Please Note: There are several acronyms that are used in the field to identify rigid steel conduit. The National Electrical Code® (NEC) uses the acronym “RMC” which includes rigid steel conduit, rigid aluminum conduit, stainless steel conduit and red brass conduit. The acronym “GRC” — galvanized rigid conduit — is also frequently used. The UL Standards refer to “Electrical Rigid Metal Conduit — Steel” or “ERMC-S. In the following FAQs, we use the term “Galvanized Steel RMC” when we need to be specific about the properties or requirements of rigid steel conduit that is galvanized.
Difference Between EMT, IMC and RMC
Question: What are the differences between electrical metallic tubing (EMT), rigid metal conduit (RMC), and intermediate metal conduit (IMC)?
Answer: The Steel Tube Institute Guidelines for Installing Steel Conduit/Tubing General Product Information address this question. Explore our installation guides here.
NEC Articles for EMT, IMC and RMC
Question: What National Electrical Code (NEC) Articles cover IMC, RMC, and EMT?
Answer: Raceway Articles were renumbered in the 2002 NEC. The following list shows the Article numbers that apply to IMC, RMC, and EMT, followed by the former Article number in parentheses.
IMC NEC Article 342 (345)
RMC NEC Article 344 (346)
EMT NEC Article 358 (348)
Steel Conduit/Tubing Standards
Question: What standards apply to steel conduit/tubing?
Answer: Steel Tube Institute members list their products to UL standards and also manufacture them in accordance with ANSI C80 standards (American National Standards Institute). ASTM International does not publish standards which specifically cover steel electrical conduit and couplings. Therefore, ASTM specifications do not apply to metal conduit for use as a metal raceway for the installation of wires and cables in accordance with the National Electrical Code. The following standards apply to steel conduit and tubing:
Electrical Metallic Tubing (EMT)
UL-797 — Standard for Electrical Metallic Tubing — Steel
ANSI C80.3 — American National Standard for Steel Electrical Metallic Tubing
Rigid Metal Conduit — Steel (RMC)
UL-6 — Standard for Electrical Rigid Metal Conduit — Steel
ANSI C80.1 — American National Standard for Electrical Rigid Steel Conduit
Intermediate Metal Conduit — Steel (IMC)
UL-1242 — Standard for Electrical Intermediate Metal Conduit — Steel
ANSI C80.6 — American National Standard for Electrical Intermediate Metal Conduit
Additional information on the titles and designations of standards or requirements that have been used for the investigation of products in a specific category can be found in the Underwriters Laboratories®, General Information for Electrical Equipment Directory. The UL product category for EMT is FJMX, for RMC is DYIX, and for IMC is DYBY.
The federal government, in an effort to reduce costs, has undertaken a process of identifying nongovernment and industry-wide practices that have been accepted previously by the Department of Defense under the Single Process Initiative (SPI) for use in lieu of a specific military or federal specification or standard. This process reduces the burden on the government to produce and maintain separate standards. To this end, federal standards which covered steel rigid metal conduit, intermediate metal conduit and electrical metallic tubing, were cancelled and replaced as follows:
WW-C-581 Class 1 Type A; replaced with UL Standard for Electrical Rigid Metal Conduit — Steel, UL-6
WW-C-581 Class 2 Type A; replaced with UL Standard for Electrical Intermediate Metal Conduit — Steel, UL-1242
WW-C-563; replaced with UL Standard for Electrical Metallic Tubing — Steel, UL-797
Direct Burial of EMT, IMC and RMC
Question: Can RMC, IMC and EMT be direct-buried?
Answer: The 2011 National Electrical Code (NEC) allows the direct burial of all three products as noted in their respective Articles:
Galvanized Steel RMC Section 344.10 (B) Corrosion Environments. Galvanized steel RMC, elbows, couplings, and fittings “shall be permitted to be installed in concrete, in direct contact with the earth, or in areas subject to severe corrosive influences where protected by corrosion protection and judged suitable for the condition.”
