CLASS 1 LOCATIONS

70-475 SNM cable with approved termination fittings shall be the wiring method employed. Type PLTC, MI, MC, MV, TC, or SNM cable shall be permitted to be installed in cable tray systems and shall be installed in a manner to avoid tensile stress at the termination fittings. Boxes, fittings, and joints shall not be required to be explosionproof except as required by Sections 501-3(b)(1), 501-6(b)(1), and 501-14(b)(1). Where provision must be made for limited flexibility, as at motor terminals, flexible metal fittings, flexible metal conduit with approved fittings, liquidtight flexible metal conduit with approved fittings, liquidtight flexible nonmetallic conduit with approved fittings, or flexible cord approved for extra-hard usage and provided with approved bushed fittings shall be used. An additional conductor for grounding shall be included in the flexible cord. (FPN): See Section 501-16(b) for grounding requirements where flexible conduit is used. Exception: Wiring in nonincendive circuits shall be permitted using any of the methods suitable for wiring in ordinary locations. 501-5. Sealing and Drainage. Seals in conduit and cable systems shall comply with (a) through (f) below. Sealing compound shall be of a type approved for the conditions and use. Sealing compound shall be used in Type MI cable termination fittings to exclude moisture and other fluids from the cable insulation. (FPN No. 1): Seals are provided in conduit and cable systems to minimize the passage of gases and vapors and prevent the passage of flames from one portion of the electrical installation to another through the conduit. Such communication through Type MI cable is inherently prevented by construction of the cable. Unless specifically designed and tested for the purpose, conduit and cable seals are not intended to prevent the passage of liquids, gases, or vapors at a continuous pressure differential across the seal. Even at differences in pressure across the seal equivalent to a few inches of water, there may be a slow passage of gas or vapor through a seal, and through conductors passing through the seal. See Section 501-5(e)(2). Temperature extremes and highly corrosive liquids and vapors can affect the ability of seals to perform their intended function. See Section 501-5(c)(2). (FPN No. 2): Gas or vapor leakage and propagation of flames may occur through the interstices between the strands of standard stranded conductors larger than No. 2. Special conductor constructions, e.g., compacted strands or sealing of the individual strands, are means of reducing leakage and preventing the propagation of flames. (a) Conduit Seals, Class I, Division 1. In Class I, Division: 1 locations, conduit seals shall be located as follows: (1) In each conduit run entering an enclosure for switches, circuit breakers, fuses, relays, resistors, or other apparatus that may produce arcs, sparks, or high temperatures. Seals shall be installed within 18 inches (457 mm) from such enclosures. Explosionproof unions, couplings, reducers, elbows, capped elbows, and conduit bodies similar to "L," "T," and "Cross" types shall be the only enclosures or fittings permitted between the sealing fitting and the enclosure. The conduit bodies shall not be larger than the largest trade size of the conduits. Exception: Conduit 1½ inches and smaller entering an explosionproof enclosure for switches, circuit breakers, fuses, relays, or other apparatus that may produce arcs or sparks shall not be required to be sealed if the current-interrupting contacts are: 70-476 NATIONAL ELECTRICAL CODE a. Enclosed within a chamber hermetically sealed against the entrance of gases or vapors, or b. Immersed in oil in accordance with Section 501-6(b)(1)(2). (2) In each conduit of 2-inch size or larger entering the enclosure or fitting housing terminals, splices, or taps and within 18 inches (457 mm) of such enclosure or fitting. (FPN): See Section 500-3(c)(2). (3) Where two or more enclosures for which seals are required under (a)(1) and (a)(2) above are connected by nipples or by runs of conduit not more than 36 inches (914 mm) long, a single seal in each such nipple connection or run of conduit shall be considered sufficient if located not more than 18 inches (457 mm) from either enclosure. (4) In each conduit run leaving the Class I, Division 1 location. The sealing fitting shall be permitted on either side of the boundary of such location but shall be so designed and installed to minimize the amount of gas or vapor that may have entered the conduit system within the Division 1 location from being