ARTICLE HAZARDOUS (CLASSIFIED) LOCATIONS
ARTICLE500HAZARDOUS (CLASSIFIED)LOCATIONSIntroduction to Article 500—Hazardous (Classified) LocationsA hazardous (classified) location is an area where the possibility of fire or explosion can be created by the presence of flammable or combustible gases or vapors, combustible dusts, or easily ignitable fibers/flyings. Electric arcs, sparks, and/orheated surfaces can serve as a source of ignition in such environments.Article 500 provides a foundation for applying Article 501 (Class I Locations), Article 502 (Class II Locations), Article 503(Class III Locations), and Article 504 (Intrinsically Safe Systems)—all of which immediately follow Article 500. This articlealso provides a foundation for applying Articles 510 through 516.Before you apply any of the articles just mentioned, you must understand and apply Article 500 which is fairly long anddetailed. You’ll notice when studying this article that there are many Informational Notes that you should review. AlthoughInformational Notes aren’t NEC requirements [90.5(C)], they contain information that help Code users better understand therelated NEC rules.A Fire Triangle (fuel, oxygen, and energy source) helps illustrate the concept of how combustion occurs. Figure 500–1 Fuel. Flammable gases or vapors, combustibledusts, and easily ignitable fibers/flyings. Oxygen. Air and oxidizing atmospheres. Ignition Source. Electric arcs or sparks,heat-producing equipment such as luminairesand motors, failure of transformers, coils, orsolenoids, as well as sparks caused by metaltools dropping on metal surfaces.Many of the graphics contained in Chapter 5 use twoshades of red to identify a Division location (darker redfor Division 1 and lighter red to identify Division 2). Insome cases, these color schemes are used as a background color to help you tell if the graphic applies toDivision 1, Division 2, or both (split color background).Figure 500–1Mike Holt Enterprises, Inc. www.MikeHolt.com 888.NEC.CODE (632.2633)23
500.1 Hazardous (Classified) Locations500.1 Scope—Articles 500 Through 504To prevent injury, death, or extensive damage to structures from firesor explosions, the NEC contains stringent requirements for equipment and its installation in hazardous (classified) locations where fireor explosion hazards may exist due to flammable gases, flammableliquid-produced vapors, combustible liquid-produced vapors, combustible dusts, or fibers/flyings. The specific requirements for electricalinstallations in hazardous (classified) locations are contained in: Article 501. Class I—Flammable or CombustibleLiquid-Produced Vapors or Flammable GasesArticle 502. Class II—Combustible DustArticle 503. Class III—Easily Ignitible Fibers/FlyingsArticle 504. Intrinsically Safe SystemsAuthor’s Comment:nSee the definition of “Structure” in Article 100.nLocating electrical wiring and equipment outside a classifiedlocation provides the safest electrical installation and is oftenmore cost-effective [500.5(A) Note].nnMany of the graphics contained in Chapter 5 use two shadesof red to identify a Division location (darker red for Division 1and lighter red to identify Division 2). In some cases, thesecolor schemes are used as a background color to help youtell if the graphic applies to Division 1, Division 2, or both(split color background). Article 511. Commercial Garages, Repair, and StorageArticle 513. Aircraft HangarsArticle 514. Motor Fuel Dispensing FacilitiesArticle 515. Bulk Storage PlantsArticle 516. Spray Application, Dipping, and CoatingProcessesArticle 517. Health Care Facilities500.2 DefinitionsThe definitions contained in 500.2 apply to Articles 500 through 504and Articles 510 through 516.Combustible Dust. Dust particles that are 500 microns or smaller andpresent a fire or explosion hazard when dispersed and ignited in air.Author’s Comment:nThe size of the material may be a significant factor in distinguishing dust (Class II) from fibers and flyings (Class III).Dust-Ignitionproof. Equipment enclosed in a manner that excludesdust and doesn’t permit arcs, sparks, or heat inside the enclosure toignite accumulations or suspensions of a specified dust on or in thevicinity of the enclosure.Dusttight. Enclosures constructed so that dust won’t enter underspecific test conditions. Examples of dusttight enclosures include FSboxes and bell boxes. Figure 500–2The NEC doesn’t classify specific hazardous (classified)locations, except as identified in Articles 511 through 517.Determining the classification of a specific hazardous area isthe responsibility of those who understand the dangers of theproducts being used, such as the fire marshal, plant facilityengineer, or insurance underwriter. It isn’t the responsibilityof the electrical designer, electrical contractor, or electricalinspector. Prior to performing any wiring in or near a hazardous (classified) location, contact the plant facility and designengineer to ensure that proper installation and materials areused. Be sure to review 500.4(B) for additional standardsthat might need to be consulted.Other articles in Chapter 5 containing specific hazardous installationrequirements include: Article 505. Class I, Zone 0, 1, and 2 Locations24Figure 500–2Mike Holt’s Illustrated Guide to Hazardous Locations based on the 2014 National Electrical Code
