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Article 500

Hazardous (Classified) Locations, Classes I, II, and III, Divisions 1 and 2

Informational Note: Text that is followed by a reference in brackets has been extracted from NFPA 497-2012, Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas, and NFPA 499-2013, Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classi‐ fied) Locations for Electrical Installation in Chemical Process Areas. Only editorial changes were made to the extracted text to make it consistent with this Code.

500.1 Scope — Articles 500 Through 504. Articles 500 through 504 cover the requirements for electrical and elec‐ tronic equipment and wiring for all voltages in Class I, Divi‐ sions 1 and 2; Class II, Divisions 1 and 2; and Class III, Divisions 1 and 2 locations where fire or explosion hazards may exist due to flammable gases, flammable liquid–produced vapors, combustible liquid–produced vapors, combustible dusts, or ignitible fibers/flyings.

Informational Note No. 1: The unique hazards associated with explosives, pyrotechnics, and blasting agents are not addressed in this article.

Informational Note No. 2: For the requirements for electrical and electronic equipment and wiring for all voltages in Zone 0, Zone 1, and Zone 2 hazardous (classified) locations where fire or explosion hazards may exist due to flammable gases or vapors or flammable liquids, refer to Article 505.

Informational Note No. 3: For the requirements for electrical and electronic equipment and wiring for all voltages in Zone 20, Zone 21, and Zone 22 hazardous (classified) locations where fire or explosion hazards may exist due to combustible dusts or ignitible fibers/flyings, refer to Article 506.

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  1. Other Articles. Except as modified in Articles 500through 504, all other applicable rules contained in this Code shall apply to electrical equipment and wiring installed in hazardous (classified) locations.
  2. General.
    1. Documentation. All areas designated as hazardous (classi‐ fied) locations shall be properly documented. This documenta‐ tion shall be available to those authorized to design, install, inspect, maintain, or operate electrical equipment at the loca‐ tion.
    2. Reference Standards. Important information relating to topics covered in Chapter 5 may be found in other publica‐ tions.
    Informational Note No. 1: Familiarity with the standards of the National Fire Protection Association (NFPA), the American Petroleum Institute (API), and the International Society of Automation (ISA), as well as relevant industrial experience, may be of use in the classification of various locations, the determina‐ tion of adequate ventilation, and the protection against static electricity and lightning hazards.Informational Note No. 2: For further information on the classi‐ fication of locations, see NFPA 30-2015, Flammable and Combusti‐ ble Liquids Code; NFPA 32-2011, Standard for Drycleaning Plants; NFPA 33 -2015, Standard for Spray Application Using Flammable or Combustible Materials; NFPA 34 -2015, Standard for Dipping and Coating Processes Using Flammable or Combustible Liquids; NFPA 35-2011, Standard for the Manufacture of Organic Coatings; NFPA 36-2013, Standard for Solvent Extraction Plants; NFPA 45-2015, Standard on Fire Protection for Laboratories Using Chemicals; NFPA 55-2013, Compressed Gases and Cryogenic Fluids Code; NFPA 58-2014, Liquefied Petroleum Gas Code; NFPA 59 -2015, Utility LP- Gas Plant Code; NFPA 497-2012, Recommended Practice for the Classi‐ fication of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas; NFPA 499-2013, Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electri‐ cal Installations in Chemical Process Areas; NFPA 820-2012, Standard for Fire Protection in Wastewater Treatment and Collection Facilities; ANSI/API RP 500-2012, Recommended Practice for Classification of Locations of Electrical Installations at Petroleum Facilities Classified as Class I, Division 1 and Division 2; ISA-12.10-1988, Area Classifica‐ tion in Hazardous (Classified) Dust Locations.Informational Note No. 3: For further information on protec‐ tion against static electricity and lightning hazards in hazardous (classified) locations, see NFPA 77-2014, Recommended Practice on Static Electricity; NFPA 780-2014, Standard for the Installation of Lightning Protection Systems; and API RP 2003-2008, Protection Against Ignitions Arising Out of Static Lightning and Stray Currents.Informational Note No. 4: For further information on ventila‐ tion, see NFPA 30 -2015, Flammable and Combustible Liquids Code; and ANSI/API RP 500-2012, Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Division 1 and Division 2.Informational Note No. 5: For further information on electrical systems for hazardous (classified) locations on offshore oil- and gas-producing platforms, see ANSI/API RP 14F-2013, Recommen‐ ded Practice for Design and Installation of Electrical Systems for Fixed and Floating Offshore Petroleum Facilities for Unclassified and Class I, Division 1 and Division 2 Locations.