IMC Section 342.10 (B) Corrosion Environments. Galvanized steel IMC, elbows, couplings, and fittings “shall be permitted to be installed in concrete, in direct contact with the earth, or in areas subject to severe corrosive influences where protected by corrosion protection and judged suitable for the condition.”
EMT Section 358.10 (B) Corrosion Protection. Galvanized steel EMT, elbows, couplings, and fittings “shall be permitted to be installed in concrete, in direct contact with the earth, or in areas subject to severe corrosive influences where protected by corrosion protection and approved as suitable for the condition.”
Members of the Conduit Committee of the Steel Tube Institute apply a zinc coating to galvanize steel RMC, IMC and EMT. Therefore, these products are “protected by corrosion protection” as the Code requires. However, in severe corrosive environments, the designer or AHJ may decide to require additional or supplementary protection.
Underwriters Laboratories’ Guide Information for Electrical Equipment – The White Book 2011 contains information relating to limitations or special conditions applying to products listed by UL. The directory states that galvanized steel RMC and IMC do not generally require supplementary corrosion protection when installed in soil unless: 1. Soil resistivity is less than 2000 ohm-centimeters. 2. Local experience has confirmed that the soil is extremely corrosive.
(Note: Soils producing severe corrosive effects have low electrical resistivity, expressed in ohm-centimeters. Local electric utilities commonly measure the resistivity of soils. The authority having jurisdiction (AHJ) has the authority to determine the necessity for additional protection.) EMT in direct contact with the soil generally requires supplementary corrosion protection. However, local experience in some areas of the country has shown this to be unnecessary.
Steel Raceways as Watertight Systems
Question: Can steel raceways be installed as a watertight system?
Answer: No raceway system is really “watertight”, including those that are glued or have sealed joints. Normally raceways will collect condensation. The raceway is designed to allow this moisture to drain out.
The 2011 NEC permits galvanized steel EMT, IMC and RMC to be installed in all wet locations both inside and outside. NEC Sections 342 (345).10(D), 344 (346).10(D) and 358 (348).10(C) caution the installer that all hardware items such as straps, bolts, screws etc. be of a corrosion resistant material when used to support raceways in wet locations. When threadless fittings are used with RMC and IMC, they must be identified and listed for the application. The UL standard for Conduit, Tubing and Cable Fittings, UL 514B, requires that threadless fittings intended for use in wet locations be marked “wet locations” on the fitting or its smallest unit shipping container. Threadless fittings intended for embedment in poured concrete are marked “concrete-tight” or “concrete-tight when taped.” Fittings listed as “wet locations” are also “concrete-tight”, but not all concrete-tight fittings are “wet locations” fittings. For more information, see Section 4.3.3, Fittings for use with RMC, IMC, and EMT in the publication Guidelines for Installing Steel Conduit/Tubing.
2011 NEC, Article 100, defines a wet location as follows:
“Location, Wet. Installations underground or in concrete slabs or masonry in direct contact with the Earth; in locations subject to saturation with water or other liquids, such as vehicle washing areas, and in unprotected locations exposed to weather.”
Wet Locations: Underground
All types of raceways installed underground are considered to be in a wet location. Section 300.5(B) requires that the inside of all raceways installed underground be classified as a wet location. Conductors installed in such underground locations must be listed for use in wet locations and comply with 310.10(C). It is understandable that in some cases where the ground does not percolate well, the water will also seep into the raceway.
If the presence of water is a problem, one of the following steps may help: 1. Install a typical Quazite™ concrete-polymer underground open bottom junction box over a gravel sump. This can be done by excavating a hole approximately 3′ X 3′; filling it with 1 – 2 inch gravel and placing the Quazite™ box flush with the ground prior to entering the building, or at the lowest location in the conduit run. 2. Install type ECDB or equivalent drain fittings to permit the water to drain out of the conduit. These would normally be installed in all above-the-ground locations prior to entering each building or piece of equipment.
