- Electrical Metallic Tubing (EMT)
- Electrical Nonmetallic Tubing (ENMT)
- Rigid Metal Conduit (RMC)
- Intermediate Metallic Conduit (IMC)
- Flexible Metal Conduit (FMC)
- Rigid PVC Conduit.
Six Types of Electrical Conduit
|6 Types of Electrical Conduit|
Electrical Metallic Tubing (EMT) – Thin Wall Conduit
Electrical metallic tubing (EMT), often called thin-wall conduit, is commonly used. It is lightweight, thinner, and easier to bend than rigid conduit. Because it is metallic, it can handle physical abuse. It is used wherever physical protection is needed, but PVC cannot be used because of the presence of steam pipes or other sources of heat.
Conduit usually comes in 10-foot lengths. Couplings are used to extend the overall length of a piece of conduit. Thin-wall conduit couplings are electrical fittings used to attach or couple the length of one conduit to another.
EMT does not require threads to be cut on the ends. It uses specially made connectors for the ends so that it can be attached to boxes, panels, and other devices. (Rain-tight fittings should be used outside.) How EMT is fitted into a conduit box, and how EMT fittings are installed:
1. To install connectors:
a. Insert conduit into fitting.
b. Tighten screw with screwdriver, or tighten compression nut with wrench.
c. Remove lock nut from fitting.
d. Insert fitting into knockout in outlet box.
e. Turn lock nut on fitting from inside box and tighten by lapping with a screwdriver blade until teeth bite into box.
2. To install couplings:
a. Insert conduit into fitting.
b. Tighten screws with screwdriver, or tighten compression nuts with wrench.
3. Secure conduit with straps:
a. Install minimum of one strap for every 10 feet of conduit run.
b. Always use a strap within 3 feet of a box.
4. Offset connectors are used to connect conduit that is flat against wall with an outlet box without bending the conduit.
5. Pulling elbows are made for corners and is installed onto conduit by tightening screws with screwdriver. Wires can be readily pulled through the fitting by simply removing the cover. A variety of fittings used on EMT.
Electrical Nonmetallic Tubing (ENMT)
Electrical nonmetallic tubing (ENMT) is also used as conduit. Made of the same material as PVC, ENMT is resistant to moisture and many atmospheric pollutants and is flame-retardant. Suitable for above-ground use, it is easily bent by hand but cannot be used where flexibility is needed, such as at motor terminations, to prevent noise and vibration.
|Electrical nonmetallic tubing|
NEC Article 331 deals with ENMT and lists couplings, connectors, and fittings to be used with it. Exposed ENMT cannot be used in buildings more than three stories high. Changes to this article in 2002 NEC now allow it under certain conditions. It can be used in buildings of any height when concealed.
Rigid Metal Conduit (RMC)
Rigid conduit is like pipe with thick walls and ends with screw threads. Industrial and commercial wiring must, because of the large amounts of current and high voltages required, be enclosed in large pipe to protect the wires from damage by equipment operating around them.
This large pipe, called rigid conduit, presents a number of problems, most of which arise whenever a bend has to be made. (For conduit of smaller diameter, special hand benders, or “hickeys,” are used.) Dies are used to keep large pipe from collapsing as it is bent.
The inside part of the pipe is compressed as the outside portion is stretched. It is very easy to collapse the piece of pipe if proper care is not taken during the bending operation. That is why some very elaborate bending equipment is available. The skill associated with conduit bending comes with experience.
Threads have to be cut on the ends of rigid conduit for fittings. This can be done by hand tools in some cases, but in most instances, a thread-cutting machine is used. Cutting, threading, and bending rigid conduit takes a number of years lo master. Rigid conduit has special types of boxes for switches and receptacles, known as FS boxes.
When using rigid conduit, keep in mind that the entire conduit is not filled to capacity. The number of wires in the conduit is limited by NEC rules.
Intermediate Metallic Conduit (IMC)
This is a relatively new type of conduit with wall thickness less than that of rigid metal conduit but greater than that of ENMT. It uses the same threading methods and standard fittings as rigid metal conduit and has the same general application rules as rigid metal conduit.
Intermediate metallic conduit (IMC) is a lightweight rigid steel conduit that requires about 25% less steel than heavy-wall rigid conduit. Acceptance into the Code was based on a UL fact-finding report that showed, through research and comparative tests, that IMC performs as well as rigid steel conduit in many cases and surpasses rigid aluminum and EMT in more cases.
IMC may be used in any application for which rigid metal conduit is recognized by the NEC, including use in all classes and divisions of hazardous locations, as covered in Articles 501.4, 502.4, and 503.3. Its thinner wall makes it lighter and less expensive than standard rigid metal conduit.
However, it has physical properties that give outstanding strength. It has the same outside diameter as rigid conduit and the same trade sizes. The rules for number of wires in IMC are the same as for rigid metal conduit.
Electrical Installation Articles
PVC Conduit System
PVC is also used in conduit systems. The use of this plastic conduit pipe in electrical wiring systems has decided advantages.
Non Metallic conduits weigh one-fourth to one-fifth as much as metallic systems. They can also be easily installed in less than half the time and are easily fabricated on the job. (Nonmetallic conduit and raceway systems are covered by Article 347 of the NEC.)
|Rigid PVC Conduit|
PVC has high impact resistance to protect wiring systems from physical damage. It is resistant to sunlight and approved for outdoor usage. The use of expansion fittings allows the system to expand and contract with temperature variations.
