HAZARDOUS (CLASSIFIED) LOCATIONS

500-1. Scope - Articles 500 Through 504. Articles 500 through 504 cover the requirements for electrical equipment and wiring for all voltages in locations where fire or explosion hazards may exist due to flammable gases or vapors, flammable liquids, combustible dust, or ignitible fibers or flyings. 500-2. Location and General Requirements. Locations shall be classified depending on the properties of the flammable vapors, liquids or gases, or combustible dusts or fibers that may be present and the likelihood that a flammable or combustible concentration or quantity is present. Where pyrophoric materials are the only materials used or handled, these locations shall not be classified. Each room, section, or area shall be considered individually in determining its classification. Exception: Except as modified in Articles 500 through 504, all other applicable rules contained in this Code shall apply to electric equipment and wiring installed in hazardous (classified) locations. (FPN No. 1): For definitions of "approved" and "explosionproof" as used in these articles, see Article 100; "dust-ignition-proof" is defined in Section 502-1. Intrinsically safe apparatus and wiring shall be permitted in any hazardous (classified) location for which it is approved, and the provisions of Articles 501 through 503 and 510 through 516 shall not be considered applicable to such installations except as required by Article 504. Wiring of intrinsically safe circuits shall be physically separated from wiring of all other circuits that are not intrinsically safe. Means shall be provided to minimize the passage of gases and vapors. Installation of intrinsically safe apparatus and wiring shall be in accordance with the requirements of Article 504. (FPN No. 2): Through the exercise of ingenuity in the layout of electrical installations for hazardous (classified) locations, it is frequently possible to locate much of the equipment in less hazardous or in nonhazardous locations and, thus, to reduce the amount of special equipment required. In some cases, hazards may be reduced or hazardous (classified) locations limited or eliminated by adequate positivepressure ventilation from a source of clean air in conjunction with effective safeguards against ventilation failure. For further information, see Purged and Pressurized Enclosures for Electrical Equipment, NFPA 496-1989 (ANSI), Flammable and Combustible Liquids Code, NFPA 30-1990 (ANSI), Section 5-3.3, and American Petroleum Institute, API RP 500-1991, Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities, Section 4.6. (FPN No. 3): It is important that the authority having jurisdiction be familiar with such recorded industrial experience as well as with such standards of the National Fire Protection Association as may be of use in the classification of various areas with respect to hazard. 70-466 NATIONAL ELECTRICAL CODE (FPN No. 4): For further information, see Flammable and Combustible Liquids Code, NFPA 30-1990; Drycleaning Plants, NFPA 32-1990; Manufacture of Organic Coatings, NFPA 35-1987 (ANSI); Solvent Extraction Plants, NFPA 36-1988 (ANSI); Storage and Handling of Liquefied Petroleum Gases, NFPA 58-1992; Storage and Handling of Liquefied Petroleum Gases at Utility Gas Plants, NFPA 59-1992; Classification of Class I Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas, NFPA 497A-1992 (ANSI); and Fire Protection in Wastewater Treatment and Collection Facilities, NFPA 820-1992 (ANSI). (FPN No. 5): For protection against static electricity hazards, see Recommended Practice on Static Electricity, NFPA 77-1988 (ANSI). (FPN No. 6): For electrical classification of laboratory areas, see Fire Protection for Laboratories Using Chemicals, NFPA 45-1991. All conduit referred to herein shall be threaded with an NPT standard conduit cutting die that provides ³/₄-inch taper per foot. Such conduit shall be made up wrenchtight to minimize sparking when fault current flows through the conduit system. Where it is impractical to make a threaded joint tight, a bonding jumper shall be utilized. 