Wet Location: Above Grade and Indoors
The inside of all above grade raceways installed in a wet location are also considered to be a wet location. Section 300.9 requires conductors and cables installed in these locations to be listed for use in wet locations in accordance with 310.10(C).
NEC 300.6(C) requires all metallic raceways installed indoors in a wet location to be mounted so there is at least a 1/4 inch air space between the raceway and the supporting surface to minimize the accumulation of moisture.
EMT, IMC and RMC in Concrete
Question: Can RMC, IMC and EMT be installed in concrete?
Answer: The National Electrical Code (NEC) allows all three products to be installed in concrete, as noted in their respective Articles: (Also see NEC 300.6(A)(3)).
Galvanized RMC Section 344.10 (B) Corrosion Environments. Galvanized steel RMC, elbows, couplings, and fittings “shall be permitted to be installed in concrete, in direct contact with the earth, or in areas subject to severe corrosive influences where protected by corrosion protection and judged suitable for the condition.”
IMC Section 342.10 (B) Corrosion Environments. “IMC, elbows, couplings, and fittings shall be permitted to be installed in concrete, in direct contact with the earth, or in areas subject to severe corrosive influences where protected by corrosion protection and judged suitable for the condition.”
EMT Section 358.10 (B) Corrosion Protection. “Ferrous or nonferrous EMT, elbows, couplings, and fittings shall be permitted to be installed in concrete, in direct contact with the earth, or in areas subject to severe corrosive influences where protected by corrosion protection and approved as suitable for the condition.”
Members of the Conduit Committee of the Steel Tube Institute apply a zinc coating to galvanize steel RMC, IMC and EMT. Therefore, these products are “protected by corrosion protection” as the code requires. However, in severe corrosive environments, the designer or AHJ may decide to require additional or supplementary protection. If supplementary corrosion protection is required or desired, it can be provided by a factory-applied PVC coating, paint approved for the purpose, or tape wraps approved for the application.
When steel conduit/EMT emerges from concrete into soil, we recommend that supplementary corrosion protection is applied a minimum of 4 inches on each side of the point where the conduit or EMT emerges. This link will take you to an individual page from the STI Installation Guide pertaining to concrete installation.
Underwriters Laboratories’ Guide Information for Electrical Equipment – The White Book 2011 contains information relating to limitations or special conditions applying to products listed by UL.
The UL Directory states that supplementary corrosion protection is not required on rigid metal conduit and IMC when installed in concrete.
The UL Directory states the following for EMT: “Galvanized steel electrical metallic tubing installed in concrete on grade or above generally requires no supplementary corrosion protection. Galvanized steel electrical metallic tubing in concrete slab below grade level may require supplementary corrosion protection.”
Where the concrete slab is installed on grade, it is important that the raceway be placed in between layers of rebar and above the bottom of the slab to ensure full encasement. However, there are structural concerns that must be satisfied. These concerns are related to size of the conduit or tubing to be encased and the thickness of the slab.
EMT, IMC and RMC Compliance with NFPA 130
Question: Do RMC, IMC and EMT comply with the requirements of NFPA 130?
Answer: The requirements in the 2010 edition of NFPA 130 were changed from the 2007 edition of NFPA 130.
NFPA 130 (2007 Edition): Section 5.4 Wiring Requirements state that “Materials manufactured for use as conduits, raceways, ducts, boxes, cabinets, equipment enclosures, and their surface finish materials shall be capable of being subjected to temperatures up to 500°C (932°F) for 1 hour and shall not support combustion under the same temperature condition.”
Steel conduit is zinc-galvanized for corrosion protection. Underwriters Laboratories exposed rigid metal conduit and steel intermediate metal conduit and electrical metallic tubing to a 4 hour ASTM E119 fire test at a temperature of 2000°F. The conduit and EMT were still intact at the end of the test. This information is contained in report Annular Space Protection of Openings Created by Penetrations of Tubular Steel Conduit – a Review of UL Special Services Investigation File NC546 Project 90NK111650, which is available from the Conduit Committee of the Steel Tube Institute.