    Informational Note No. 6: Portable or transportable equipmenthaving self-contained power supplies, such as battery-operated equipment, could potentially become an ignition source in hazardous (classified) locations. See ANSI/ISA-12.12.03-2011, Standard for Portable Electronic Products Suitable for Use in Class I and II, Division 2, Class I Zone 2 and Class III, Division 1 and 2 Hazard‐ ous (Classified) Locations.
  3. Classifications of Locations.
    1. General. Locations shall be classified depending on the properties of the flammable gas, flammable liquid–produced vapor, combustible liquid–produced vapors, combustible dusts, or fibers/flyings that could be present, and the likelihood that a flammable or combustible concentration or quantity is present. Each room, section, or area shall be considered indi‐ vidually in determining its classification. Where pyrophoric materials are the only materials used or handled, these loca‐ tions are outside the scope of this article.Informational Note No. 1: Through the exercise of ingenuity in the layout of electrical installations for hazardous (classified) locations, it is frequently possible to locate much of the equip‐ ment in a reduced level of classification or in an unclassified
      location and, thus, to reduce the amount of special equipment required.Refrigerant machinery rooms that contain ammonia refrig‐ eration systems and are equipped with adequate mechanicalcontinuously or for long periods of time. Examples include the following:
      1. The inside of inadequately vented enclosures containing instruments normally venting flammable gases or vaporsventilation that operates continuously or is initiated by a detec‐ tion system at a concentration not exceeding 150 ppm shall be permitted to be classified as “unclassified” locations.•Informational Note No. 2: For further information regarding


      3. to the interior of the enclosureThe inside of vented tanks containing volatile flammable liquidsThe area between the inner and outer roof sections of a floating roof tank containing volatile flammable fluidsclassification and ventilation of areas involving closed-circuit ammonia refrigeration systems, see ANSI/ASHRAE 15-2013, Safety Standard for Refrigeration Systems, and ANSI/IIAR 2–2014, Standard for Safe Design of Closed-Circuit Ammonia Refrigeration Systems.
    2. Class I Locations. Class I locations are those in which flammable gases, flammable liquid–produced vapors, or combustible liquid–produced 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 500.5(B)(1) and (B)(2).
      1. Class I, Division 1. A Class I, Division 1 location is a loca‐ tion:
        1. In which ignitible concentrations of flammable gases, flammable liquid–produced vapors, or combustible liquid–produced vapors can exist under normal operat‐ ing conditions, or
        2. In which ignitible concentrations of such flammable gases, flammable liquid–produced vapors, or combustible liquids above their flash points 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 flam‐ mable gases, flammable liquid–produced vapors, or combustible liquid–produced vapors and might also cause simultaneous failure of electrical equipment in such a way as to directly cause the electrical equipment to become a source of ignitionInformational Note No. 1: This classification usually includes the following locations:
          1. Where volatile flammable liquids or liquefied flammable gases are transferred from one container to another
          2. Interiors of spray booths and areas in the vicinity of spraying and painting operations where volatile flamma‐ ble solvents are used
          3. Locations containing open tanks or vats of volatile flam‐ mable liquids
          4. Drying rooms or compartments for the evaporation of flammable solvents
          5. Locations containing fat- and oil-extraction equipment using volatile flammable solvents
          6. Portions of cleaning and dyeing plants where flammable liquids are used
          7. Gas generator rooms and other portions of gas manufac‐ turing plants where flammable gas may escape
          8. Inadequately ventilated pump rooms for flammable gas or for volatile flammable liquids
          9. The interiors of refrigerators and freezers in which vola‐ tile flammable materials are stored in open, lightly stop‐ pered, or easily ruptured containers
          10. All other locations where ignitible concentrations of flammable vapors or gases are likely to occur in the course of normal operationsInformational Note No. 2: In some Division 1 locations, ignitible concentrations of flammable gases or vapors may be present
        4. Inadequately ventilated areas within spraying or coating operations using volatile flammable fluids