500-3. Special Precaution. Articles 500 through 504 require equipment construction and installation that will ensure safe performance under conditions of proper use and maintenance. (FPN No. 1): It is important that inspection authorities and users exercise more than ordinary care with regard to installation and maintenance. (FPN No. 2): Low ambient conditions require special consideration. Explosionproof or dust-ignitionproof equipment may not be suitable for use at temperatures lower than -25°C (-13°F) unless they are approved for low-temperature service. However, at low ambient temperatures, flammable concentrations of vapors may not exist in a location classified Class I, Division 1 at normal ambient temperature. For purposes of testing, approval, and area classification, various air mixtures (not oxygen-enriched) shall be grouped in accordance with Sections 500-3(a) and 500-3(b). Exception: Equipment approved for a specific gas, vapor, or dust. (FPN): This grouping is based on the characteristics of the materials. Facilities have been made available for testing and approving equipment for use in the various atmospheric groups. (a) Class I Group Classifications. Class I groups shall be as follows: (1) Group A. Atmospheres containing acetylene. (2) Group B: Atmospheres containing hydrogen, fuel and combustible process gases containing more than 30 percent hydrogen by volume, or gases or vapors of equivalent hazard such as butadiene, ethylene oxide, propylene oxide, and acrolein. Exception No. 1: Group D equipment shall be permitted to be used for atmospheres containing butadiene if such equipment is isolated in accordance with Section 501-5(a) by sealing all conduit ¹/₂-inch size or larger. Exception No. 2: Group C equipment shall be permitted to be used for atmospheres containing ethylene oxide, propylene oxide, and acrolein if such equipment is isolated in accordance with Section 501-5(a) by sealing all conduit ¹/₂-inch size or larger. (3) Group C: Atmospheres such as ethyl ether, ethylene, or gases or vapors of equivalent hazard. ARTICLE 500 - HAZARDOUS (CLASSIFIED) LOCATIONS 70-467 (4) Group D: Atmospheres such as acetone, ammonia, benzene, butane, cyclopropane, ethanol, gasoline, hexane, methanol, methane, natural gas, naphtha, propane, or gases or vapors of equivalent hazard. Exception: For atmospheres containing ammonia, the authority having jurisdiction for enforcement of this Code shall be permitted to reclassify the location to a less hazardous location or a nonhazardous location. (FPN No. 1): For additional information on the properties and group classification of Class I materials, see Classification of Gases, Vapors, and Dusts for Electrical Equipment in Hazardous (Classified) Locations, NFPA 497M-1991, and Fire Hazard Properties of Flammable Liquids, Gases, and Volatile Solids, NFPA 325M-1991. (FPN No. 2): The explosion characteristics of air mixtures of gases or vapors vary with the specific material involved. For Class I locations, Groups A, B, C, and D, the classification involves determinations of maximum explosion pressure and maximum safe clearance between parts of a clamped joint in an enclosure. It is necessary, therefore, that equipment be approved not only for class but also for the specific group of the gas or vapor that will be present. (FPN No. 3): Certain chemical atmospheres may have characteristics that require safeguards beyond those required for any of the above groups. Carbon disulfide is one of these chemicals because of its low ignition temperature [100°C (212°F)] and the small joint clearance permitted to arrest its flame. (FPN No. 4): For classification of areas involving ammonia atmosphere, see Safety Code for Mechanical Refrigeration, ANSI/ASHRAE 15-1978, and Safety Requirements for the Storage and Handling of Anhydrous Ammonia, ANSI/CGA G2.1-1981. (b) Class II Group Classifications. Class II groups shall be as follows: (1)x Group E: Atmospheres containing combustible metal dusts, including aluminum, magnesium, and their commercial alloys, or other combustible dusts whose particle size, abrasiveness, and conductivity present similar hazards in the use of electrical equipment. (FPN): Certain metal dusts may have characteristics that require safeguards beyond those required for atmospheres containing the dusts of aluminum, magnesium, and their commercial alloys. For example, zirconium, thorium, and uranium dusts have extremely low ignition temperatures [as low as 20°C (68°F)] and minimum ignition energies lower than any material classified in any of the Class I or Class II Groups. (2)* Group F: Atmospheres containing combustible carbonaceous dusts, including carbon black, charcoal, coal, or coke dusts that have more than 8 percent total entrapped voltatiles, or dusts that have been sensitized by other materials so that they present an explosion hazard. (FPN): See ASTM D 3175-89, Standard Test Method for Volatile Material in the Analysis Sample for Coal and Coke. (3) Group G: Atmospheres containing combustible dusts not included in Group E or F, including flour, grain, wood, plastic, and chemicals. (FPN No. 1): For additional information on group classification of Class II materials, see Classification of Gases, Vapors, and Dusts for Electrical Equipment in Hazardous (Classified) Locations, NFPA 497M-1991. (FPN No. 2): The explosion characteristics of air mixtures of dust vary with the materials involved. For Class II locations, Groups E, F, and G, the