Since the melting point of zinc is around 800°F, the galvanizing may be compromised but the conduit and EMT would still be intact after the E119 fire and would not be “capable of supporting combustion”. We cannot verify the condition of the conductors within the conduit. However, fire resistive circuit integrity cables can be used within conduit to obtain a 2-hour fire rating.
NFPA 130 (2010): Section 5.4.2 states that “conduits, raceways, ducts, boxes, cabinets, and equipment enclosures shall be constructed of noncombustible materials in accordance with the requirements of ASTM E 136.” Steel conduit and steel EMT are noncombustible products and comply with this requirement.
Steel Conduit/EMT Coefficient for Expansion
Question: What is the coefficient of expansion for steel conduit/EMT and why is this significant?
Answer: The coefficient of expansion for steel conduit/EMT is 6.5×10-6 in./in./°F. This is significant as it relates to whether or not expansion fittings would be required in a particular application. Expansion fittings are installed where significant temperature differentials are anticipated. These temperature shifts cause materials to expand and contract and could result in the conduit being pulled apart at the joint. Expansion fittings are not normally required with steel conduit/tubing because their coefficient of expansion is identical to or similar to that of other common building materials. However, when steel conduit is installed on bridges or rooftops or as an outdoor raceway span between buildings, expansion fittings may be required. In these types of installations, there is a probability that expansion and contraction would occur, resulting from the direct heat of the sun coupled with significant temperature drops at night. Expansion characteristics of steel conduit/tubing are shown in Table 3, at 5°F to 200°F in 5°F increments. The table also shows the length changes for steel conduit at each temperature differential. This will help determine the need for expansion fittings. For information about the use of expansion fittings, contact your fittings supplier.
Steel Conduit/EMT Metric Designators
Question: What are the metric designators for steel conduit/EMT?
Answer: The trade sizes and metric designator equivalents for RMC, IMC, and EMT are:
|Trade Size||Metric Designator|
Note: The trade sizes and metric designators are for identification purposes only and are not actual dimensions. Product dimensions do not change.
Steel Conduit/EMT Fire Rating
Question: What is the fire rating of steel conduit/EMT?
Answer: Steel conduit and tubing are considered noncombustible by the building codes. They do not have fire ratings. This question is usually asked relative to the penetration of a fire-rated assembly or use in an emergency circuit, fire pump, or mass transit vehicle (see Question 4 for information on NFPA 130 “Standard for Fixed Guideway Transit and Passenger Rail Systems”). The NEC does not require these thermal protection methods where conduit is installed in a fully sprinklered building.
The NEC and building codes require the sealing of openings around raceways that penetrate a fire rated assembly. This requirement is to prevent smoke, gases and flames from migrating from one area to another. There are many listed penetration fire-stopping systems that can be used to seal openings; the listing instructions should be strictly followed. Most building codes permit the openings around galvanized steel RMC, IMC and EMT in concrete or masonry to be filled with cement, mortar, or grout. However, since local codes sometimes vary, these requirements should be checked prior to installation.
2.) Emergency circuits (NEC 700.10(D)(1) and Fire Pump Circuits (NEC 695.6(A)(2)(d)
Prior to installing these circuits, the NEC and local or state code requirements should be reviewed. Steel raceways withstand fire (see Question 4); however, ordinary conductor insulation may be compromised when exposed to elevated temperatures, and a short circuit can be created. This is the reason for special protection of emergency and fire-pump circuits. Methods of fire protection include enclosing the raceways in a fire-rated enclosure, embedding them in concrete, wrapping them with a listed wrap system for protection from fire, or installing them as part of a listed Electrical Circuit Protective System. The UL Fire Resistive Directory contains information on listed fire-rated assemblies that include steel conduit and EMT. It is the entire assembly that has the fire-rating, not the individual component.