        5. The interior of an exhaust duct that is used to vent igniti‐ ble concentrations of gases or vapors
        Experience has demonstrated the prudence of avoiding the installation of instrumentation or other electrical equipment in these particular areas altogether or where it cannot be avoided because it is essential to the process and other locations are not feasible [see 500.5(A), Informational Note] using electrical equipment or instrumentation approved for the specific applica‐ tion or consisting of intrinsically safe systems as described in Article 504.(2) Class I, Division 2. A Class I, Division 2 location is a loca‐ tion:
        1. In which volatile flammable gases, flammable liquid– produced vapors, or combustible liquid–produced vapors 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 flammable gases, flammable liquid–produced vapors, or combustible liquid–produced vapors are normally prevented by posi‐ tive 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 flammable gases, flam‐ mable liquid–produced vapors, or combustible liquid– produced vapors above their flash points might occasion‐ ally 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.
        Informational Note 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.Informational Note 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. Depending on factors such as the quantity and size of the containers and ventilation, locations used for the storage of flammable liquids or liquefied or compressed gases in sealed containers may be considered either hazardous (classified) or unclassified locations. See NFPA 30-2015, Flammable and Combus‐ tible Liquids Code, and NFPA 58-2014, Liquefied Petroleum Gas Code.
    3. 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 500.5(C)(1) and (C)(2).
      1. Class II, Division 1. A Class II, Division 1 location is a loca‐ tion:
        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 elec‐ trical equipment, through operation of protection devi‐ ces, or from other causes, or
        3. In which Group E combustible dusts may be present in quantities sufficient to be hazardous.
        Informational Note: Dusts containing magnesium or aluminum are particularly hazardous, and the use of extreme precaution is necessary to avoid ignition and explosion.(2) Class II, Division 2. A Class II, Division 2 location is a loca‐ tion:
        1. In which combustible dust due to abnormal operations may be present in the air in quantities sufficient to produce explosive or ignitible mixtures; or
        2. Where combustible dust accumulations are present but are normally insufficient to interfere with the normal operation of electrical equipment or other apparatus, but could as a result of infrequent malfunctioning of handling or processing equipment become suspended in the air; or
        3. In which combustible dust accumulations on, in, or in the vicinity of the electrical equipment could be sufficient to interfere with the safe dissipation of heat from electrical equipment, or could be ignitible by abnormal operation or failure of electrical equipment.
        Informational Note 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.Informational Note 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.
    4. Class III Locations. Class III locations are those that are hazardous because of the presence of easily ignitible fibers or where materials producing combustible flyings are handled, manufactured, or used, but in which such fibers/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 500.5(D)(1) and (D)(2).
      1. Class III, Division 1. A Class III, Division 1 location is a location in which easily ignitible fibers/flyings are handled, manufactured, or used.Informational Note No. 1: Such locations usually include some parts of rayon, cotton, and other textile mills; combustible fibers/flyings manufacturing and processing plants; cotton gins and cotton-seed mills; flax-processing plants; clothing manufac‐ turing plants; woodworking plants; and establishments and industries involving similar hazardous processes or conditions.Informational Note No. 2: Easily ignitible fibers/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.
      2. Class III, Division 2. A Class III, Division 2 location is a location in which easily ignitible fibers/flyings are stored or handled other than in the process of manufacture.
  4. Material Groups. For purposes of testing, approval, and area classification, various air mixtures (not oxygen-enriched) shall be grouped in accordance with 500.6(A) and (B).Exception: Equipment identified for a specific gas, vapor, dust, or fiber/ flying.Informational Note: This grouping is based on the characteris‐ tics of the materials. Facilities are available for testing and identi‐ fying equipment for use in the various atmospheric groups.