Hazardous (Classified) Locations 500.4Hermetically Sealed. Equipment sealed against the entrance of anexternal atmosphere. Figure 500–3500.4 General(A) Classification Documentation. All hazardous (classified) locationsmust be properly documented. The documentation must be availableto those who are authorized to design, install, inspect, maintain, oroperate the electrical equipment.Author’s Comment:nProper documentation of hazardous areas assists thedesigner, installer, and authority having jurisdiction in ensuring compliance with the stringent requirements contained inArticles 501–517 of the Code.nTo ensure compliance with the above requirements, someauthorities having jurisdiction require drawings that indicatehazardous (classified) location areas and their classification.(B) Other Standards. Important information related to topics coveredin Chapter 5 may be found in other publications.Figure 500–3Oil Immersion. Electrical equipment immersed in a protective liquid.Purged and Pressurized.(1) Purging (Class I). Supplying an enclosure with a protective gas ata sufficient positive pressure to reduce the concentration of any flammable gas or vapor.(2) Pressurization (Class I or II). Supplying an enclosure with a protective gas with or without continuous flow at sufficient pressure toprevent the entrance of a flammable gas or vapor, a combustible dust,or an ignitible fiber.Note 1: To ensure a proper and safe installation, the authority havingjurisdiction should be familiar with the industry involved and standards ofthe National Fire Protection Association (NFPA), the American PetroleumInstitute (API), and the International Society of Automation (ISA).Note 2: For further information on the classification of locations, see: 500.3 Other Articles Except as modified in Articles 500 through 504, all installation requirements contained in Chapters 1 through 4 of the NEC apply to electricalequipment and wiring installed in hazardous (classified) locations. Recommended Practice for the Classification of Flammable Liquids,Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas, NFPA 497Standard for Dipping and Coating Processes Using Flammable orCombustible Liquids, NFPA 34Area Classification in Hazardous (Classified) Dust Locations,ISA 12.10Flammable and Combustible Liquids Code, NFPA 30Recommended Practice for Classification of Locations of ElectricalInstallations at Petroleum Facilities Classified as Class I, Division 1and Division 2, ANSI/API RP 500Standard for Spray Application Using Flammable or CombustibleMaterials, NFPA 33Liquefied Petroleum Gas Code, NFPA 58Standard for Fire Protection in Wastewater Treatment and CollectionFacilities, NFPA 820Note 3: For further information on protection against static electricityand lightning hazards in hazardous (classified) locations, see:Mike Holt Enterprises, Inc. www.MikeHolt.com 888.NEC.CODE (632.2633)25
500.5 Hazardous (Classified) Locations Standard for the Installation of Lightning Protection Systems, NFPA780Recommended Practice on Static Electricity, NFPA 77Protection Against Ignitions Arising Out of Static Lightning and StrayCurrents, API RP 2003–1998500.5 Classifications of LocationsScan the QR code for a video clip of Mike explaining thistopic; this is a sample from the DVDs that accompany thistextbook.(A) Classifications of Locations. Locations are classified accordingto the properties of the flammable gases, flammable liquid-producedvapors, combustible liquid-produced vapors, combustible dusts, oreasily ignitable fibers/flyings that may be present, and the likelihoodthat a flammable or combustible concentration will be present.Author’s Comment:nThe same building or structure might contain any combination of Class I, Division 1 and 2 locations; Class II, Division 1and 2 locations; and/or Class III, Division 1 and 2 locations.nSee the definitions of “Building” and “Structure” in Article100.(B) Identification of a Class I Location. A Class I location is an areawhere flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors may be present in quantities sufficientto produce explosive or ignitible mixtures.(1) Class I, Division 1 Location. A Class I, Division 1 location is alocation in which: Figure 500–5Note: To reduce expensive equipment and expensive wiring methods, locate as much electrical equipment as possible in an unclassifiedlocation.Each room, section, or area is considered individually in determiningits classification. Figure 500–4Figure 500–5(1) Ignitible concentrations of flammable gases, flammable liquidproduced vapors, or combustible liquid-produced vapors mayexist in the course of normal operations, or(2) Ignitible concentrations of flammable gases, flammable liquidproduced vapors, or combustible liquids above their flash pointsmay exist frequently due to repair or maintenance operations orbecause of leakage, orFigure 500–426(3) Breakdown or faulty equipment releases ignitible concentrationsof flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors and the electrical equipmentbecomes a source of ignition.Mike Holt’s Illustrated Guide to Hazardous Locations based on the 2014 National Electrical Code