IMC as a Service Mast
Question: Can IMC be used as a service mast?
Answer: Yes. According to Section 230.28 of the 2011 NEC the only requirement is that the service mast “…shall be of adequate strength or be supported by braces or guys to withstand safely the strain imposed by the service drop.” IMC meets this requirement. The local utility should be consulted for service mast requirements.
Physical Protection of IMC vs. RMC
Question: Does IMC provide as much physical protection as rigid?
Answer: Yes, even though IMC is lighter and has a thinner wall than rigid metal conduit, it provides equivalent protection to conductors. This is achieved through the steel processing during manufacturing. The NEC allows IMC to be used in all of the same atmospheres and occupancies as rigid metal conduit. Articles 342 for intermediate metal conduit and 344 (for rigid metal conduit contain identical installation requirements. Both products are UL listed to their respective standards.
Question: How do you thread IMC?
Answer: Cutting and threading of IMC is covered extensively in the Steel Tube Institute publication Guidelines for Installing Steel Conduit/Tubing under Section 4, General Installation Practices. You can jump to that section by clicking here to download a pdf file.
Type of Thread Cut for IMC and RMC
Question: What type of thread is cut on RMC and IMC?
Answer: The NEC requires that the threads of both RMC and IMC be cut with a 3/4 inch taper per foot (1 in 16) per ANSI/ASME B.1.20.1 Standards for Pipe Threads, General Purpose (Inch). This applies to both factory and field threads. This is the same taper as standard plumbing pipe.
Protecting Field-cut Threads from Corrosion
Question: How can field-cut threads be protected against corrosion?
Answer: Factory-cut threads have corrosion protection applied at the factory. Field cut threads are required to be coated “with an approved electrically conductive, corrosion-resistant compound “where corrosion protection is necessary.” (see NEC 2011 300.6(A)). Where installed in wet or outdoor locations, we recommend that all field-cut threads and any threads exposed after installation be protected against corrosion. We recommend that all field-cut threads be protected against corrosion where they will be installed in wet or outdoor locations. The thread surface should be protected with conductive rust-resistant coating. Zinc-rich paint is a typical coating, but there are other conductive coatings that can be used. Field threads should be cut one thread short. This will ensure a good connection and allow the entire thread surface to be inside the coupling.
RMC and IEC Standards
Question: Does RMC conform to IEC standards?
Answer: The rigid metal conduit used in the U.S. is manufactured to UL Safety Standard No. 6 and to ANSI C80.1. There is one IEC standard entitled Extra Heavy-Duty Rigid Conduit (IEC 981) which is very similar to the rigid metal conduit listed to UL-6, with the exception of some dimensional differences. At the current time, no one lists product to IEC 981.
RMC and Schedule 40
Question: Is RMC the same as Schedule 40 pipe?
Answer: No. The term “Schedule 40” originated with an ASTM specification and has come into general usage. It is still addressed in a number of ASTM specifications, whose requirements typically include dimensional and strength parameters.
Rigid conduit is intended for electrical applications, not structural ones. Its requirements are governed by UL and ANSI specifications, not by ASTM. The main issue here is that, whereas dimensions may be similar between the two, rigid conduit is not intended for, is not designed for, and is not tested for any strength or structural requirements
If a job specification calls for “schedule 40 conduit” or “schedule 40 rigid” confirm that the requirement is for rigid steel conduit, since “schedule 40” is still a term that applies to PVC conduit.
Usage of EMT with 600V+
Question: Can EMT be used over 600 volts?
Answer: Underwriters Laboratories Guide Information for Electrical Equipment – The White Book 2011 states that EMT is listed for “installation of conductors in circuits rated above or below 600V, nominal, and in accordance with ANSI/NFPA 70, National Electrical Code”.