    1. Class I Group Classifications. Class I groups shall be according to 500.6(A)(1) through (A)(4).Informational Note No. 1: Informational Note Nos. 2 and 3 apply to 500.6(A).Informational Note 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 classi‐ fication involves determinations of maximum explosion pres‐ sure and maximum safe clearance between parts of a clamped joint in an enclosure. It is necessary, therefore, that equipment be identified not only for class but also for the specific group of the gas or vapor that will be present.Informational Note No. 3: Certain chemical atmospheres may have characteristics that require safeguards beyond those required for any of the Class I groups. Carbon disulfide is one of these chemicals because of its low autoignition temperature (90°C) and the small joint clearance permitted to arrest its flame.(1) Group A. Acetylene. [497:3.3.5.1.1]
      1. Group B. Flammable gas, flammable liquid–produced vapor, or combustible liquid–produced vapor mixed with air that may burn or explode, having either a maximum experi‐ mental safe gap (MESG) value less than or equal to 0.45 mm or a minimum igniting current ratio (MIC ratio) less than or equal to 0.40. [497:3.3.5.1.2]Informational Note: A typical Class I, Group B material is hydro‐ gen.Exception No. 1: Group D equipment shall be permitted to be used for atmospheres containing butadiene, provided all conduit runs into explosionproof equipment are provided with explosionproof seals instal‐ led within 450 mm (18 in.) of the enclosure.Exception No. 2: Group C equipment shall be permitted to be used for atmospheres containing allyl glycidyl ether, n-butyl glycidyl ether, ethyl‐ ene oxide, propylene oxide, and acrolein, provided all conduit runs into explosionproof equipment are provided with explosionproof seals instal‐ led within 450 mm (18 in.) of the enclosure.
      2. Group C. Flammable gas, flammable liquid–produced vapor, or combustible liquid–produced vapor mixed with air that may burn or explode, having either a maximum experi‐ mental safe gap (MESG) value greater than 0.45 mm and less than or equal to 0.75 mm, or a minimum igniting current ratio (MIC ratio) greater than 0.40 and less than or equal to 0.80. [497:3.3.5.1.3]Informational Note: A typical Class I, Group C material is ethyl‐ ene.
      3. Group D. Flammable gas, flammable liquid–produced vapor, or combustible liquid–produced vapor mixed with air that may burn or explode, having either a maximum experi‐ mental safe gap (MESG) value greater than 0.75 mm or a mini‐ mum igniting current (MIC) ratio greater than 0.80. [497:3.3.5.1.4]
      Informational Note No. 1: A typical Class I, Group D material is propane. [497:3.3.5.1.4]Informational Note No. 2: For classification of areas involving ammonia atmospheres, see ANSI/ASHRAE 15-2013, Safety Stand‐ ard for Refrigeration Systems.
    2. Class II Group Classifications. Class II groups shall be in accordance with 500.6(B)(1) through (B)(3).
      1. Group E. Atmospheres containing combustible metal dusts, including aluminum, magnesium, and their commercial alloys, or other combustible dusts whose particle size, abrasive‐ ness, and conductivity present similar hazards in the use of electrical equipment. [499:3.3.4.1]Informational Note: Certain metal dusts may have characteris‐ tics that require safeguards beyond those required for atmos‐ pheres 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 carbona‐ ceous dusts that have more than 8 percent total entrapped vola‐ tiles (see ASTM D3175-11, Standard Test Method for Volatile Matter in the Analysis Sample for Coal and Coke, for coal and coke dusts) or that have been sensitized by other materials so that they present an explosion hazard. [499:3.3.4.2] Coal, carbon black, charcoal, and coke dusts are examples of carbonaceous dusts. [499:A.3.3.4.2]Informational Note: Testing of specific dust samples, following established ASTM testing procedures, is a method used to iden‐ tify the combustibility of a specific dust and the need to classify those locations containing that material as Group F.