Hazardous (Classified) Locations 500.5Note 1: Class I, Division 1 locations include:(1) Areas where volatile flammable liquids or liquefied flammable gasesare transferred from one container to another, such as at gasolinestorage and dispensing areas.(2) Interiors of spray booths and areas in the vicinity of spraying andpainting operations where volatile flammable solvents are used tocoat products with paint or plastics.(3) Locations containing open tanks or vats of volatile flammableliquids, or dip tanks for parts cleaning or other operations.Author’s Comment:nSee 500.5(B)(1)(3) Notes 1 and 2 in the NEC for moreinformation.(2) Class I, Division 2 Location. An area where volatile flammablegases, or combustible or flammable liquid-produced vapors, wouldbecome hazardous only in case of an accident or of some unusualoperating condition, or under any of the following conditions: Figure500–6(2) If ignitible concentrations of flammable gases, flammableliquid-produced vapors, or combustible liquid-produced vaporsare normally prevented by positive mechanical ventilation, butmight become hazardous through failure or abnormal operation ofthe ventilating equipment.(3) Areas adjacent to a Class I, Division 1 location and to whereflammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors might occasionally be communicatedunless prevented by adequate positive-pressure ventilation witheffective safeguards against ventilation failure.Note 1: The quantity of flammable gases, flammable liquid-producedvapors, or combustible liquid-produced vapors that might escape incase of accident, the adequacy of ventilating equipment, the total areainvolved, and the record of the industry with respect to explosions orfires are all factors that should be taken into consideration.(C) Identification of a Class II Location. Class II locations are thosewhere the presence of combustible dust may be suspended in theair or accumulates on electrical equipment in quantities sufficient toignite or explode. Figure 500–7Figure 500–6Figure 500–7(1) If flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors are handled, processed, or used,but are normally confined within closed containers and the gaseswould only escape in the case of accidental rupture or breakdown,or in case of abnormal operation of equipment.(1) Class II, Division 1 Location. A Class II, Division 1 location is anarea where combustible dust may exist under any of the followingconditions:Mike Holt Enterprises, Inc. www.MikeHolt.com 888.NEC.CODE (632.2633)27
500.6 Hazardous (Classified) Locations(1) Nonconductive combustible dust is continuously or periodicallysuspended in the air in sufficient quantities to produce mixturesthat will ignite or explode, or(2) If faulty equipment releases ignitible mixtures of dust and theequipment becomes a source of ignition.500.6 Material GroupsFor purposes of testing, approval, and area classification, various airmixtures must be grouped in accordance with 500.6(A) and (B). Figure500–8(2) Class II, Division 2 Location. An area where combustible dustwould become hazardous under any of the following conditions:(1) If combustible dust, due to abnormal operations, may be present inthe air in quantities sufficient to produce explosive or ignitible mixtures, or(2) If combustible dust accumulation is normally insufficient to interfere with the normal operation of electrical equipment, but wheremalfunctioning of equipment may result in combustible dust beingsuspended in the air, or(3) If combustible dust accumulations on, in, or near electrical equipment could be sufficient to interfere with the safe dissipation ofheat from electrical equipment, or could be ignitible by abnormaloperation or failure of electrical equipment.Note 1: The quantity of combustible dust that may be present and theadequacy of dust removal systems should be considered when determining the area classification.(D) Identification of a Class III Location. A Class III location is anarea where easily ignitible fibers or materials producing combustibleflyings are handled, manufactured, or used and aren’t likely to be suspended in the air in quantities sufficient to produce ignitible mixtures.(1) Class III, Division 1 Location. A Class III, Division 1 location is anarea where easily ignitible fibers/flyings are manufactured, handled, orused.Note 1: Such locations usually include some parts of rayon, cotton, andother textile mills or clothing manufacturing plants, as well as facilitiesthat create sawdust and flyings by pulverizing or cutting wood.(2) Class III, Division 2 Location. A Class III, Division 2 location is anarea where easily ignitible fibers/flyings are stored or handled otherthan in the manufacturing process.Figure 500–8Author’s Comment:nRefer to 500.6(A) and (B) in the NEC to be certain that equipment is listed for the proper group.nThe Cooper Crouse-Hinds Code Digest contains a substantiallist of different products and their group designations. It canbe downloaded from l.500.7 Protection TechniquesAcceptable protection techniques for electrical and electronic equipment in hazardous (classified) locations include:(A) Explosionproof Equipment. Explosionproof equipment is permitted in any Class 1 location. Figure 500–928Mike Holt’s Illustrated Guide to Hazardous Locations based on the 2014 National Electrical Code
