Article 501

Class I LocationsPart I. General501.1 Scope. Article 501 covers the requirements for electri‐ cal and electronic equipment and wiring for all voltages in Class I, Division 1 and 2 locations where fire or explosion hazards may exist due to flammable gases or vapors or flamma‐ ble liquids.Informational Note: For the requirements for electrical and electronic equipment and wiring for all voltages in Zone 0, Zone 1, or Zone 2 hazardous (classified) locations where fire or explo‐ sion hazards may exist due to flammable gases or vapors or flam‐ mable liquids, refer to Article 505.501.5 Zone Equipment. Equipment listed and marked in accordance with 505.9(C)(2) for use in Zone 0, 1, or 2 loca‐ tions shall be permitted in Class I, Division 2 locations for the same gas and with a suitable temperature class. Equipment listed and marked in accordance with 505.9(C)(2) for use in Zone 0 locations shall be permitted in Class I, Division 1 or Division 2 locations for the same gas and with a suitable temperature class.
Part II. Wiring501.10 Wiring Methods. Wiring methods shall comply with 501.10(A) or (B).
1. Class I, Division 1.
  1. General. In Class I, Division 1 locations, the wiring meth‐ ods in (a) through (f) shall be permitted.
    1. Threaded rigid metal conduit or threaded steel inter‐ mediate metal conduit.Exception: Type PVC conduit, Type RTRC conduit, and Type HDPE conduit shall be permitted where encased in a concrete envelope a mini‐ mum of 50 mm (2 in.) thick and provided with not less than 600 mm (24 in.) of cover measured from the top of the conduit to grade. The concrete encasement shall be permitted to be omitted where subject to the provisions of 514.8, Exception No. 2, or 515.8(A). Threaded rigid metal conduit or threaded steel intermediate metal conduit shall be used for the last 600 mm (24 in.) of the underground run to emergence or to the point of connection to the aboveground raceway. An equipment grounding conductor shall be included to provide for electrical continu‐ ity of the raceway system and for grounding of non–current-carrying metal parts.
    2. Type MI cable terminated with fittings listed for the loca‐ tion. Type MI cable shall be installed and supported in a manner to avoid tensile stress at the termination fittings.
    3. In industrial establishments with restricted public access, where the conditions of maintenance and supervision ensure that only qualified persons service the installation, Type MC-HL cable listed for use in Class I, Zone 1 or Division 1 locations, with a gas/vaportight continuous corrugated metallic sheath, an overall jacket of suitable polymeric material, and a separate equipment grounding conductor(s) in accordance with 250.122, and terminated with fittings listed for the application.Type MC-HL cable shall be installed in accordance with the provisions of Article 330, Part II.
    4. In industrial establishments with restricted public access, where the conditions of maintenance and supervision ensure that only qualified persons service the installation, Type ITC-HL cable listed for use in Class I, Zone 1 or Division 1 loca‐ tions, with a gas/vaportight continuous corrugated metallic sheath and an overall jacket of suitable polymeric material, and terminated with fittings listed for the application, and installed in accordance with the provisions of Article 727.
    5. Optical fiber cable Types OFNP, OFCP, OFNR, OFCR, OFNG, OFCG, OFN, and OFC shall be permitted to be instal‐ led in raceways in accordance with 501.10(A). These optical fiber cables shall be sealed in accordance with 501.15.
  2. Flexible Connections. Where necessary to employ flexible connections, as at motor terminals, one of the following shall be permitted:
    1. Flexible fittings listed for the location
    2. Flexible cord in accordance with the provisions of 501.140, terminated with cord connectors listed for the location
    3. In industrial establishments with restricted public access, where the conditions of maintenance and supervision ensure that only qualified persons service the installation, for applications limited to 600 volts, nominal, or less, and where protected from damage by location or a suitable guard, listed Type TC-ER-HL cable with an overall jacket and a separate equipment grounding conductor(s) in accordance with 250.122 that is terminated with fittings listed for the location
    (3) Boxes and Fittings. All boxes and fittings shall be approved for Class I, Division 1.Informational Note: For entry into enclosures required to be explosionproof, see the information on construction, testing, and marking of cables, explosionproof cable fittings, and explo‐ sionproof cord connectors in ANSI/UL 2225-2011, Cables and Cable-Fittings for Use in Hazardous (Classified) Locations.
2. Class I, Division 2.(1) Rigid metal conduit (RMC) and intermediate metalconduit (IMC) with listed threadless fittings.
  1. General. In Class I, Division 2 locations, all wiring meth‐ ods permitted in 501.10(A) and the following wiring methods shall be permitted:
- (2) Enclosed gasketed busways and enclosed gasketed wire‐ ways.
1. Type PLTC and Type PLTC-ER cable in accordance with the provisions of Article 725, including installation in cable tray systems. The cable shall be terminated with listed fittings.
2. Type ITC and Type ITC-ER cable as permitted in 727.4 and terminated with listed fittings.
3. Type MC, MV, TC, or TC-ER cable, including installation in cable tray systems. The cable shall be terminated with listed fittings.
4. In industrial establishments with restricted public access, where the conditions of maintenance and supervision ensure that only qualified persons service the installation and where metallic conduit does not provide sufficient corrosion resistance, listed reinforced thermosetting resin conduit (RTRC), factory elbows, and associated fittings, all marked with the suffix -XW, and Schedule 80 PVC conduit, factory elbows, and associated fittings shall be permitted.
5. Optical fiber cable Types OFNP, OFCP, OFNR, OFCR, OFNG, OFCG, OFN, and OFC shall be permitted to be installed in cable trays or any other raceway in accord‐ ance with 501.10(B). Optical fiber cables shall be sealed in accordance with 501.15.
6. Cablebus.
Where seals are required for boundary conditions as defined in 501.15(A)(4), the Division 1 wiring method shall extend into the Division 2 area to the seal, which shall be located on the Division 2 side of the Division 1–Division 2 boundary.(2) Flexible Connections. Where provision must be made for flexibility, one or more of the following shall be permitted:
1. Listed flexible metal fittings.
2. Flexible metal conduit with listed fittings.
3. Interlocked armor Type MC cable with listed fittings.
4. Liquidtight flexible metal conduit with listed fittings.
5. Liquidtight flexible nonmetallic conduit with listed fittings.
6. Flexible cord listed for extra-hard usage and terminated with listed fittings. A conductor for use as an equipment grounding conductor shall be included in the flexible cord.
7. For elevator use, an identified elevator cable of Type EO, ETP, or ETT, shown under the “use” column in Table
400.4 for “hazardous (classified) locations” and termina‐ ted with listed fittings.Informational Note: See 501.30(B) for grounding requirements where flexible conduit is used.(3) Nonincendive Field Wiring. Nonincendive field wiring shall be permitted using any of the wiring methods permitted for unclassified locations. Nonincendive field wiring systems shall be installed in accordance with the control drawing(s). Simple apparatus, not shown on the control drawing, shall be permitted in a nonincendive field wiring circuit, provided the simple apparatus does not interconnect the nonincendive field wiring circuit to any other circuit.Informational Note: Simple apparatus is defined in 504.2.Separate nonincendive field wiring circuits shall be installed in accordance with one of the following:
