NEC 500-516 — Hazardous Locations (Washington)

NEC 500–516 — Hazardous Locations (Washington Journeyman)

Learning Objectives

1.1 Classes, Divisions, Documentation, and Scope (500.1, 500.4, 500.5)

Section 500.1(A) covers area classification and general requirements for electrical and electronic equipment and wiring rated at all voltages where fire or explosion hazards might exist due to flammable gases, flammable liquid–produced vapors, combustible liquid–produced vapors, combustible dusts, and ignitible fibers/flyings in Class I, Class II, and Class III, Division 1 or Division 2 locations.

Section 500.1(B) is the exclusion list. Article 500 does not cover Zone 0, Zone 1, or Zone 2 locations [500.1(B)(1)]; Zone 20, Zone 21, or Zone 22 locations [500.1(B)(2)]; locations subject to the unique risk and explosion hazards associated with explosives, pyrotechnics, and blasting agents [500.1(B)(3)]; locations where pyrophoric materials are the only materials used or handled [500.1(B)(4)]; or features of equipment involving nonelectrical potential sources of ignition [500.1(B)(5)].

Section 500.5 defines the classes: Class I, flammable gases or flammable and combustible liquid–produced vapors in quantities sufficient to produce explosive or ignitible mixtures; Class II, combustible dust; Class III, easily ignitible fibers or flyings. Division 1 means ignitible concentrations can exist under normal operating conditions, or the hazard arises from repair and maintenance, leakage, or breakdown that simultaneously makes electrical equipment an ignition source. Division 2 means the material is normally confined in closed containers or systems and escapes only on accidental rupture or breakdown, or ignitible concentrations are normally prevented by positive mechanical ventilation. Section 500.6 sets the groups that markings must match: Class I Groups A through D, where A is acetylene; Class II Groups E (combustible metal dusts), F (carbonaceous dusts with more than 8 percent total entrapped volatiles), and G; Class III fibers and flyings are not grouped.

Section 500.4 requires that areas designated as hazardous (classified) locations, or determined to be unclassified, be documented on an area classification drawing and other associated documentation available to the authority having jurisdiction and to authorized persons.

1.2 Protection Techniques and Equipment Identification (500.7, 500.8)

Section 500.7 requires equipment in hazardous (classified) locations to be protected by one or more of the techniques in 500.7(A) through 500.7(U); Chapter 9 Table 13 shows suitability for specific locations. Notation below: I/1 means Class I, Division 1, and so on.

Section 500.8(B)(1) requires identification 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 present. Class I equipment must not have an exposed surface operating above the autoignition temperature of the specific gas or vapor; Class II equipment must not have an external temperature higher than that specified in 500.8(D)(2); Class III equipment must not exceed the surface temperatures of 503.5. Luminaires, switches, circuit breakers, and plugs and receptacles are potential sources of ignition and are investigated for suitability in classified locations.

Section 500.8(B)(2) permits equipment identified for a Division 1 location in a Division 2 location of the same class, group, and temperature class; under 500.8(B)(2)(b), equipment required to be explosionproof shall incorporate seals in accordance with 501.15(A) or 501.15(D) when the wiring methods of 501.10(B) are employed. Section 500.8(B)(3) permits general-purpose equipment in Division 2 locations where specifically permitted in Article 501, Article 502, and Article 503 Part III. Marking under 500.8(C) covers class, division, group, temperature class, and ambient range. The limits under 500.8(D) split by class: Class I, not above the autoignition temperature of the specific gas or vapor; Class II, less than the ignition temperature of the dust and, for organic dusts that might dehydrate or carbonize, not more than the lower of that temperature or 165°C (329°F); Class III, a 165°C (329°F) cap and 120°C (248°F) for equipment that might be overloaded.