Hazardous (Classified) Locations 500.7Figure 500–9Figure 500–11Author’s Comment:nAuthor’s Comment:Explosionproof equipment is designed to be capable of withstanding and containing the force of an internal explosion,and the hot gases within the enclosure cool as they escape[Article 100—Definitions]. Figure 500–10nDust-ignitionproof enclosures are designed to exclude dustsand won’t permit arcs, sparks, or heat within the enclosure tocause ignition of exterior dust [500.2].(C) Dusttight Enclosures. Dusttight enclosures are permitted in ClassII, Division 2 or any Class III location. Figure 500–12Figure 500–10Figure 500–12(B) Dust-Ignitionproof Enclosures. Dust-ignitionproof enclosures arepermitted in any Class II location. Figure 500–11Mike Holt Enterprises, Inc. www.MikeHolt.com 888.NEC.CODE (632.2633)29
500.8 Hazardous (Classified) Locations(D) Purged and Pressurized Systems. These systems are permittedfor equipment in any hazardous (classified) locations for which they’reidentified. Figure 500–13Author’s Comment:nThe NEC is silent on the meaning of this section. The key isthe use of the term “Identified,” which is defined in Article100 as, “Recognizable as suitable for the specific purpose,function, use, environment, application, and so forth, wheredescribed in a particular Code requirement.”500.8 Equipment(A) Suitability of Equipment. Suitability of identified equipment mustbe determined by one of the following:(1) Equipment listing or labeling. Figure 500–14Figure 500–13(E) Intrinsically Safe Systems. These systems are allowed in anyhazardous (classified) location. None of the requirements containedin Articles 501 through 503, or 510 through 516 apply to intrinsicallysafe system installations, except as required by Article 504.Author’s Comment:nIntrinsically safe systems are incapable of releasing sufficientelectrical or thermal energy to cause ignition of flammablegases or vapors [504.2].(I) Oil-Immersed Contacts. Oil-immersed make-and-break contactscan be installed in a Class I, Division 2 location.(J) Hermetically Sealed Contacts. Hermetically sealed contacts canbe installed in Class I, Division 2; Class II, Division 2; or Class III, Division 1 and 2 Locations.(L) Other Protection Techniques. Other protection techniques usedin equipment identified for use in hazardous (classified) locations arepermitted.30Figure 500–14(2) Evidence of evaluation by a qualified testing laboratory or inspection agency concerned with product evaluation.(3) Evidence approved by the authority having jurisdiction, suchas a manufacturer’s self-evaluation or an owner’s engineeringjudgment.Note: Additional documentation may include certificates demonstratingcompliance with applicable equipment standards, indicating special conditions of use, and other pertinent information.Mike Holt’s Illustrated Guide to Hazardous Locations based on the 2014 National Electrical Code
Hazardous (Classified) Locations 500.8(B) Approval for Class and Properties.(1) Identified for Use. Equipment installed in any hazardous (classified) location must be identified for the class and explosive,combustible, or ignitible properties of the specific gas, vapor, dust, orfiber/flyings that will be present. Equipment in Class I locations mustnot have any exposed surfaces that reach temperatures higher thanthe autoignition temperature of the gas or vapor present.(2) Divisions. Equipment identified for Class I, Division 1 locations canbe installed in a Class I, Division 2 location of the same class, group,and temperature class.Author’s Comment:n500.5(B) through (D) explains the differences between ClassI, Class II, and Class III locations, as well as the differencesbetween Division 1 and Division 2.(3) General-Purpose Enclosures. General-purpose enclosures notcontaining make-and-break contacts can be installed in a Class I, Division 2 location. Figure 500–15(D) Temperature.
Article 505. Class I, Zone 0, 1, and 2 Locations Figure 500–2. Mike Holt Enterprises, Inc. www.MikeHolt.com 888.NEC.CODE (632.2633) 25 Hazardous (Classified) Locations 500.4 500.4 General (A) Classification Documentation. All hazardous (classified) locations must be properly documented. The documentation must be available to those who are authorized to design, install, inspect .
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