1. In separate cables
2. In multiconductor cables where the conductors of each circuit are within a grounded metal shield
3. In multiconductor cables or in raceways, where the conductors of each circuit have insulation with a mini‐ mum thickness of 0.25 mm (0.01 in.)
4. Boxes and Fittings. Boxes and fittings shall not be required to be explosionproof except as required by 501.105(B)(2), 501.115(B)(1) , and 501.150(B)(1).
Informational Note: For entry into enclosures required to be explosionproof, see the information on construction, testing, and marking of cables, explosionproof cable fittings, and explo‐ sionproof cord connectors in ANSI/UL 2225-2011, Cables and Cable-Fittings for Use in Hazardous (Classified) Locations.501.15 Sealing and Drainage. Seals in conduit and cable systems shall comply with 501.15(A) through (F). Sealing compound shall be used in Type MI cable termination fittings to exclude moisture and other fluids from the cable insulation.Informational Note No. 1: Seals are provided in conduit and cable systems to minimize the passage of gases and vapors and prevent the passage of flames from one portion of the electrical installation to another through the conduit. Such communica‐ tion through Type MI cable is inherently prevented by construc‐ tion of the cable. Unless specifically designed and tested for the purpose, conduit and cable seals are not intended to prevent the passage of liquids, gases, or vapors at a continuous pressure differential across the seal. Even at differences in pressure across the seal equivalent to a few inches of water, there may be a slow passage of gas or vapor through a seal and through conductors passing through the seal. Temperature extremes and highly corrosive liquids and vapors can affect the ability of seals to perform their intended function.Informational Note No. 2: Gas or vapor leakage and propaga‐ tion of flames may occur through the interstices between the strands of standard stranded conductors larger than 2 AWG. Special conductor constructions, such as compacted strands or sealing of the individual strands, are means of reducing leakage and preventing the propagation of flames.
1. Conduit Seals, Class I, Division 1. In Class I, Division 1 locations, conduit seals shall be located in accordance with 501.15(A)(1) through (A)(4).
  1. Entering Enclosures. Each conduit entry into an explo‐ sionproof enclosure shall have a conduit seal where either of the following conditions apply:
    1. The enclosure contains apparatus, such as switches, circuit breakers, fuses, relays, or resistors that may produce arcs, sparks, or temperatures that exceed 80 percent of the autoignition temperature, in degrees Celsius, of the gas or vapor involved in normal operation.Exception: Seals shall not be required for conduit entering an enclosure under any one of the following conditions:
      1. The switch, circuit breaker, fuse, relay, or resistor is enclosed within a chamber hermetically sealed against the entrance of gases or vapors.
      2. The switch, circuit breaker, fuse, relay, or resistor is immersed in oil in accordance with 501.115(B)(1)(2).
      3. The switch, circuit breaker, fuse, relay, or resistor is enclosed within an enclosure, identified for the location, and marked “Leads Factory Sealed,” or “Factory Sealed,” “Seal not Required,” or equiv‐ alent.
      4. The switch, circuit breaker, fuse, relay, or resistor is part of a nonin‐ cendive circuit.
    2. The entry is metric designator 53 (trade size 2) or larger, and the enclosure contains terminals, splices, or taps.
    An enclosure, identified for the location, and marked “Leads Factory Sealed”, or “Factory Sealed,” or “Seal not Required,” or equivalent shall not be considered to serve as a seal for another adjacent enclosure that is required to have a conduit seal.Conduit seals shall be installed within 450 mm (18 in.) from the enclosure or as required by the enclosure marking. Onlyexplosionproof unions, couplings, reducers, elbows, and capped elbows that are not larger than the trade size of the conduit shall be permitted between the sealing fitting and the explosionproof enclosure.
    1. Pressurized Enclosures. Conduit seals shall be installed within 450 mm (18 in.) of the enclosure in each conduit entry into a pressurized enclosure where the conduit is not pressur‐ ized as part of the protection system.Informational Note No. 1: Installing the seal as close as possible to the enclosure will reduce problems with purging the dead airspace in the pressurized conduit.Informational Note No. 2: For further information, see NFPA 496-2013, Standard for Purged and Pressurized Enclosures for Electri‐ cal Equipment.
    2. Two or More Explosionproof Enclosures. Where two or more explosionproof enclosures that require conduit seals are connected by nipples or runs of conduit not more than 900 mm (36 in.) long, a single conduit seal in each such nipple connection or run of conduit shall be considered sufficient if the seal is located not more than 450 mm (18 in.) from either enclosure.
    3. Class I, Division 1 Boundary. A conduit seal shall be required in each conduit run leaving a Division 1 location. The sealing fitting shall be permitted to be installed on either side of the boundary within 3.05 m (10 ft) of the boundary, and it shall be designed and installed to minimize the amount of gas or vapor within the portion of the conduit installed in the Divi‐ sion 1 location that can be communicated beyond the seal. The conduit run between the conduit seal and the point at which the conduit leaves the Division 1 location shall contain no union, coupling, box, or other fitting except for a listed explo‐ sionproof reducer installed at the conduit seal.
    Exception No. 1: Metal conduit that contains no unions, couplings, boxes, or fittings, that passes completely through a Division 1 location with no fittings installed within 300 mm (12 in.) of either side of the boundary, shall not require a conduit seal if the termination points of the unbroken conduit are located in unclassified locations.Exception No. 2: For underground conduit installed in accordance with 300.5 where the boundary is below grade, the sealing fitting shall be permitted to be installed after the conduit emerges from below grade, but there shall be no union, coupling, box, or fitting, other than listed explosionproof reducers at the sealing fitting, in the conduit between the sealing fitting and the point at which the conduit emerges from below grade.
2. Conduit Seals, Class I, Division 2. In Class I, Division 2 locations, conduit seals shall be located in accordance with 501.15(B)(1) and (B)(2).
  1. Entering Enclosures. For connections to enclosures that are required to be explosionproof, a conduit seal shall be provi‐ ded in accordance with 501.15(A)(1)(1) and (A)(3). All portions of the conduit run or nipple between the seal and enclosure shall comply with 501.10(A).
  2. Class I, Division 2 Boundary. A conduit seal shall be required in each conduit run leaving a Class I, Division 2 loca‐ tion. The sealing fitting shall be permitted to be installed on either side of the boundary within 3.05 m (10 ft) of the boun‐ dary and it shall be designed and installed to minimize the amount of gas or vapor within the portion of the conduit instal‐ led in the Division 2 location that can be communicated