1.3 Class I Locations: Wiring Methods, Sealing, and Bonding (501.10, 501.15, 500.30)

Article 501 applies to Class I, Division 1 and Division 2. Under 501.10(A)(1), one or more of the wiring methods of Table 501.10(A)(1) shall be used in accordance with the sections in the Reference column:

Table 501.10(A)(1) permits Type MC-HL cable (gas/vaportight continuous corrugated metallic sheath, overall jacket, listed fittings, separate equipment grounding conductor), Type MI cable and Type ITC-HL cable (listed fittings), Type TC-ER-HL cable (not subject to physical damage, limited to 600 volts nominal or less), and Type P cable, plus IMC and RMC with threaded connections. PVC conduit is permitted only when encased in a concrete envelope at least 50 mm (2 in.) thick with cover not less than 600 mm (24 in.) measured from the top of the conduit to grade, with RMC or IMC for the last 600 mm (24 in.) of the underground run and an equipment grounding conductor included.

Where flexibility is necessary in Division 1 — to minimize vibration, allow movement after installation, or allow movement during maintenance — 501.10(A)(2) requires one or more of the wiring methods of Table 501.10(A)(2): Type TC-ER-HL cable, Type P cable, flexible cord per Table 400.4 complying with 501.140(B), and flexible metal fittings listed for the location. Boxes and fittings must be identified for Class I, Division 1 [501.10(A)(3)].

Section 501.15 governs sealing. Under 501.15(A), each conduit entry into an explosionproof enclosure requires a conduit seal where the enclosure contains switches, circuit breakers, fuses, relays, or resistors that may produce arcs, sparks, or temperatures exceeding 80 percent of the autoignition temperature of the gas or vapor, or where the entry is metric designator 53 (trade size 2) or larger with terminals, splices, or taps. Seals are installed within 450 mm (18 in.) from the enclosure, with only threaded couplings or explosionproof fittings not larger than the conduit trade size between seal and enclosure. Where two or more explosionproof enclosures requiring seals are connected by nipples or runs of conduit not more than 900 mm (36 in.) long, a single seal in each nipple or run is sufficient if located not more than 450 mm (18 in.) from either enclosure. A seal is required in each conduit run leaving a Division 1 location, on either side of the boundary within 3.05 m (10 ft) of it, with the Division 1 wiring method extended to the seal when the seal is not on the Division 1 side and no union, coupling, box, or other fitting between seal and boundary except a listed explosionproof reducer at the seal. Exception No. 1 allows unbroken metal conduit with no such fittings, and none within 300 mm (12 in.) of the boundary, to pass unsealed into unclassified terminations. Division 2 conduit seals are in 501.15(B); cable seals for Division 1 are in 501.15(D), the counterpart that 500.8(B)(2)(b) names alongside 501.15(A); seal construction is in 501.15(C).

Section 500.30 governs bonding. Under 500.30(A), regardless of system voltage, bonding of normally non-current-carrying metal parts of equipment, raceways, metal-clad cable, and metal enclosures through the point of grounding for service equipment or a separately derived system must be made by wrenchtight threaded couplings, threaded entries, or listed threaded hubs; threadless couplings and connectors made up tight; or other listed devices such as bonding-type locknuts, bushings, or bushings with bonding jumpers. Section 500.30(B) prohibits locknut-bushing and double-locknut connections and bonding locknuts around impaired connections such as reducing washers or oversized, concentric, or eccentric knockouts. Section 500.30(C)(1) requires a wire-type equipment bonding jumper for FMC and LFMC sized in accordance with 250.122; 500.30(C)(2) waives that jumper only for listed LFMC 1.8 m (6 ft) or less with fittings listed for grounding, overcurrent protection of 10 amperes or less, and a meter, instrument, or relay circuit load.

1.4 Class II Locations: Combustible Dust (502.10, 502.15, 502.25, 502.150)

Article 502 covers Class II, Division 1 and Division 2, where dust enters enclosures, blankets equipment, and erodes ignition-temperature margin, so the rules cluster around dusttight and dust-ignitionproof construction.

Under 502.10(A)(3), boxes and fittings must have threaded bosses for connection to conduit or cable terminations and must be dusttight. Boxes and fittings in which taps, joints, or terminal connections are made, or that are used in Group E locations, shall be identified for Class II locations; general-purpose boxes are not permitted, and Class I, Division 1 identification is not a substitute.