      3. Group G. Atmospheres containing combustible dusts not included in Group E or Group F, including flour, grain, wood, plastic, and chemicals. [499:3.3.4.3]
    Informational Note No. 1: For additional information on group classification of Class II materials, see NFPA 499-2013, Recommen‐ ded Practice for the Classification of Combustible Dusts and of Hazard‐ ous (Classified) Locations for Electrical Installations in Chemical Process Areas.Informational Note 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 maycause overheating, and the ignition temperature of the dust. It is necessary, therefore, that equipment be identified not only for the class but also for the specific group of dust that will be present.Informational Note No. 3: Certain dusts may require additional precautions due to chemical phenomena that can result in the generation of ignitible gases. See ANSI/IEEE C2-2012, National Electrical Safety Code, Section 127A, Coal Handling Areas.
  5. Protection Techniques. Section 500.7(A) through (L) shall be acceptable protection techniques for electrical and electronic equipment in hazardous (classified) locations.
    1. Explosionproof Equipment. This protection technique shall be permitted for equipment in Class I, Division 1 or 2 locations.
    2. Dust Ignitionproof. This protection technique shall be permitted for equipment in Class II, Division 1 or 2 locations.
    3. Dusttight. This protection technique shall be permitted for equipment in Class II, Division 2 or Class III, Division 1 or 2 locations.
    4. Purged and Pressurized. This protection technique shall be permitted for equipment in any hazardous (classified) loca‐ tion for which it is identified.
    5. Intrinsic Safety. This protection technique shall be permitted for equipment in Class I, Division 1 or 2; or Class II, Division 1 or 2; or Class III, Division 1 or 2 locations. The provi‐ sions of Articles 501 through 503 and Articles 510 through 516 shall not be considered applicable to such installations, except as required by Article 504, and installation of intrinsically safe apparatus and wiring shall be in accordance with the require‐ ments of Article 504.
    6. Nonincendive Circuit. This protection technique shall be permitted for equipment in Class I, Division 2; Class II, Division 2; or Class III, Division 1 or 2 locations.
    7. Nonincendive Equipment. This protection technique shall be permitted for equipment in Class I, Division 2; Class II, Division 2; or Class III, Division 1 or 2 locations.
    8. Nonincendive Component. This protection technique shall be permitted for equipment in Class I, Division 2; Class II, Division 2; or Class III, Division 1 or 2 locations.
    9. Oil Immersion. This protection technique shall be permit‐ ted for current-interrupting contacts in Class I, Division 2 loca‐ tions as described in 501.115(B)(1)(2).
    10. Hermetically Sealed. This protection technique shall be permitted for equipment in Class I, Division 2; Class II, Division 2; or Class III, Division 1 or 2 locations.
    11. Combustible Gas Detection System. A combustible gas detection system shall be permitted as a means of protection in industrial establishments with restricted public access and where the conditions of maintenance and supervision ensure that only qualified persons service the installation. Where such a system is installed, equipment specified in 500.7(K)(1), (K)(2), or (K)(3) shall be permitted.The type of detection equipment, its listing, installation loca‐ tion(s), alarm and shutdown criteria, and calibration frequency shall be documented where combustible gas detectors are used as a protection technique.
      Informational Note No. 1: For further information, see ANSI/ ISA-60079-29-1 (12.13.01)-2013, Explosive Atmospheres — Part 29-1: Gas detectors — Performance requirements of detectors for flammable gases.Informational Note No. 2: For further information, see ANSI/API RP 500–2012, Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Division I or Division 2.Informational Note No. 3: For further information, see ANSI/ ISA-60079-29-2 (12.13.02)-2012, Explosive Atmospheres — Part 29-2: Gas detectors — Selection, installation, use and maintenance of detectors for flammable gases and oxygen.Informational Note No. 4: For further information, see ISA- TR12.13.03-2009, Guide for Combustible Gas Detection as a Method of Protection.
