Types of Electrical Conduits

1- Introduction to Electrical Conduits

Definition: An electrical conduit (EC): is a piping system that is used to carry electrical wiring for either power or communication. These piping systems are commonly referred to as raceways.

Advantages of using Electrical conduits than other wiring methods Electrical conduit provides very good protection to enclosed conductors from impact, moisture, and chemical vapors. Electrical Conduit is widely used to protect cables that carry various amounts of electrical current. For example, Rigid conduit used is used to guard against crushing and shorting of cables.  Conduit prevents accidental damage to the insulation. Varying numbers, sizes, and types of conductors can be pulled into a conduit, which simplifies design and construction compared to multiple runs of cables or the expense of customized composite cable. Wiring systems in buildings may be subject to frequent alterations. Frequent wiring changes are made simpler and safer through the use of electrical conduit, as existing conductors can be withdrawn and new conductors installed, with little disruption along the path of the conduit. Metal conduit can be used to shield sensitive circuits from electromagnetic interference, and also can prevent emission of such interference from enclosed power cables. When installed with proper sealing fittings, a conduit will not permit the flow of flammable gases and vapors, which provides protection from fire and explosion hazard in areas handling volatile substances. Both metal and plastic conduit can be bent at the job site to allow a neat installation without excessive numbers of manufactured fittings. This is particularly advantageous when following irregular or curved building profiles. Some types of conduit are approved for direct encasement in concrete. This is commonly used in commercial buildings to allow electrical and communication outlets to be installed in the middle of large open areas.

Disadvantages of using Electrical conduits The cost of conduit installation is higher than other wiring methods due to the cost of materials and labor. Conductors installed within conduit cannot dissipate heat as readily as those installed in open wiring, so the current capacity of each conductor must be reduced if many are installed in one conduit. It is impractical, and prohibited by wiring regulations, to have more than 360 degrees of total bends in a run of conduit, so special outlet fittings must be provided to allow conductors to be installed without damage in such runs. Some types of metal conduit offer a useful bonding conductor for grounding (earthing), but wiring regulations may also dictate workmanship standards or supplemental means of grounding for certain types. While metal conduit can be used as a grounding conductor, the circuit length is limited. A long run of conduit as grounding conductor will not allow proper operation of overcurrent devices on a fault.

2- How to Verify Correct Installation of Conduit

2.1 Types of Conduit Conduit systems are classified by the wall thickness, mechanical stiffness, and material used to make the tubing. The most common types of conduit are: First: Metal Conduit Rigid Metal Conduit (RMC), Galvanized rigid conduit (GRC), Intermediate Metal Conduit (IMC), Electrical metallic tubing (EMT), Aluminum conduit. Second: Non-metal Conduits PVC conduit, Rigid Nonmetallic Conduit (RNMC), Electrical Nonmetallic Tubing (ENT). Third: Flexible Conduits  Flexible Metallic Conduit (FMC), Liquidtight Flexible Metal Conduit (LFMC), Flexible Metallic Tubing (FMT), Liquidtight Flexible Nonmetallic Conduit (LFNC).

First: Metal Conduit   A- Rigid Metal Rigid metal conduit provides the best protection for cables.  It is strong enough to protect against damage from nails and impact.  It may be installed outdoors or indoors in all types of buildings, exposed or concealed in any location, including hazardous locations.  Fire protection is provided by rigid metal conduit, as arcing sparks cannot escape the conduit system.   This heavyweight, thick-walled metal conduit provides maximum strength and, when properly installed, also provides an excellent equipment ground. Where possible, boxes and fittings must be of the same material to prevent galvanic action. When cut, the ends of rigid metal conduit must be reamed carefully to protect the wire insulation from abrasion. Figure 1.  Rigid Metal Conduit Do not use rigid metal conduit in an area subject to permanent moisture unless it is of the corrosion-resistant type.  Corrosion-resistant metal conduit is available in three (3) types: Aluminum, bronze alloy and rigid steel that are PVC-coated. Aluminum is most useful in areas where certain chemical fumes and vapors are present, which has little effect on aluminum, yet, are extremely corrosive to steel.  Do not embed unprotected aluminum in concrete or bury it in the earth.  Do not use aluminum fittings.  The bronze-alloy conduit, known as Everdur, has special corrosive-resistance characteristics.  It is used along the seacoast, in chemical plants, oil refineries and for underwater fixtures. Plastic-coated conduit is rigid steel coated with a seamless coating of polyvinyl chloride.  It is highly resistant to oils, grease, acids, alkalis, and moisture.  It is also flame-retardant. B- Galvanized rigid conduit (GRC): It is galvanized steel tubing, with a tubing wall that is thick enough to allow it to be threaded. Its common applications are in commercial and industrial construction. C- Intermediate Metal Conduit (IMC): It is steel tubing heavier than EMT but lighter than RMC. It may be threaded. D- Electrical metallic tubing (EMT): It is sometimes called thin-wall, is commonly used instead of galvanized rigid conduit (GRC), as it is less costly and lighter than GRC. EMT itself may not be threaded, but can be used with threaded fittings that clamp to it.  Lengths of conduit are connected to each other and to equipment with clamp-type fittings.  Like GRC, EMT is more common in commercial and industrial buildings than in residential applications. EMT is generally made of coated steel, though it may be aluminum. EMT is about 40% lighter than rigid.  Its couplings and connectors may be either compression or setscrew types.  EMT may be used in most locations, but because of its lighter weight, thin wall, and threadless or setscrew connectors, it is restricted to installations where it will not be subjected to severe physical damage.  Just like rigid conduit, when cut in the field, the ends must be reamed to remove all sharp edges. Referred to as “thin-walled" conduit, electrical metallic tubing shown in Figure 2 shows the difference between the thin walled EMT and the thick walled rigid conduit. Figure 2.  EMT Conduit EMT can be installed in both exposed and concealed work. If installed in wet locations, it must be installed so as to prevent water from entering the tubing.  If installed in a location where corrosive fumes or vapors exist, special corrosive-resistant tubing must be used. E- Aluminum conduit: It is similar to galvanized steel conduit, is a rigid conduit, generally used in commercial and industrial applications, where a higher resistance to corrosion is needed. Such locations would include food processing plants, where large amounts of water and cleaning chemicals would make galvanized conduit unsuitable. Aluminum cannot be directly embedded in concrete, since the metal reacts with the alkalis in cement.  The conduit may be coated to prevent corrosion by incidental contact with concrete.  The extra cost of aluminum is somewhat offset by the lower labor cost to install, since a length of aluminum conduit will have about one-third the weight of an equally-sized rigid steel conduit. In extreme corrosion environments where plastic coating of the tubing is insufficient, conduits may be made from stainless steel, bronze or brass.