Where flexibility is necessary in Division 1, 502.10(A)(2) requires one or more of the wiring methods of Table 502.10(A)(2): flexible connectors shall be dusttight; Type MC cable requires listed fittings and interlocked armor with an overall jacket of suitable polymeric material; LFMC requires listed fittings and bonding in accordance with 500.30; Type EO, ETP, and ETT cables are permitted for elevator use with listed dusttight fittings; and Type TC-ER-HL and Type P cables carry the table's listing, termination, and 600-volt limits. The main Division 1 table, 502.10(A)(1), adds Type MC-HL, Type MI, Type ITC-HL, Type P, IMC, and RMC, plus optical fiber cables installed in raceways per 722.162 and sealed per 502.15. Division 2 permits any Division 1 method plus the methods of Table 502.10(B)(1) and nonincendive field wiring installed per the control drawing(s) [502.10(B)(3)].

Section 502.15 requires suitable means to prevent the entrance of dust into a dust-ignitionproof enclosure where a raceway provides communication with an enclosure that is not, and permits a permanent and effective seal; a horizontal raceway not less than 3.05 m (10 ft) long; a vertical raceway not less than 1.5 m (5 ft) long extending downward from the dust-ignitionproof enclosure; a raceway equivalent to the last two that extends only horizontally and downward; or electrical sealing putty. A vertical raceway extending upward is not a permitted means. Sealing fittings must be accessible and need not be explosionproof, and no seal is required to an enclosure in an unclassified location.

Section 502.25 allows no uninsulated exposed parts — conductors, buses, terminals, or components — operating at more than 30 volts, or 15 volts in wet locations; where such parts exist they must additionally be protected by a protection technique according to 500.7(E), 500.7(F), or 500.7(G) suitable for the location. Grounding under Table 250.122 answers a different requirement. For process control instruments on flexible cord, attachment plug, and receptacle in Division 2, 502.150(B)(5)(2) requires the cord not to exceed 900 mm (3 ft) and to be listed for extra-hard usage, or, if listed for hard usage, to be protected by location.

1.5 Class III Locations: Fibers and Flyings (503.1, 503.5, 503.10)

Article 503 covers Class III, Division 1 and Division 2 locations where fire or explosion hazards might exist due to nonmetal combustible fibers/flyings or ignitible fibers/flyings [503.1]. The characteristic hazard is accumulation: fibers settle on equipment, insulate it, and become highly susceptible to spontaneous ignition once carbonized or excessively dry. Section 503.5 therefore requires equipment in Class III locations to function at full rating without developing surface temperatures high enough to cause excessive dehydration or gradual carbonization of accumulated fibers/flyings — the basis for the Class III limits applied by 500.8(B)(1).

In Division 1, one or more of the wiring methods of Table 503.10(A)(1) shall be used, all boxes and fittings shall be dusttight [503.10(A)(2)], flexible connections follow Table 503.10(A)(3), and nonincendive field wiring may use any unclassified-location method installed per the control drawing(s) [503.10(A)(4)]. Under 503.10(B)(2), open wiring on insulators is permitted only in sections, compartments, or areas that do not contain machinery and are used solely for storage, installed in accordance with Article 398 Part II, including the protection required by 398.15(C) where conductors are not run in roof spaces and are well out of reach of physical damage. Elsewhere, 503.115 requires dusttight enclosures for switches, circuit breakers, motor controllers, and fuses, and 503.140 requires flexible cords listed for extra-hard usage with an equipment grounding conductor and a listed dusttight cord connector.

1.6 Intrinsically Safe Systems (504.1, 504.2, 504.4, 504.10, 504.20, 504.30)

Article 504 covers the installation of intrinsically safe (I.S.) apparatus, wiring, and systems for hazardous (classified) locations [504.1]. Its Informational Note points to ANSI/ISA RP 12.06.01 for additional information, but the installation requirement is the control drawing, not the recommended practice.