    beyond the seal. Rigid metal conduit or threaded steel inter‐ mediate metal conduit shall be used between the sealing fitting and the point at which the conduit leaves the Division 2 loca‐ tion, and a threaded connection shall be used at the sealing fitting. The conduit run between the conduit seal and the point at which the conduit leaves the Division 2 location shall contain no union, coupling, box, or other fitting except for a listed explosionproof reducer installed at the conduit seal. Such seals shall not be required to be explosionproof but shall be identified for the purpose of minimizing the passage of gases permitted under normal operating conditions and shall be accessible.Informational Note: For further information, refer to ANSI/ UL 514B-2012, Conduit, Tubing, and Cable Fittings.Exception No. 1: Metal conduit that contains no unions, couplings, boxes, or fittings, that passes completely through a Division 2 location with no fittings installed within 300 mm (12 in.) of either side of the boundary, shall not be required to be sealed if the termination points of the unbroken conduit are located in unclassified locations.Exception No. 2: Conduit systems terminating in an unclassified loca‐ tion where the metal conduit transitions to cable tray, cablebus, ventila‐ ted busway, or Type MI cable, or to cable not installed in any cable tray or raceway system, shall not be required to be sealed where passing from the Division 2 location into the unclassified location under the follow‐ ing conditions:
    1. The unclassified location is outdoors, or the unclassified location is indoors and the conduit system is entirely in one room.
    2. The conduits shall not terminate at an enclosure containing an ignition source in normal operation.Exception No. 3: Conduit systems passing from an enclosure or a room that is unclassified, as a result of pressurization, into a Division 2 loca‐ tion shall not require a seal at the boundary.Informational Note: For further information, refer to NFPA 496-2013, Standard for Purged and Pressurized Enclosures for Electri‐ cal Equipment.Exception No. 4: Segments of aboveground conduit systems shall not be required to be sealed where passing from a Division 2 location into an unclassified location if all of the following conditions are met:
      1. No part of the conduit system segment passes through a Division 1 location where the conduit segment contains unions, couplings, boxes, or fittings that are located within 300 mm (12 in.) of the Division 1 location.
      2. The conduit system segment is located entirely in outdoor loca‐ tions.
      3. The conduit system segment is not directly connected to canned pumps, process or service connections for flow, pressure, or analy‐ sis measurement, and so forth, that depend on a single compres‐ sion seal, diaphragm, or tube to prevent flammable or combustible fluids from entering the conduit system.
      4. The conduit system segment contains only threaded metal conduit, unions, couplings, conduit bodies, and fittings in the unclassified location.
      5. The conduit system segment is sealed at its entry to each enclosure or fitting located in the Division 2 location that contains termi‐ nals, splices, or taps.
3. Class I, Divisions 1 and 2. Seals installed in Class I, Divi‐ sion 1 and Division 2 locations shall comply with 501.15(C)(1) through (C)(6).Exception: Seals that are not required to be explosionproof by 501.15(B)(2) or 504.70 shall not be required to comply with 501.15(C).
  1. Fittings. Enclosures that contain connections or equip‐ ment shall be provided with an integral sealing means, or seal‐ ing fittings listed for the location shall be used. Sealing fittings shall be listed for use with one or more specific compounds and shall be accessible.
  2. Compound. The compound shall provide a seal to mini‐ mize the passage of gas and/or vapors through the sealing fitting and shall not be affected by the surrounding atmos‐ phere or liquids. The melting point of the compound shall not be less than 93°C (200°F).
  3. Thickness of Compounds. The thickness of the sealing compound installed in completed seals, other than listed cable sealing fittings, shall not be less than the metric designator (trade size) of the sealing fitting expressed in the units of meas‐ urement employed; however, in no case shall the thickness of the compound be less than 16 mm (5∕8 in.).
  4. Splices and Taps. Splices and taps shall not be made in fittings intended only for sealing with compound; nor shall other fittings in which splices or taps are made be filled with compound.
  5. Assemblies. An entire assembly shall be identified for the location where the equipment that may produce arcs, sparks, or high temperatures is located in a compartment that is sepa‐ rate from the compartment containing splices or taps, and an integral seal is provided where conductors pass from one compartment to the other. In Division 1 locations, seals shall be provided in conduit connecting to the compartment contain‐ ing splices or taps where required by 501.15(A)(1)(2).
  6. Conductor or Optical Fiber Fill. The cross-sectional area of the conductors or optical fiber tubes (metallic or nonmetal‐ lic) permitted in a seal shall not exceed 25 percent of the cross- sectional area of a rigid metal conduit of the same trade size unless the seal is specifically identified for a higher percentage of fill.
4. Cable Seals, Class I, Division 1. In Division 1 locations, cable seals shall be located according to 501.15(D)(1) through (D)(3).
  1. At Terminations. Cables shall be sealed with sealing fittings that comply with 501.15(C) at all terminations. Type MC-HL cables with a gas/vaportight continuous corrugated metallic sheath and an overall jacket of suitable polymeric material shall be sealed with a listed fitting after the jacket and any other covering have been removed so that the sealing compound can surround each individual insulated conductor in such a manner as to minimize the passage of gases and vapors.Seals for cables entering enclosures shall be installed within 450 mm (18 in.) of the enclosure or as required by the enclo‐ sure marking. Only explosionproof unions, couplings, reduc‐ ers, elbows, and capped elbows that are not larger than the trade size of the enclosure entry shall be permitted between the sealing fitting and the enclosure.Exception: Shielded cables and twisted pair cables shall not require the removal of the shielding material or separation of the twisted pairs, provided the termination is sealed by an approved means to minimize
    the entrance of gases or vapors and prevent propagation of flame into the cable core.
  2. Cables Capable of Transmitting Gases or Vapors. Cables with a gas/vaportight continuous sheath capable of transmit‐ ting gases or vapors through the cable core, installed in conduit, shall be sealed in the Class 1, Division 1 location after the jacket and any other coverings have been removed so that the sealing compound can surround each individual insulated conductor or optical fiber tube and the outer jacket.Exception: Multiconductor cables with a gas/vaportight continuous sheath capable of transmitting gases or vapors through the cable core shall be permitted to be considered as a single conductor by sealing the cable in the conduit within 450 mm (18 in.) of the enclosure and the cable end within the enclosure by an approved means to minimize the entrance of gases or vapors and prevent the propagation of flame into the cable core, or by other approved methods. It shall not be required to remove the shielding material or separate the twisted pairs of shielded cables and twisted pair cables.