classification involves the tightness of the joints of assembly and shaft openings to prevent the entrance of dust in the dust-ignitionproof enclosure, the blanketing effect of layers of dust on the equipment that may cause overheating, and the ignition temperature 70-468 NATIONAL ELECTRICAL CODE of the dust. It is necessary, therefore, that equipment be approved not only for the class, but also for the specific group of dust that will be present. (FPN No. 3): Certain dusts may require additional precautions due to chemical phenomena that can result in the generation of ignitible gases. See National Electrical Safety Code, ANSI C2-1990, Section 127A-Coal Handling Areas. (c) Approval for Class and Properties. Equipment shall be approved not only for the class of location but also for the explosive, combustible, or ignitible properties of the specific gas, vapor, dust, fiber, or flyings that will be present. In addition, Class I equipment shall not have any exposed surface that operates at a temperature in excess of the ignition temperature of the specific gas or vapor. Class II equipment shall not have an external temperature higher than that specified in Section 500-3(f). Class III equipment shall not exceed the maximum surface temperatures specified in Section 503-1. Equipment that has been approved for a Division 1 location shall be permitted in a Division 2 location of the same class and group. Where specifically permitted in Articles 501 through 503, generalpurpose equipment or equipment in general-purpose enclosures shall be permitted to be installed in Division 2 locations if the equipment does not constitute a source of ignition under normal operating conditions. Unless otherwise specified, normal operating conditions for motors shall be assumed to be rated full-load steady conditions. Where flammable gases or combustible dusts are or may be present at the same time, the simultaneous presence of both shall be considered when determining the safe operating temperature of the electrical equipment. (FPN): The characteristics of various atmospheric mixtures of gases, vapors, and dusts depend on the specific material involved. (d) Marking. Approved equipment shall be marked to show the Class, Group, and operating temperature or temperature range referenced to a 40°C ambient. The temperature range, if provided, shall be indicated in identification numbers, as shown in Table 500-3(d). Identification numbers marked on equipment nameplates shall be in accordance with Table 500-3(d). Equipment that is approved for Class I and Class II shall be marked with the maximum safe operating temperature, as determined by simultaneous exposure to the combinations of Class I and Class II conditions. Exception No. 1: Equipment of the nonheat-producing type, such as junction boxes, conduit, and fittings and equipment of the heat-producing type having a maximum temperature not more than 100°C (212°F) shall not be required to have a marked operating temperature or temperature range. Exception No. 2: Fixed lighting fixtures marked for use in Class I, Division 2 or Class II, Division 2 locations only shall not be required to be marked to indicate the group. Exception No. 3: Fixed general-purpose equipment in Class I locations, other than fixed lighting fixtures, that is acceptable for use in Class I, Division 2 locations shall not be required to be marked with the Class, Group, Division, or operating temperature. ARTICLE 500 - HAZARDOUS (CLASSIFIED) LOCATIONS 70-469 Exception No. 4: Fixed dusttight equipment other than fixed lighting fixtures that are acceptable for use in Class II, Division 2 and Class III locations shall not be required to be marked with the Class, Group, Division, or operating temperature. Table 500-3(d). Identification Numbers I Maximum Temperature Identification Degrees C Degrees F Number 450 842 T1 300 572 T2 280 536 T2A 260 500 T2B 230 446 T2C 215 419 T2D 200 392 T3 180 356 T3A 165 329 T3B 160 320 T3C 135 275 T4 120 248 T4A 100 212 T5 85 185 T6 (FPN): Since there is no consistent relationship between explosion properties and ignition temperature, the two are independent requirements. (e) Class I Temperature. The temperature marking specified in (d) above shall not exceed the ignition temperature of the specific gas or vapor to be encountered. (FPN): For information regarding ignition temperatures of gases and vapors, see Classification of Gases, Vapors, and Dusts for Electrical Equipment in Hazardous (Classified) Locations, NFPA 497M-1991, and Fire Hazard Properties of Flammable Liquids, Gases, and Volatile Solids, NFPA 325M-1991. Formerly, the temperature limit of each group was assumed to be the lowest ignition temperature of any material in the group, i.e., 280°C (536°F) for Group D and 180°C (356°F) for Group