Second: Non-metal Conduits A- PVC conduit PVC conduit is the lightest in weight compared to other conduit materials, and usually lower in cost than other forms of conduit.  In North American electrical practice, it is available in three different wall thicknesses, with the thin-wall variety only suitable for embedded use in concrete and heavier grades suitable for direct burial and exposed work.  The various fittings made for metal conduit are also made for PVC. The plastic material resists moisture and many corrosive substances, but since the tubing is non-conductive an extra bonding (grounding) conductor must be pulled into each conduit. PVC conduit may be heated and bent in the field.  Joints to fittings are made with slip-on solvent-welded connections, which set up rapidly after assembly and attain full strength in about one day. Since slip-fit sections do not need to be rotated during assembly, the special union fittings used with threaded conduit (Ericson) are not required.  Since PVC conduit has a higher thermal coefficient of expansion than other types, it must be mounted so as to allow for expansion and contraction of each run.  Care should be taken when installing PVC underground in multiple or parallel run configurations due to mutual heating effect of cable.  Figure 3.  PVC Conduit B- Rigid Nonmetallic Conduit (RNMC) It is a non-metallic (PVC) unthreaded tubing. C- Electrical Nonmetallic Tubing (ENT) It is thin-walled corrugated tubing that is moisture-resistant and flame retardant.  It is pliable such that it can be bent by hand and is often flexible although the fittings are not. It is not threaded due to its corrugated shape although the fittings might be.

Third: Flexible Conduits A- Flexible Metallic Conduit (FMC) It is often informally called greenfield or flex.  FMC is used primarily in dry areas where it would be impractical to install EMT or other non-flexible conduit, yet where metallic strength to protect conductors is still required. The flexible tubing does not maintain any permanent bend.  FMC may be used as an equipment grounding conductor if specific provisions are met regarding the trade size and length of FMC used in addition to the amperage of the circuits contained in the conduit. In general an equipment grounding conductor must be pulled through the FMC with an ampacity suitable to carry the fault current likely imposed on the largest circuit contained within the FMC. B- Liquidtight Flexible Metal Conduit (LFMC) It is a metallic flexible conduit covered by a waterproof plastic coating. The interior is similar to FMC.  Liquidtight flexible metal conduit is a single strip of aluminum or galvanized steel. It is spiral wound and interlocked. It provides great flexibility and strength.   Flexible metal conduit requires no elbow fittings. Flexible metal may fit when rigid metal conduit is out of the question. Covered with a liquid-tight plastic, liquid-tight flexible conduit is impervious to water, grease, oil, chemicals, etc.  Liquidtight flexible conduit may be installed as a fixed-wiring method.  However, an equipment grounding conductor must be installed when it is used as a fixed-wiring method. When used for flexibility or in runs of less than 6 feet, the conduit can serve as the ground when used in the same ground return path.  The NEC specifies that liquid-tight flexible conduit must not be exposed to physical damage.  This type of conduit must not be installed where the ambient temperature or the conductor rating is above the rating of the conduit.  Figure 4.  LFMC Conduit C- Flexible Metallic Tubing (FMT)  It is not the same as Flexible Metallic Conduit (FMC) which is described in National Electrical Code (NEC) Article 348.  FMT is a raceway, but not a conduit and is described in a separate NEC Article 360.  It only comes in 1/2" & 3/4" trade sizes, whereas FMC is sized 1/2" ~ 4" trade sizes. NEC 360.2 describes it as: "A raceway that is circular in cross section, flexible, metallic and liquidtight without a nonmetallic jacket." D- Liquid tight Flexible Nonmetallic Conduit (LFNC) It refers to several types of flame-resistant non-metallic tubing. Interior surfaces may be smooth or corrugated.  There may be integral reinforcement within the conduit wall.  It is also known as FNMC. Figure 5.  LFNC Conduit