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Ứng dụng, đặc điểm ống luồn dây điện trơn EMT - CVL:
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CAT VAN LOI STEEL CONDUIT & FITTINGS CATALOG 2012
VIDEO HOW TO BEND STEEL CONDUIT
The Steel Tube Institute Guidelines for Installing Steel Conduit/Tubing General Product Information addresses this question. Click here to link to the section containing a downloadable pdf file.
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 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 Standards 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 Inc.®, 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 of 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 by: 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
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.
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® Sectons 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 to 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: Abovegrade and Indoors The inside of all abovegrade 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.
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 be 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 steel 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 insure 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.
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 steel 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 a 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
The coefficient of expansion for steel conduit/EMT is 6.5x10-6 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 Table3, 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
The Trade Sizes and Metric Designator equivalents for RMC, IMC, and EMT are:
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"). 1. Penetrations The NEC® and building codes require the sealing of openings around raceways that penetrate a firerated assembly. This requirement is to prevent smoke, gases and flames from migrating from one area to another. There are many listed penetration firestopping 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. The NEC does not require these thermal protection methods where conduit is installed in a fully sprinklered building.
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.
Yes, even though IMC is lighter and has a thinner wall than rigid steel 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 steel 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.
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.
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.
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 insure a good connection and allow the entire thread surface to be inside the coupling.
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