communicated to the conduit beyond the seal. There shall be no union, coupling, box, or fitting in the conduit between the sealing fitting and the point at which the conduit leaves the Division 1 location. Exception: Metal conduit containing no unions, couplings, boxes, or fittings that passes completely through a Class I, Division 1 location with no fittings less than 12 inches (305 mm) beyond each boundary shall not be required to be sealed if the termination points of the unbroken conduit are in unclassified locations. (b) Conduit Seals, Class I, Division 2. In Class I, Division 2 locations, conduit seals shall be located as follows: (1) For connections to explosionproof enclosures that are required to be approved for Class I locations, seals shall be provided in accordance with (a)(1), (a)(2), and (a)(3) above. All portions of the conduit run or nipple between the seal and such enclosure shall comply with Section 501-4(a). (2) In each conduit run passing from a Class I, Division 2 location into an unclassified location. The sealing fitting shall be permitted on either side of the boundary of such location but shall be so designed and installed to minimize the amount of gas or vapor that may have entered the conduit system within the Division 2 location from being communicated to the conduit beyond the seal. Rigid metal conduit or threaded steel intermediate metal conduit shall be used between the sealing fitting and the point at which the conduit leaves the Division 2 location, and a threaded connection shall be used at the sealing fitting. There shall be no union, coupling, box, or fitting in the conduit between the sealing fitting and the point at which the conduit leaves the Division 2 location. Exception No. 1: Metal conduit containing no unions, couplings, boxes, or fittings that passes completely through a Class I, Division 2 location with no fittings less than 12 inches (305 mm) beyond each boundary shall not be required to be sealed if the cermination points of the unbroken conduit are in unclassified locations. ARTICLE 501 - CLASS I LOCATIONS 70-477 Exception No. 2: Conduit systems terminating at an outdoor unclassified location where a wiring method transition is made to cable tray, cablebus, ventilated busway, Type MI cable, or open wiring shall not be required to be sealed where passing from the Class I, Division 2 location into the unclassified area. The conduits shall not terminate at an enclosure containing an ignition source. (c) Class I, Divisions 1 and 2. Where required, seals in Class I, Division 1 and 2 locations shall comply with the following: (1) Fittings. Enclosures for connections or equipment shall be provided with an approved integral means for sealing, or sealing fittings approved for Class I locations shall be used. Sealing fittings shall be accessible. (2) Compound. Sealing compound shall be approved and shall provide a seal against passage of gas or vapors through the seal fitting, shall not be affected by the surrounding atmosphere or liquids, and shall not have a melting point of less than 93°C (200°F). (3) Thickness of Compounds. In a completed seal, the minimum thickness of the sealing compound shall not be less than the trade size of the conduit and, in no case, less than 5/₈ inch (16 mm). (4) Splices and Taps. Splices and taps shall not be made in fittings intended only for sealing with compound, nor shall other fittings in which splices or taps are made be filled with compound. (5) Assemblies. In an assembly where equipment that may produce arcs, sparks, or high temperatures is located in a compartment separate from the compartment containing splices or taps, and an integral seal is provided where conductors pass from one compartment to the other, the entire assembly shall be approved for Class I locations. Seals in conduit connections to the compartment containing splices or taps shall be provided in Class I, Division 1 locations where required by (a)(2) above. (d) Cable Seals, Class I, Division 1. In Class I, Division 1 locations, each multiconductor cable in conduit shall be considered as a single conductor if the cable is incapable of transmitting gases or vapors through the cable core. These cables shall be sealed in accordance with (a) above. Cables with a gas/vapor-tight continuous sheath capable of transmitting gases or vapors through the cable core shall be sealed in the Division 1 location after removing the jacket and any other coverings so that the sealing compound will surround each individual insulated conductor and the outer jacket. Exception: Multiconductor cables with a gas/vapor-tight