PVC conduit will expand or contract approximately four to five times as much as steel and two and one-fourth times as much as aluminum. Installations where the expected temperature exceeds 14°C (25°F) should use expansion joints.
The manufacturer furnishes the formulas for figuring out the expansion joint size and how often it is needed in any given installation. Any plastic conduit should always be installed away from steam lines and other sources of heat. Support straps should be tightened only enough to allow for linear movement caused by expansion and contraction.
The PVC conduit that is widely used in the United States is called Plus 40. It is UL listed for use underground, encased in concrete or direct burial, and for exposed or concealed use in most conduit applications above ground. Plus 80 is designed for above-ground and underground applications where PVC conduit with extra heavy wall is needed. It is frequently used in situations where severe abuse may occur, such as for pole risers, bridge crossings, and heavy traffic areas.
Typical applications are around loading docks, in high-traffic areas, and where threaded connections are required. PVC conduit and fittings are similar to those used on rigid conduit.
Installation of these is as follows:
1. To bond fittings to conduit:
a. Cut conduit to desired length and deburr ends.
b. Pipe any contaminant or shavings from end of conduit
c. Mate (unthreaded) end of fitting
d. Apply cement to clean end of conduit and fitting.
e. While making a one-quarter rotation, push conduit into fitting until it “bottoms.” Allow 10 minutes to set properly (more time is required in temperatures below 60°F).
2. To install terminal adapters:
a. Follow procedure in item 1 mentioned previously.
b. Insert threaded end of bonded terminal adapter through knockout into box.
c. Turn metallic lock nut onto terminal adapter threads from inside box and tighten by tapping with a screwdriver blade until locknut teeth bite into box.
3. To install couplings
Follow procedure in item 1. (Note that special fittings, called expansion couplings, may be required if a PVC run is in an area subjected to drastic changes in temperature.)
4. Secure conduit with straps
a. Install a minimum of one strap for every 10 feet of conduit run.
b. Always use a strap within 3 feet of box.
5. Pulling elbows are made for corners and are installed onto conduit by following steps of item 1 mentioned previously.
6. To connect PVC conduit to a piece of rigid metallic pipe, use a PVC female adapter.
a. Follow item 1 mentioned previously to bond PVC female adapter to PVC conduit.
b. Turn female end of bonded PVC fitting onto rigid metallic threads.
c. Be certain that ground continuity is properly maintained.
7. An LB fitting is used when running wires inside conduit through an outside building wall.
a. Open holes at each end will accept conduit.
b. Wires can be readily pulled through the fitting by simply removing the cover.
c. The gasket under the cover is designed to keep out moisture.
8. When installing FS-type boxes follow procedure outlined in item 1 for bonding to conduit.
9. To mount a weatherproof cover to a box, place the gasket and the cover together on the box and attach with screws provided.
The purpose of the previous diagram and text is only to illustrate certain uses of PVC products. The manufacturer and publisher cannot be responsible for any actual electrical installation. The previous diagram and text are compiled on information that is current at the time of printing. Recommendations for product use, as well as permissible applications, are subject to change.
Electrical equipment should be selected and be applied in accordance with the NEC or other local authority having jurisdiction over the installation.
Working with Conduit
Three types of conduit may need to be bent. This bending is in order to serve their intended purpose. That purpose is the protection of electrical wiring against physical harm.
Installation of rigid metal conduit, IMC, and EMT, sometimes referred to as thin-wall conduit, requires that provision for making changes of direction in the conduit runs—ranging from simple offsets at the point of termination at outlet boxes and cabinets to complicated angular offsets at columns, beams, cornices, and so forth.
Sometimes contract specifications dictate otherwise, but changes in direction are made, particularly in the case of small diameter conduit, by bending the tubing as required. In the case of one and half inch and larger sizes, right-angle changes of direction are sometimes installed with the use of factory bent elbows or conduit bodies.
In most cases, however, such changes in direction are made more economically by using conduit bent in the field.
A good reason for making on-the-job bends is multiple runs of the larger conduit sizes. This is because truer parallel alignment of multiple runs can be maintained by using on-the-job conduit bends rather than factory elbows.
Such bends can all be made from the same center, using bends of the largest conduit in the turn as the pattern for all the other bends. Most conduit and raceway systems are run exposed. Learning to install neat-looking conduit systems in an efficient and workmanlike manner is the hallmark of a good electrician.
Every electrician working on commercial and industrial electrical installations must learn how to calculate and fabricate conduit bends. That means the operation of both hand and power conduit benders take pride in performing the best work possible.
National Electrical Code
The NEC requires that metal conduit bends must be made so that the conduit will not be damaged during the installation and during operation.
The Code also requires that the internal diameter of the conduit will not be effectively reduced in size. To accomplish this, the Code, further specifies that the minimum radius of the inner edge curve of a conduit bend be at least six times the internal diameter of the conduit when conductors without lead sheath are installed.
The reason for this rule is: when the inside of an elbow is less than six times the inside diameter, wire pulling becomes difficult. That means the insulation on the conductors may be damaged. When lead sheath conductors are to be installed, the inside must be increased according to NEC Table 346-10.
The NEC further states that no more than four bends (360°) total may be made in any one conduit run between boxes, cabinets, panels, or junction boxes. That is, between pull points.
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