  3. Cables Incapable of Transmitting Gases or Vapors. Each multiconductor cable installed in conduit shall be considered as a single conductor if the cable is incapable of transmitting gases or vapors through the cable core. These cables shall be sealed in accordance with 501.15(A).
5. Cable Seals, Class I, Division 2. In Division 2 locations, cable seals shall be located in accordance with 501.15(E)(1) through (E)(4).Exception: Cables with an unbroken gas/vaportight continuous sheath shall be permitted to pass through a Division 2 location without seals.
  1. Terminations. Cables entering enclosures that are required to be explosionproof shall be sealed at the point of entrance. The sealing fitting shall comply with 501.15(B)(1) or be explosionproof. Multiconductor or optical multifiber cables with a gas/vaportight continuous sheath capable of transmit‐ ting gases or vapors through the cable core that are installed in a Division 2 location shall be sealed with a listed fitting after the jacket and any other coverings have been removed, so that the sealing compound can surround each individual insulated conductor or optical fiber tube in such a manner as to mini‐ mize the passage of gases and vapors. Multiconductor or opti‐ cal multifiber cables installed in conduit shall be sealed as described in 501.15(D).Exception No. 1: Cables leaving an enclosure or room that is unclassi‐ fied as a result of Type Z pressurization and entering into a Division 2 location shall not require a seal at the boundary.Exception No. 2: Shielded cables and twisted pair cables shall not require the removal of the shielding material or separation of the twisted pairs, provided the termination is by an approved means to minimize the entrance of gases or vapors and prevent propagation of flame into the cable core.
  2. Cables That Do Not Transmit Gases or Vapors. Cables that have a gas/vaportight continuous sheath and do not trans‐ mit gases or vapors through the cable core in excess of the quantity permitted for seal fittings shall not be required to be sealed except as required in 501.15(E)(1). The minimum length of such a cable run shall not be less than the length needed to limit gas or vapor flow through the cable core, excluding the interstices of the conductor strands, to the rate permitted for seal fittings [200 cm3/hr (0.007 ft3/hr) of air at a pressure of 1500 pascals (6 in. of water)].
  3. Cables Capable of Transmitting Gases or Vapors. Cables with a gas/vaportight continuous sheath capable of transmit‐ ting gases or vapors through the cable core shall not be required to be sealed except as required in 501.15(E)(1), unless the cable is attached to process equipment or devices that may cause a pressure in excess of 1500 pascals (6 in. of water) to be exerted at a cable end, in which case a seal, a barrier, or other means shall be provided to prevent migration of flammables into an unclassified location.
  4. Cables Without Gas/Vaportight Sheath. Cables that do not have a gas/vaportight continuous sheath shall be sealed at the boundary of the Division 2 and unclassified location in such a manner as to minimize the passage of gases or vapors into an unclassified location.
6. Drainage.
  1. Control Equipment. Where there is a probability that liquid or other condensed vapor may be trapped within enclo‐ sures for control equipment or at any point in the raceway system, approved means shall be provided to prevent accumula‐ tion or to permit periodic draining of such liquid or condensed vapor.
  2. Motors and Generators. Where liquid or condensed vapor may accumulate within motors or generators, joints and conduit systems shall be arranged to minimize the entrance of liquid. If means to prevent accumulation or to permit periodic draining are necessary, such means shall be provided at the time of manufacture and shall be considered an integral part of the machine.
501.17 Process Sealing. This section shall apply to process- connected equipment, which includes, but is not limited to, canned pumps, submersible pumps, flow, pressure, tempera‐ ture, or analysis measurement instruments. A process seal is a device to prevent the migration of process fluids from the designed containment into the external electrical system. Process-connected electrical equipment that incorporates a single process seal, such as a single compression seal, diaphragm, or tube to prevent flammable or combustible fluids from entering a conduit or cable system capable of transmit‐ ting fluids, shall be provided with an additional means to miti‐ gate a single process seal failure, The additional means may include, but is not limited to, the following:
1. A suitable barrier meeting the process temperature and pressure conditions that the barrier will be subjected to upon failure of the single process seal. There shall be a vent or drain between the single process seal and the suit‐ able barrier. Indication of the single process seal failure shall be provided by visible leakage, an audible whistle, or other means of monitoring.
2. A listed Type MI cable assembly, rated at not less than 125 percent of the process pressure and not less than125 percent of the maximum process temperature (in degrees Celsius), installed between the cable or conduit and the single process seal.
3. A drain or vent located between the single process seal and a conduit or cable seal. The drain or vent shall be sufficiently sized to prevent overpressuring the conduit or cable seal above 6 in. water column (1493 Pa). Indication of the single process seal failure shall be provided by visi‐ ble leakage, an audible whistle, or other means of moni‐ toring.
4. An add-on secondary seal marked “secondary seal” and rated for the pressure and temperature conditions to which it will be subjected upon failure of the single proc‐ ess seal.
Process-connected electrical equipment that does not rely on a single process seal or is listed and marked “single seal” or “dual seal” shall not be required to be provided with an addi‐ tional means of sealing.Informational Note: For construction and testing requirements for process sealing for listed and marked single seal, dual seal, or secondary seal equipment, refer to ANSI/ISA-12.27.01-2011, Requirements for Process Sealing Between Electrical Systems and Flam‐ mable or Combustible Process Fluids.501.20 Conductor Insulation, Class I, Divisions 1 and 2. Where condensed vapors or liquids may collect on, or come in contact with, the insulation on conductors, such insulation shall be of a type identified for use under such conditions; or the insulation shall be protected by a sheath of lead or by other approved means.501.25 Uninsulated Exposed Parts, Class I, Divisions 1 and 2. There shall be no uninsulated exposed parts, such as electrical conductors, buses, terminals, or components, that operate at more than 30 volts (15 volts in wet locations). These parts shall additionally be protected by a protection technique according to 500.7(E), (F), or (G) that is suitable for the location.501.30 Grounding and Bonding, Class I, Divisions 1 and 2. Regardless of the voltage of the electrical system, wiring and equipment in Class I, Division 1 and 2 locations shall be groun‐ ded as specified in Article 250 and in accordance with the requirements of 501.30(A) and (B).