Under 504.2, all intrinsically safe apparatus and associated apparatus shall be listed. The exception is narrow: simple apparatus, as described on the control drawing, shall not be required to be listed. Associated apparatus gets no such exception — its listing is not conditioned on location, division, or internal components. Under 504.4, except as modified by Article 504, all applicable articles of the code shall apply.

Under 504.10(A), intrinsically safe apparatus, associated apparatus, and other equipment shall be installed in accordance with the control drawing(s), and the control drawing identification is marked on the apparatus. A simple apparatus, whether or not shown on the control drawing(s), shall be permitted to be installed provided the simple apparatus does not interconnect intrinsically safe circuits — if it does interconnect intrinsically safe circuits, the permission does not apply and it must be shown on the control drawing(s). Sections 504.10(B) and 504.10(C) permit intrinsically safe and associated apparatus in any hazardous (classified) location for which they are identified and allow general-purpose enclosures unless the manufacturer's documentation says otherwise; 504.10(D) sets the temperature basis for simple apparatus from the output power of the associated apparatus, its thermal resistance, and Table 500.8(C)(4).

Under 504.20, any wiring method suitable for unclassified locations, including Chapter 7 and Chapter 8 methods, is permitted for intrinsically safe apparatus, while sealing follows 504.70 and separation follows 504.30. Under 504.30(A), conductors of intrinsically safe circuits shall not be placed in any raceway, cable tray, or cable with conductors of any nonintrinsically safe circuit unless they are separated by at least 50 mm (2 in.) and secured; by a grounded metal partition 0.91 mm (0.0359 in.) or thicker; by an approved insulating partition; or by placing all intrinsically safe or all nonintrinsically safe conductors in Type MC or Type MI cable.

1.7 Zones and the Sibling Articles (505, 506)

Because Article 500 excludes zone locations, exam items test which article owns them: Article 505 governs Zone 0, Zone 1, and Zone 2 locations for flammable gases and vapors, and Article 506 governs Zone 20, Zone 21, and Zone 22 locations for combustible dusts and ignitible fibers/flyings. The two systems are bridged by markings: 501.5 permits equipment listed and marked per 505.9(C)(2) for Zone 0, 1, or 2 use to be installed in Class I, Division 2 locations for the same gas and with a suitable temperature class, and Zone 0 equipment may be used in Class I, Division 1 or Division 2. Sections 502.6 and 503.6 carry the corresponding allowances for equipment marked under 506.9(C)(2). A zone answer in an Article 500 scope question points to 500.1(B).

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Practical Field Points

Summary

Article 500 sets the frame for the rest of the chapter: 500.1 states what the article covers and excludes Zone 0, 1, and 2 locations, Zone 20, 21, and 22 locations, explosives and pyrotechnics, pyrophoric materials, and nonelectrical ignition features; 500.4 requires an area classification drawing available to the authority having jurisdiction; 500.5 defines the classes and divisions; 500.6 defines the material groups; 500.7 lists the protection techniques with the class and division each may serve; and 500.8 fits equipment to the hazard through identification, surface temperature limits, and marking.

The article family then splits by class. Article 501 covers gas and vapor locations, where the questions are which wiring method the table permits and how conduit and cable systems are sealed — seals within 450 mm (18 in.) of an explosionproof enclosure, one seal for nipples not more than 900 mm (36 in.) long, and boundary seals within 3.05 m (10 ft) of the boundary. Article 502 covers combustible dust, where everything serves keeping dust out: identified boxes and fittings, dusttight flexible connectors, LFMC bonded per 500.30, and the 502.15 geometry of 3.05 m (10 ft) horizontal or 1.5 m (5 ft) downward. Article 503 covers fibers and flyings, where equipment must run at full rating without carbonizing accumulations.

Article 504 runs across all three classes, requiring listed apparatus, the control drawing as the installation document, and separation of intrinsically safe conductors, while 500.30 names the bonding means that are permitted and those that are prohibited. Learn the exclusion list, the technique table, the sealing distances, and the dust-entry geometry, and the rest of the chapter reads as consequences of those four.