Part II of Article 300 (Wiring Methods) of the National Electrical Code covers “Requirements for Over 600 Volts, Nominal.” In Section 300.37 of Part II entitled “Aboveground Wiring Methods,” EMT is listed as one of the permitted wiring methods.
In 1996, Underwriters Laboratories published a report of research entitled Electrical Metallic Tubing for Use Over 600 Volts. To view a copy of the report, click here.
Steel Conduit/Tubing as Equipment Grounding Conductors
Question: Can steel conduit/tubing be used as equipment grounding conductors or do you have to use a supplementary conductor?
Answer: Yes, RMC, IMC and EMT are all permitted as equipment grounding conductors in accordance with NEC 250.118. It is permitted to add a supplementary equipment grounding conductor sized in accordance with NEC 250.122. This is a systems design consideration. If a supplementary equipment grounding conductor is used, it is still very important to comply with NEC 300.10 and 300.12, since approximately 90% to 95% of the current will flow on the conduit and not in a supplementary conductor. For information on the equipment grounding capabilities of RMC, IMC and EMT, click here for a link to the section with the results of a research study performed by the Georgia Institute of Technology.
Electromagnetic Interference (EMI) Shielding
Question: Does steel conduit/tubing shield against electromagnetic interference (EMI)?
Answer: Yes. A copy of the study conducted by Georgia Institute of Technology titled Modeling and Evaluation of Conduit Systems for Harmonics and Electromagnetic Fields is the basis for the Grounding Electromagnetic Interference (GEMI) Analysis software, which is available for downloading.
Advantages of Steel Conduit/Tubing
Question: What are the advantages of using steel conduit/tubing?
Answer: Steel conduit and tubing offer several advantages over alternative wiring methods:
- EMI Shielding
- Physical protection of conductors
- Proven equipment grounding conductor
- Low coefficient of expansion — infrequent need for expansion fittings
- Impact resistant — high tensile strength
- Adaptable to future wiring changes
- Low life-cycle costs
- Recyclable (For more documentation, click here.)
EMT on Rooftops
Question: Can EMT be used on roof-tops?
Answer: There is no prohibition in the NEC. In some installations, there may be a concern about physical damage or corrosion protection. This is often a judgment call by the authority having jurisdiction. The NEC does not allow EMT to be used where, “during installation or afterward, it will be subject to severe physical damage.” The NEC also requires the EMT to be “securely fastened in place” and provides requirements for supports. The AHJ may determine more supports are necessary. If the AHJ approves the installation with EMT, consideration should be given to compliance with NEC 110.12, 300.6, 300.7(A) & (B), 358.10 and 358.30.
EMT Fittings in Wet Locations
Question: Are special EMT fittings required for use in wet locations?
Answer: Yes. Per 2011 NEC Article 358, all fittings are required to be listed (358.6) and suitable for a wet location (358.42). In 358.42, it is stated that “Couplings and connectors used with EMT shall be made up tight. Where buried in masonry or concrete, they shall be concrete tight type. Where installed in wet locations, they shall comply with 314.15(A).” Article 314 includes installation requirements for fittings used to join raceways and to connect raceways and cables to boxes and conduit bodies. 314.15(A) states that in damp or wet locations, boxes, conduit bodies, and fittings “shall be placed or equipped so as to prevent moisture from entering or accumulating within the box, conduit body, or fitting. Boxes, conduit bodies and fittings installed in wet locations shall be listed for use in wet locations.”
Recycled Content and LEED Points
Question: What is the recycled content of steel conduit/tubing? Can I get LEED points for using these products in my project?
Answer: Unlike other materials, steel contains recycled material and is also fully recyclable. In fact, according to the Steel Recycling Institute (SRI), steel is the world’s most recycled material. All of our member conduit producers use recycled steel in their conduit and tubing products. The amount of recycled material in steel conduit/tubing products depends upon the process used by our various steel suppliers.
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