1. Bonding. The locknut-bushing and double-locknut types of contacts shall not be depended on for bonding purposes, but bonding jumpers with proper fittings or other approved means of bonding shall be used. Such means of bonding shall apply to all intervening raceways, fittings, boxes, enclosures, and so forth between Class I locations and the point of ground‐ ing for service equipment or point of grounding of a separately derived system.Exception: The specific bonding means shall be required only to the nearest point where the grounded circuit conductor and the grounding electrode are connected together on the line side of the building or struc‐ ture disconnecting means as specified in 250.32(B), provided the branch-circuit overcurrent protection is located on the load side of the disconnecting means.
2. Types of Equipment Grounding Conductors. Flexible metal conduit and liquidtight flexible metal conduit shall include an equipment bonding jumper of the wire type in compliance with 250.102.
Exception: In Class I, Division 2 locations, the bonding jumper shall be permitted to be deleted where all of the following conditions are met:(1) Listed liquidtight flexible metal conduit 1.8 m (6 ft) or less infor Class I, Division 1 locations. Surge-protective capacitors shall be of a type designed for specific duty.(B) Class I, Division 2. Surge arresters and surge-protective devices shall be nonarcing, such as metal-oxide varistor (MOV) sealed type, and surge-protective capacitors shall be of a type designed for specific duty. Enclosures shall be permitted to be of the general-purpose type. Surge protection of types other than described in this paragraph shall be installed in enclo‐ sures identified for Class I, Division 1 locations.
Part III. Equipment501.100 Transformers and Capacitors.
1. Class I, Division 1. In Class I, Division 1 locations, trans‐ formers and capacitors shall comply with 501.100(A)(1) and (A)(2).
  1. Containing Liquid That Will Burn. Transformers and capacitors containing a liquid that will burn shall be installed only in vaults that comply with 450.41 through 450.48 and with(1) through (4) as follows:
    1. There shall be no door or other communicating opening between the vault and the Division 1 location.
    2. Ample ventilation shall be provided for the continuous removal of flammable gases or vapors.
    3. Vent openings or ducts shall lead to a safe location outside of buildings.
    4. Vent ducts and openings shall be of sufficient area to relieve explosion pressures within the vault, and all portions of vent ducts within the buildings shall be of reinforced concrete construction.
    (2) Not Containing Liquid That Will Burn. Transformers and capacitors that do not contain a liquid that will burn shall be installed in vaults complying with 501.100(A)(1) or be identi‐ fied for Class I locations.
2. Class I, Division 2. In Class I, Division 2 locations, trans‐ formers shall comply with 450.21 through 450.27, and capaci‐ tors shall comply with 460.2 through 460.28.
501.105 Meters, Instruments, and Relays.
1. Class I, Division 1. In Class I, Division 1 locations, meters, instruments, and relays, including kilowatt-hour meters, instru‐ ment transformers, resistors, rectifiers, and thermionic tubes, shall be provided with enclosures identified for Class I, Division 1 locations. Enclosures for Class I, Division 1 locations include explosionproof enclosures and purged and pressurized enclo‐ sures.
  Informational Note: See NFPA 496-2013, Standard for Purged andPressurized Enclosures for Electrical Equipment.
2. Class I, Division 2. In Class I, Division 2 locations, meters, instruments, and relays shall comply with 501.105(B)(2) through (B)(6).
(2)
(3)length, with fittings listed for grounding, is used.Overcurrent protection in the circuit is limited to 10 amperes or less.The load is not a power utilization load.
1. General-Purpose Assemblies. Where an assembly is made up of components for which general-purpose enclosures are acceptable as provided in 501.105(B)(1), (B)(2), and (B)(3), a single general-purpose enclosure shall be acceptable for the501.35 Surge Protection.(A) Class I, Division 1. Surge arresters, surge-protective devi‐ ces, and capacitors shall be installed in enclosures identifiedassembly. Where such an assembly includes any of the equip‐ ment described in 501.105(B)(1), 501.105(B)(2), and 501.105(B)(3), the maximum obtainable surface temperature of any component of the assembly that exceeds 100°C shall be
  clearly and permanently indicated on the outside of the enclo‐ sure. Alternatively, equipment shall be permitted to be marked to indicate the temperature class for which it is suitable, using the temperature class (T Code) of Table 500.8(C).
2. Contacts. Switches, circuit breakers, and make-and-break contacts of pushbuttons, relays, alarm bells, and horns shall have enclosures identified for Class I, Division 1 locations in accordance with 501.105(A).
Exception: General-purpose enclosures shall be permitted if current- interrupting contacts comply with one of the following:
1. Are immersed in oil
2. Are enclosed within a chamber that is hermetically sealed against the entrance of gases or vapors
3. Are in nonincendive circuits
4. Are listed for Division 2
1. Resistors and Similar Equipment. Resistors, resistance devices, thermionic tubes, rectifiers, and similar equipment that are used in or in connection with meters, instruments, and relays shall comply with 501.105(A).Exception: General-purpose-type enclosures shall be permitted if such equipment is without make-and-break or sliding contacts [other than as provided in 501.105(B)(2)] and if the marked maximum operating temperature of any exposed surface will not exceed 80 percent of the autoignition temperature in degrees Celsius of the gas or vapor involved or has been tested and found incapable of igniting the gas or vapor. This exception shall not apply to thermionic tubes.
2. Without Make-or-Break Contacts. Transformer windings, impedance coils, solenoids, and other windings that do not incorporate sliding or make-or-break contacts shall be provided with enclosures. General-purpose-type enclosures shall be permitted.
3. Fuses. Where general-purpose enclosures are permitted in 501.105(B)(2) through (B)(4), fuses for overcurrent protec‐ tion of instrument circuits not subject to overloading in normal use shall be permitted to be mounted in general-purpose enclosures if each such fuse is preceded by a switch complying with 501.105(B)(2).
4. Connections. To facilitate replacements, process control instruments shall be permitted to be connected through flexi‐ ble cord by means of attachment plug and receptacle, provided that all of the following conditions apply:
(1) The attachment plug and receptacle are listed for use inClass I, Division 2 locations and for use with flexible cords and shall be of the locking and grounding type.Exception: A Class I, Division 2 listing shall not be required if the circuit is nonincendive field wiring.
1. Unless the attachment plug and receptacle are inter‐ locked mechanically or electrically, or otherwise designed so that they cannot be separated when the contacts are energized and the contacts cannot be energized when the plug and socket outlet are separated, a switch complying with 501.105(B)(2) is provided so that the attachment plug or receptacle is not depended on to interrupt current.Exception: The switch shall not be required if the circuit is nonincen‐ dive field wiring.•
2. The flexible cord does not exceed 900 mm (3 ft) and is of a type listed for extra-hard usage or for hard usage if protected by location, if applicable.
(4) Only necessary receptacles are provided.
(5) The circuit has a maximum current of 3 amps.