      1. Inadequate Ventilation. In a Class I, Division 1 location that is so classified due to inadequate ventilation, electrical equipment suitable for Class I, Division 2 locations shall be permitted. Combustible gas detection equipment shall be listed for Class I, Division 1, for the appropriate material group, and for the detection of the specific gas or vapor to be encoun‐ tered.
      2. Interior of a Building. In a building located in, or with an opening into, a Class I, Division 2 location where the interior does not contain a source of flammable gas or vapor, electrical equipment for unclassified locations shall be permitted. Combustible gas detection equipment shall be listed for Class I, Division 1 or Class I, Division 2, for the appropriate material group, and for the detection of the specific gas or vapor to be encountered.
      3. Interior of a Control Panel. In the interior of a control panel containing instrumentation utilizing or measuring flam‐ mable liquids, gases, or vapors, electrical equipment suitable for Class I, Division 2 locations shall be permitted. Combustible gas detection equipment shall be listed for Class I, Division 1, for the appropriate material group, and for the detection of the specific gas or vapor to be encountered.
    12. Other Protection Techniques. Other protection techni‐ ques used in equipment identified for use in hazardous (classi‐ fied) locations.
  6. Equipment. Articles 500 through 504 require equip‐ ment construction and installation that ensure safe perform‐ ance under conditions of proper use and maintenance.Informational Note No. 1: It is important that inspection author‐ ities and users exercise more than ordinary care with regard to installation and maintenance.Informational Note No. 2: Since there is no consistent relation‐ ship between explosion properties and ignition temperature, the two are independent requirements.Informational Note No. 3: Low ambient conditions require special consideration. Explosionproof or dust-ignitionproof equipment may not be suitable for use at temperatures lower than -25ºC (-13ºF) unless they are identified for low-temperature service. However, at low ambient temperatures, flammable concentrations of vapors may not exist in a location classified as Class I, Division 1 at normal ambient temperature.
    1. Suitability. Suitability of identified equipment shall be determined by one of the following:
      1. Equipment listing or labeling
      2. Evidence of equipment evaluation from a qualified test‐ ing laboratory or inspection agency concerned with prod‐ uct evaluation
      3. Evidence acceptable to the authority having jurisdiction such as a manufacturer’s self-evaluation or an owner’s engineering judgment
      Informational Note: Additional documentation for equipment may include certificates demonstrating compliance with applica‐ ble equipment standards, indicating special conditions of use, and other pertinent information. Guidelines for certificates may be found in ANSI/UL 120002:2014, Certificate Standard for AEx Equipment for Hazardous (Classified) Locations.
    2. Approval for Class and Properties.
      1. Equipment shall be identified not only for the class of location but also for the explosive, combustible, or ignitible properties of the specific gas, vapor, dust, or fibers/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 autoignition temperature of the specific gas or vapor. Class II equipment shall not have an external temperature higher than that specified in 500.8(D)(2). Class III equipment shall not exceed the maximum surface temperatures specified in 503.5.Informational Note: Luminaires and other heat-producing appa‐ ratus, switches, circuit breakers, and plugs and receptacles are potential sources of ignition and are investigated for suitability in classified locations. Such types of equipment, as well as cable terminations for entry into explosionproof enclosures, are avail‐ able as listed for Class I, Division 2 locations. Fixed wiring, however, may utilize wiring methods that are not evaluated with respect to classified locations. Wiring products such as cable, raceways, boxes, and fittings, therefore, are not marked as being suitable for Class I, Division 2 locations. Also see 500.8(C)(6)(a).
      2. Equipment that has been identified for a Division 1 loca‐ tion shall be permitted in a Division 2 location of the same class, group, and temperature class and shall comply with (a) or (b) as applicable.
        1. Intrinsically safe apparatus having a control drawing requiring the installation of associated apparatus for a Division 1 installation shall be permitted to be installed in a Division 2 location if the same associated apparatus is used for the Divi‐ sion 2 installation.
        2. Equipment that is required to be explosionproof shall incorporate seals in accordance with 501.15(A) or (D) when the wiring methods of 501.10(B) are employed.