C. (FPN): To avoid revising this limit as new gases are added (see hexane in Group D and acetaldehyde in Group C), temperature will be specified in future markings. The ignition temperature for which equipment was approved prior to this requirement shall be assumed to be as follows: Group A - 280°C (536°F) Group C - 180°C (356°F) Group B - 280°C (536°F) Group D - 280°C (536°F) 70-470 NATIONAL ELECTRICAL CODE (f) Class II Temperature. The temperature marking specified in (d) above shall be less than the ignition temperature of the specific dust to be encountered. For organic dusts that may dehydrate or carbonize, the temperature marking shall not exceed the lower of either the ignition temperature or 165°C (329°F). See Classification of Gases, Vapors, and Dusts for Electrical Equipment in Hazardous (Classified) Locations, NFPA 497M- 1991, for minimum ignition temperatures of specific dusts. The ignition temperature for which equipment was approved prior to this requirement shall be assumed to be as shown in Table 500-3(f). Table 500-3(f) Equipment (such as Motors or Power Transformers) that May Be Overloaded Equipment that is Not Subject to Overloading Normal Operation Abnormal Operation Class II Degrees Degrees Degrees Degrees Degrees Degrees Group C F C F C F E 200 392 200 392 200 392 F 200 392 150 302 200 392 G 165 329 120 248 165 329 500-4. Specific Occupancies. Articles 510 through 517 cover garages, aircraft hangars, gasoline dispensing and service stations, bulk storage plants, spray application, dipping and coating processes, and health care facilities. 500-5. Class I Locations. Class I locations are those in which flammable gases or vapors are or may be present in the air in quantities sufficient to produce explosive or ignitible mixtures. Class I locations shall include those specified in (a) and (b) below. (a) Class I, Division 1. A Class I, Division 1 location is a location (1) in which ignitible concentrations of flammable gases or vapors can exist under normal operating conditions; or (2) in which ignitible concentrations of such gases or vapors may exist frequently because of repair or maintenance operations or because of leakage; or (3) in which breakdown or faulty operation of equipment or processes might release ignitible concentrations of flammable gases or vapors, and might also cause simultaneous failure of electric equipment. (FPN): This classification usually includes locations where volatile flammable liquids or liquefied flammable gases are transferred from one container to another; interiors of spray booths and areas in the vicinity of spraying and painting operations where volatile flammable solvents are used; locations containing open tanks or vats of volatile flammable liquids; drying rooms or compartments for the evaporation of flammable solvents; locations containing fat and oil extraction equipment using volatile flammable solvents; portions of cleaning and dyeing plants where flammable liquids are used; gas generator rooms and other portions of gas manufacturing ARTICLE 500 - HAZARDOUS (CLASSIFIED) LOCATIONS 70-471 plants where flammable gas may escape; inadequately ventilated pump rooms for flammable gas or for volatile flammable liquids; the interiors of refrigerators and freezers in which volatile flammmable materials are stored in open, lightly stoppered, or easily ruptured containers; and all other locations where ignitible concentrations of flammable vapors or gases are likely to occur in the course of normal operations. (b) Class I, Division 2. A Class I, Division 2 location is a location (1) in which volatile flammable liquids or flammable gases are handled, processed, or used, but in which the liquids, vapors, or gases will normally be confined within closed containers or closed systems from which they can escape only in case of accidental rupture or breakdown of such containers or systems, or in case of abnormal operation of equipment; or (2) in which ignitible concentrations of gases or vapors are normally prevented by positive mechanical ventilation, and which might become hazardous through failure or abnormal operation of the ventilating equipment; or (3) that is adjacent to a Class I, Division 1 location, and to which ignitible concentrations of gases or vapors might occasionally be communicated unless such communication is prevented by adequate positive-pressure ventilation from a source of clean air, and effective safeguards against ventilation failure are provided. (FPN No. 1): This classification usually includes locations where volatile flammable liquids or flammable gases or vapors are used but that, in the judgment of the authority having jurisdiction, would become hazardous only in case of an accident or of some unusual operating condition. The quantity of