501.115 Switches, Circuit Breakers, Motor Controllers, and Fuses.

Class I, Division 1. In Class I, Division 1 locations, switches, circuit breakers, motor controllers, and fuses, includ‐ ing pushbuttons, relays, and similar devices, shall be provided with enclosures, and the enclosure in each case, together with the enclosed apparatus, shall be identified as a complete assem‐ bly for use in Class I locations.
Class I, Division 2. Switches, circuit breakers, motor controllers, and fuses in Class I, Division 2 locations shall comply with 501.115(B)(1) through (B)(4).
1. Type Required. Circuit breakers, motor controllers, and switches intended to interrupt current in the normal perform‐ ance of the function for which they are installed shall be provi‐ ded with enclosures identified for Class I, Division 1 locations in accordance with 501.105(A), unless general-purpose enclo‐ sures are provided and any of the following apply:

The interruption of current occurs within a chamber hermetically sealed against the entrance of gases and vapors.
The current make-and-break contacts are oil-immersed and of the general-purpose type having a 50-mm (2-in.) minimum immersion for power contacts and a 25-mm (1- in.) minimum immersion for control contacts.
The interruption of current occurs within an enclosure, identified for the location, and marked “Leads Factory Sealed”, or “Factory Sealed”, or “Seal not Required”, or equivalent.
The device is a solid state, switching control without contacts, where the surface temperature does not exceed 80 percent of the autoignition temperature in degrees Celsius of the gas or vapor involved.