      3. Where specifically permitted in Articles 501 through 503, general-purpose equipment or equipment in general-purpose enclosures shall be permitted to be installed in Division 2 loca‐ tions if the equipment does not constitute a source of ignition under normal operating conditions.
      4. Equipment that depends on a single compression seal, diaphragm, or tube to prevent flammable or combustible fluids from entering the equipment shall be identified for a Class I, Division 2 location even if installed in an unclassified location. Equipment installed in a Class I, Division 1 location shall be identified for the Class I, Division 1 location.Informational Note: Equipment used for flow measurement is an example of equipment having a single compression seal, diaphragm, or tube.
      5. Unless otherwise specified, normal operating conditions for motors shall be assumed to be rated full-load steady condi‐ tions.
      6. Where flammable gases, flammable liquid–produced vapors, combustible liquid–produced vapors, 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.Informational Note: The characteristics of various atmospheric mixtures of gases, vapors, and dusts depend on the specific material involved.
    3. Marking. Equipment shall be marked to show the envi‐ ronment for which it has been evaluated. Unless otherwise specified or allowed in (C)(6), the marking shall include the information specified in (C)(1) through (C)(5).
      1. Class. The marking shall specify the class(es) for which the equipment is suitable.
      2. Division. The marking shall specify the division if the equipment is suitable for Division 2 only. Equipment suitable for Division 1 shall be permitted to omit the division marking.Informational Note: Equipment not marked to indicate a divi‐ sion, or marked “Division 1” or “Div. 1,” is suitable for both Divi‐ sion 1 and 2 locations; see 500.8(B)(2). Equipment marked “Division 2” or “Div. 2” is suitable for Division 2 locations only.
      3. Material Classification Group. The marking shall specify the applicable material classification group(s) or specific gas, vapor, dust, or fiber/flying in accordance with 500.6.Exception: Fixed luminaires marked for use only in Class I, Division 2 or Class II, Division 2 locations shall not be required to indicate the group.
        Informational Note: A specific gas, vapor, dust, or fiber/flying istypically identified by the generic name, chemical formula, CAS number, or combination thereof.
      4. Equipment Temperature. The marking shall specify the temperature class or operating temperature at a 40°C ambient temperature, or at the higher ambient temperature if the equipment is rated and marked for an ambient temperature of greater than 40°C. For equipment installed in a Class II, Divi‐ sion 1 location, the temperature class or operating temperature shall be based on operation of the equipment when blanketed with the maximum amount of dust that can accumulate on the equipment. The temperature class, if provided, shall be indica‐ ted using the temperature class (T codes) shown in Table 500.8(C). Equipment 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: Equipment of the non–heat-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 shall not be required to have a marked operating temperature or temperature class.Informational Note: More than one marked temperature class or operating temperature, for gases and vapors, dusts, and different ambient temperatures, may appear.
      5. Ambient Temperature Range. Electrical equipment designed for use in the ambient temperature range between – 25°C to +40°C shall require no ambient temperature marking. For equipment rated for a temperature range other than –25°CTable 500.8(C) Classification of Maximum Surface Temperature
        image°C°F(T Code)450842T1300572T2280536T2A260500T2B230446T2C215419T2D200392T3180356T3A165329T3B160320T3C135275T4120248T4A100212T585185T6Maximum Temperature Temperature Class
        to +40°C, the marking shall specify the special range of ambi‐ ent temperatures in degrees Celsius. The marking shall include either the symbol “Ta” or “Tamb.”Informational Note: As an example, such a marking might be “–30°C ≤ Ta ≤ +40°C.”
      6. Special Allowances.
        1. General-Purpose Equipment. Fixed general-purpose equipment in Class I locations, other than fixed luminaires, that is acceptable for use in Class I, Division 2 locations shall not be required to be marked with the class, division, group, temperature class, or ambient temperature range.
        2. Dusttight Equipment. Fixed dusttight equipment, other than fixed luminaires, that is acceptable for use in Class II, Division 2 and Class III locations shall not be required to be marked with the class, division, group, temperature class, or ambient temperature range.