continuous sheath capable of transmitting gases or vapors through the cable core shall be permitted to be considered as a single conductor by sealing the cable in the conduit within 18 inches (457 mm) of the enclosure and the cable end within the enclosure by an approved means to prevent the entrance of gases or vapors or propagation of flame into the cable core, or by other approved methods. (e) Cable Seals, Class I, Division 2. In Class I, Division 2 locations, cable seals shall be located as follows: 70-478 NATIONAL ELECTRICAL CODE (1) Cables entering enclosures that are required to be approved for Class I locations shall be sealed at the point of entrance. The sealing fitting shall comply with (b)(1) above. Multiconductor cables with a gas/vaportight continuous sheath capable of transmitting gases or vapors through the cable core shall be sealed in an approved fitting in the Division 2 location after removing the jacket and any other coverings so that the sealing compound will surround each individual insulated conductor in such a manner as to minimize the passage of gases and vapors. Multiconductor cables in conduit shall be sealed as described in (d) above. (2) Cables with a gas/vapor-tight continuous sheath and that will not transmit gases or vapors through the cable core in excess of the quantity permitted for seal fittings shall not be required to be sealed except as required in (e)(1) above. The minimum length of such cable run shall not be less than that length that limits gas or vapor flow through the cable core to the rate permitted for seal fittings [0.007 cubic feet per hour (198 cubic centimeters per hour) of air at a pressure of 6 inches of water (1493 pascals)]. (FPN No. 1): See Outlet Boxes and Fittings for Use in Hazardous Locations, ANSI/UL 886-1985. (FPN No. 2): The cable core does not include the interstices of the conductor strands. (3) Cables with a gas/vapor-tight continuous sheath capable of transmitting gases or vapors through the cable core shall not be required to be sealed except as required in (e)(1) above, unless the cable is attached to process equipment or devices that may cause a pressure in excess of 6 inches (1493 pascals) of water to be exerted at a cable end, in which case a seal, barrier, or other means shall be provided to prevent migration of flammables into an unclassified area. Exception: Cables with an unbroken gas/vapor-tight continuous sheath shall be permitted to pass through a Class I, Division 2 location without seals. (4) Cables that do not have gas/vapor-tight continuous sheath shall be sealed at the boundary of the Division 2 and unclassified location in such a manner as to minimize the passage of gases or vapors into an unclassified location. (FPN): The sheath mentioned in (d) and (e) above may be either metal or a nonmetallic material. (f) Drainage. (1) Control Equipment. Where there is a probability that liquid or other condensed vapor may be trapped within enclosures for control equipment or at any point in the raceway system, approved means shall be provided to prevent accumulation or to permit periodic draining of such liquid or condensed vapor. (2) Motors and Generators. Where the authority having jurisdiction judges that there is a probability that liquid or condensed vapor may accumulate within motors or generators, joints and conduit systems shall be arranged to minimize entrance of liquid. If means to prevent accumulation or to permit periodic draining are judged necessary, such means shall be provided at the time of manufacture and shall be considered an integral part of the machine. ARTICLE 501 - CLASS I LOCATIONS 70-479 (3) Canned Pumps, Process or Service Connections, Etc. For canned pumps, process or service connections for flow, pressure, or analysis measurement, etc., that depend upon a single compression seal, diaphragm, or tube to prevent flammable or combustible fluids from entering the electrical conduit system, an additional approved seal, barrier, or other means shall be provided to prevent the flammable or combustible fluid from entering the conduit system beyond the additional devices or means, if the primary seal fails. The additional approved seal or barrier and the interconnecting enclosure shall meet the temperature and pressure conditions to which they will be subjected upon failure of the primary seal, unless other approved means are provided to accomplish the purpose above. Drains, vents, or other devices shall be provided so that primary seal leakage will be obvious. (FPN): See also the last paragraph of Section 500-5(b) and Fine Print Notes to