sun. Unlike metal conduit, PVC conduit does not corrode when it is in contact with water or damp soil, and it does not need to be painted. It can protect buried wires transitioning from below ground to above-ground destinations PVC conduit and fittings can be joined together to form assemblies with a solvent glue similar to that used for PVC plumbing pipe. Because it is nonconductive, a grounding wire is required within the PVC conduit. If used underground, it must be buried at a depth of at least 18 in., but if it is covered by a concrete slab, the depth can be shallower.