Isolating Switches. Fused or unfused disconnect and isolating switches for transformers or capacitor banks that are not intended to interrupt current in the normal performance of the function for which they are installed shall be permitted to be installed in general-purpose enclosures.
Fuses. For the protection of motors, appliances, and lamps, other than as provided in 501.115(B)(4), standard plug or cartridge fuses shall be permitted, provided they are placed within enclosures identified for the location; or fuses shall be permitted if they are within general-purpose enclosures, and if they are of a type in which the operating element is immersed in oil or other approved liquid, or the operating element is enclosed within a chamber hermetically sealed against the entrance of gases and vapors, or the fuse is a nonindicating, filled, current-limiting type.
Fuses Internal to Luminaires. Listed cartridge fuses shall be permitted as supplementary protection within luminaires.

501.120 Control Transformers and Resistors. Transformers, impedance coils, and resistors used as, or in conjunction with, control equipment for motors, generators, and appliances shall comply with 501.120(A) and (B).

Class I, Division 1. In Class I, Division 1 locations, trans‐ formers, impedance coils, and resistors, together with any switching mechanism associated with them, shall be provided with enclosures identified for Class I, Division 1 locations in accordance with 501.105(A).
Class I, Division 2. In Class I, Division 2 locations, control transformers and resistors shall comply with 501.120(B)(1) through (B)(3).
1. Switching Mechanisms. Switching mechanisms used in conjunction with transformers, impedance coils, and resistors shall comply with 501.115(B).
2. Coils and Windings. Enclosures for windings of transform‐ ers, solenoids, or impedance coils shall be permitted to be of the general-purpose type.
3. Resistors. Resistors shall be provided with enclosures; and the assembly shall be identified for Class I locations, unless resistance is nonvariable and maximum operating temperature, in degrees Celsius, will not exceed 80 percent of the autoigni‐ tion temperature of the gas or vapor involved or the resistor has been tested and found incapable of igniting the gas or vapor.

501.125 Motors and Generators.

Class I, Division 1. In Class I, Division 1 locations, motors, generators, and other rotating electrical machinery shall be one of the following:
1. Identified for Class I, Division 1 locations
2. Of the totally enclosed type supplied with positive- pressure ventilation from a source of clean air with discharge to a safe area, so arranged to prevent energiz‐ ing of the machine until ventilation has been established and the enclosure has been purged with at least 10 volumes of air, and also arranged to automatically de- energize the equipment when the air supply fails
3. Of the totally enclosed inert gas–filled type supplied with a suitable reliable source of inert gas for pressurizing the enclosure, with devices provided to ensure a positive pres‐ sure in the enclosure and arranged to automatically de- energize the equipment when the gas supply fails
4. For machines that are for use only in industrial establish‐ ments with restricted public access, where the conditions of maintenance and supervision ensure that only quali‐ fied persons service the installation, the machine is permitted to be of a type designed to be submerged in a liquid that is flammable only when vaporized and mixed with air, or in a gas or vapor at a pressure greater than atmospheric and that is flammable only when mixed with air; and the machine is so arranged to prevent energizing it until it has been purged with the liquid or gas to exclude air, and also arranged to automatically de- energize the equipment when the supply of liquid or gas or vapor fails or the pressure is reduced to atmospheric
Totally enclosed motors of the types specified in 501.125(A)(2) or (A)(3) shall have no external surface with an operating temperature in degrees Celsius in excess of 80 percent of the autoignition temperature of the gas or vapor involved. Appropriate devices shall be provided to detect and automatically de-energize the motor or provide an adequate alarm if there is any increase in temperature of the motor beyond designed limits. Auxiliary equipment shall be of a type identified for the location in which it is installed.
Class I, Division 2. In Class I, Division 2 locations, motors, generators, and other rotating electrical machinery shall comply with (1), (2), or (3). They shall also comply with (4) and (5), if applicable.

Be identified for Class I, Division 2 locations, or
Be identified for Class I, Division 1 locations where slid‐ ing contacts, centrifugal or other types of switching mech‐ anism (including motor overcurrent, overloading, and overtemperature devices), or integral resistance devices, either while starting or while running, are employed, or
Be open or nonexplosionproof enclosed motors, such as squirrel-cage induction motors without brushes, switching mechanisms, or similar arc-producing devices that are not identified for use in a Class I, Division 2 location.
The exposed surface of space heaters used to prevent condensation of moisture during shutdown periods shall not exceed 80 percent of the autoignition temperature in degrees Celsius of the gas or vapor involved when oper‐ ated at rated voltage, and the maximum space heater surface temperature [based on a 40°C or higher marked ambient] shall be permanently marked on a visible name‐ plate mounted on the motor. Otherwise, space heaters shall be identified for Class I, Division 2 locations.

(5) A sliding contact shaft bonding device used for the

purpose of maintaining the rotor at ground potential, shall be permitted where the potential discharge energy is determined to be nonincendive for the application. The shaft bonding device shall be permitted to be installed on the inside or the outside of the motor.