flammable material that might escape in case of accident, the adequacy of ventilating equipment, the total area involved, and the record of the industry or business with respect to explosions or fires are all factors that merit consideration in determining the classification and extent of each location. (FPN No. 2): Piping without valves, checks, meters, and similar devices would not ordinarily introduce a hazardous condition even though used for flammable liquids or gases. Locations used for the storage of flammable liquids or of liquefied or compressed gases in sealed containers would not normally be considered hazardous unless subject to other hazardous conditions also. Electrical conduits and their associated enclosures separated from process fluids by a single seal or barrier shall be classed as a Division 2 location if the outside of the conduit and enclosures is an unclassified location. 500-6. Class II Locations. Class II locations are those that are hazardous because of the presence of combustible dust. Class II locations shall include those specified in (a) and (b) below. (a) Class II, Division 1. A Class II, Division 1 location is a location (1) in which combustible dust is in the air under normal operating conditions in quantities sufficient to produce explosive or ignitible mixtures; or (2) where mechanical failure or abnormal operation of machinery or equipment might cause such explosive or ignitible mixtures to be produced, and might also provide a source of ignition through simultaneous failure of electric equipment, operation of protection devices, or from other causes; or (3) in which combustible dusts of an electrically conductive nature may be present in hazardous quantities. (FPN): Combustible dusts that are electrically nonconductive include dusts produced in the handling and processing of grain and grain products, pulverized sugar and cocoa, dried egg and milk powders, pulverized spices, starch and pastes, potato 70-472 NATIONAL ELECTRICAL CODE and woodflour, oil meal from beans and seed, dried hay, and other organic materials that may produce combustible dusts when processed or handled. Only Group E dusts are considered to be electrically conductive for classification purposes. Dusts containing magnesium or aluminum are particularly hazardous, and the use of extreme precaution will be necessary to avoid ignition and explosion. (b) Class II, Division 2. A Class II, Division 2 location is a location where combustible dust is not normally in the air in quantities sufficient to produce explosive or ignitible mixtures, and dust accumulations are normally insufficient to interfere with the normal operation of electrical equipment or other apparatus, but combustible dust may be in suspension in the air as a result of infrequent malfunctioning of handling or processing equipment and where combustible dust accumulations on, in, or in the vicinity of the electrical equipment may be sufficient to interfere with the safe dissipation of heat from electrical equipment or may be ignitible by abnormal operation or failure of electrical equipment. (FPN No. 1): The quantity of combustible dust that may be present and the adequacy of dust removal systems are factors that merit consideration in determining the classification and may result in an unclassified area. (FPN No. 2): Where products such as seed are handled in a manner that produces low quantities of dust, the amount of dust deposited may not warrant classification. 500-7. Class III Locations. Class III locations are those that are hazardous because of the presence of easily ignitible fibers or flyings, but in which such fibers or flyings are not likely to be in suspension in the air in quantities sufficient to produce ignitible mixtures. Class III locations shall include those specified in (a) and (b) below. (a) Class III, Division 1. A Class III, Division 1 location is a location in which easily ignitible fibers or materials producing combustible flyings are handled, manufactured, or used. (FPN No. 1): Such locations usually include some parts of rayon, cotton, and other textile mills; combustible fiber manufacturing and processing plants; cotton gins and cotton-seed mills; flax-processing plants; clothing manufacturing plants; woodworking plants; and establishments and industries involving similar hazardous processes or conditions. (FPN No. 2): Easily ignitible fibers and flyings include rayon, cotton (including cotton linters and cotton waste), sisal or henequen, istle, jute, hemp, tow, cocoa fiber, oakum, baled waste kapok, Spanish moss, excelsior, and other materials of similar nature. (b) Class III, Division 2. A Class III, Division 2 location is a location in which easily ignitible fibers are stored or handled. Exception: In process of manufacture.