        3. Associated Apparatus. Associated intrinsically safe apparatus and associated nonincendive field wiring apparatus that are not protected by an alternative type of protection shall not be marked with the class, division, group, or temperature class. Associated intrinsically safe apparatus and associated nonincendive field wiring apparatus shall be marked with the class, division, and group of the apparatus to which it is to be connected.
        4. Simple Apparatus. “Simple apparatus” as defined in Article 504, shall not be required to be marked with class, divi‐ sion, group, temperature class, or ambient temperature range.
    4. Temperature.
      1. Class I Temperature. The temperature marking specified in 500.8(C) shall not exceed the autoignition temperature of the specific gas or vapor to be encountered.Informational Note: For information regarding autoignition temperatures of gases and vapors, see NFPA 497-2013, Recommen‐ ded Practice for the Classification of Flammable Liquids, Gases, or Vapors, and of Hazardous (Classified) Locations for Electrical Installa‐ tions in Chemical Process Areas.
      2. Class II Temperature. The temperature marking specified in 500.8(C) 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).Informational Note: See NFPA 499-2013, Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas, for minimum ignition temperatures of specific dusts.•
    5. Threading. The supply connection entry thread formshall be NPT or metric. Conduit and fittings shall be made wrenchtight to prevent sparking when fault current flows through the conduit system, and to ensure the explosionproof integrity of the conduit system where applicable. Equipment provided with threaded entries for field wiring connections shall be installed in accordance with 500.8(E)(1) or (E)(2) and with (E)(3).
      1. Equipment Provided with Threaded Entries for NPT- Threaded Conduit or Fittings. For equipment provided with threaded entries for NPT-threaded conduit or fittings, listed conduit, listed conduit fittings, or listed cable fittings shall be used. All NPT-threaded conduit and fittings shall be threaded with a National (American) Standard Pipe Taper (NPT) thread.NPT-threaded entries into explosionproof equipment shall be made up with at least five threads fully engaged.Exception: For listed explosionproof equipment, joints with factory- threaded NPT entries shall be made up with at least four and one-half threads fully engaged.Informational Note No. 1: Thread specifications for male NPT threads are located in ASME B1.20.1-2013, Pipe Threads, General Purpose (Inch).Informational Note No. 2: Female NPT-threaded entries use a modified National Standard Pipe Taper (NPT) thread with thread form per ASME B1.20.1-2013, Pipe Threads, General Purpose (Inch). See ANSI/UL 1203-2009, Explosionproof and Dust- Ignition-Proof Electrical Equipment for Use in Hazardous (Classified) Locations.
      2. Equipment Provided with Threaded Entries for Metric– Threaded Fittings. For equipment with metric-threaded entries, listed conduit fittings or listed cable fittings shall be used. Such entries shall be identified as being metric, or listed adapters to permit connection to conduit or NPT-threaded fittings shall be provided with the equipment and shall be used for connection to conduit or NPT-threaded fittings.Metric-threaded fittings installed into explosionproof equip‐ ment shall have a class of fit of at least 6g/6H and shall be made up with at least five threads fully engaged.Informational Note: Threading specifications for metric- threaded entries are located in ISO 965-1-2013, ISO general purpose metric screw threads — Tolerances — Part 1: Principles and basic data, and ISO 965-3-1998, ISO general purpose metric screw threads — Tolerances — Part 3: Deviations for constructional screw threads.
      3. Unused Openings. All unused openings shall be closed with listed metal close-up plugs. The plug engagement shall comply with 500.8(E)(1) or (E)(2).
    6. Optical Fiber Cables. An optical fiber cable, with or with‐ out current-carrying conductors (composite optical fiber cable), shall be installed to address the associated fire hazard and sealed to address the associated explosion hazard in
    accordance with the requirements of Article 500, 501, 502, or 503, as applicable.
  7. Specific Occupancies. Articles 510 through 517 cover garages, aircraft hangars, motor fuel dispensing facilities, bulk storage plants, spray application, dipping and coating processes, and health care facilities.

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