Informational Note No. 1: It is important to consider the temperature of internal and external surfaces that may be exposed to the flammable atmosphere.

Informational Note No. 2: It is important to consider the risk of ignition due to currents arcing across discontinuities and over‐ heating of parts in multisection enclosures of large motors and generators. Such motors and generators may need equipotential bonding jumpers across joints in the enclosure and from enclo‐ sure to ground. Where the presence of ignitible gases or vapors is suspected, clean-air purging may be needed immediately prior to and during start-up periods.

Informational Note No. 3: For further information on the appli‐ cation of electric motors in Class I, Division 2 hazardous (classi‐ fied) locations, see IEEE 1349-2011, IEEE Guide for the Application of Electric Motors in Class I, Division 2 and Class I, Zone 2 Hazardous (Classified) Locations.

Informational Note No. 4: Reciprocating engine–driven genera‐ tors, compressors, and other equipment installed in Class I, Divi‐ sion 2 locations may present a risk of ignition of flammable materials associated with fuel, starting, compression, and so forth, due to inadvertent release or equipment malfunction by the engine ignition system and controls. For further information on the requirements for ignition systems for reciprocating engines installed in Class I, Division 2 hazardous (classified) locations, see ANSI/UL 122001-2014, General Requirements for Electrical Ignition Systems for Internal Combustion Engines in Class I, Division 2 or Zone 2, Hazardous (Classified) Locations.

Informational Note No. 5: For details of the evaluation process

to determine incendivity, refer to Annex A and Figure A1 of UL 1836–2014, Outline of Investigation for Electric Motors and Generators for Use in Class I, Division 2, Class I, Zone 2, Class II, Divi‐ sion 2 and Zone 22 Hazardous (Classified) Locations.

501.130 Luminaires. Luminaires shall comply with 501.130(A) or (B).

Class I, Division 1. In Class I, Division 1 locations, lumin‐ aires shall comply with 501.130(A)(1) through (A)(4).
1. Luminaires. Each luminaire shall be identified as a complete assembly for the Class I, Division 1 location and shall be clearly marked to indicate the maximum wattage of lamps for which it is identified. Luminaires intended for portable use shall be specifically listed as a complete assembly for that use.
2. Physical Damage. Each luminaire shall be protected against physical damage by a suitable guard or by location.
3. Pendant Luminaires. Pendant luminaires shall be suspen‐ ded by and supplied through threaded rigid metal conduit stems or threaded steel intermediate conduit stems, and threa‐ ded joints shall be provided with set-screws or other effective means to prevent loosening. For stems longer than 300 mm (12 in.), permanent and effective bracing against lateral displacement shall be provided at a level not more than 300 mm (12 in.) above the lower end of the stem, or flexibility in the form of a fitting or flexible connector identified for the Class I, Division 1 location shall be provided not more than 300 mm (12 in.) from the point of attachment to the support‐ ing box or fitting.
4. Supports. Boxes, box assemblies, or fittings used for the support of luminaires shall be identified for Class I locations.
Class I, Division 2. In Class I, Division 2 locations, lumin‐ aires shall comply with 501.130(B)(1) through (B)(6).
1. Luminaires. Where lamps are of a size or type that may, under normal operating conditions, reach surface tempera‐ tures exceeding 80 percent of the autoignition temperature in degrees Celsius of the gas or vapor involved, luminaires shall comply with 501.130(A)(1) or shall be of a type that has been tested in order to determine the marked operating tempera‐ ture or temperature class (T code).
2. Physical Damage. Luminaires shall be protected from physical damage by suitable guards or by location. Where there is danger that falling sparks or hot metal from lamps or lumin‐ aires might ignite localized concentrations of flammable vapors or gases, suitable enclosures or other effective protective means shall be provided.
3. Pendant Luminaires. Pendant luminaires shall be suspen‐ ded by threaded rigid metal conduit stems, threaded steel intermediate metal conduit stems, or other approved means. For rigid stems longer than 300 mm (12 in.), permanent and effective bracing against lateral displacement shall be provided at a level not more than 300 mm (12 in.) above the lower end of the stem, or flexibility in the form of an identified fitting or flexible connector shall be provided not more than 300 mm (12 in.) from the point of attachment to the supporting box or fitting.
4. Portable Lighting Equipment. Portable lighting equip‐ ment shall comply with 501.130(B)(1).Exception: Where portable lighting equipment is mounted on movable stands and is connected by flexible cords, as covered in 501.140, it shall be permitted to comply with 501.130(B)(1), where mounted in any position, provided that it also complies with 501.130(B)(2).
5. Switches. Switches that are a part of an assembled fixture or of an individual lampholder shall comply with 501.115(B)(1).
6. Starting Equipment. Starting and control equipment for electric-discharge lamps shall comply with 501.120(B).

Exception: A thermal protector potted into a thermally protected fluores‐ cent lamp ballast if the luminaire is identified for the location.

501.135 Utilization Equipment.

Class I, Division 1. In Class I, Division 1 locations, all utilization equipment shall be identified for Class I, Division 1 locations.
Class I, Division 2. In Class I, Division 2 locations, all utilization equipment shall comply with 501.135(B)(1) through (B)(3).
1. Heaters. Electrically heated utilization equipment shall conform with either item (1) or item (2):

The heater shall not exceed 80 percent of the autoigni‐ tion temperature in degrees Celsius of the gas or vapor involved on any surface that is exposed to the gas or vapor when continuously energized at the maximum rated ambient temperature. If a temperature controller is not provided, these conditions shall apply when the heater is operated at 120 percent of rated voltage.Exception No. 1: For motor-mounted anticondensation space heaters, see 501.125 .Exception No. 2: Where a current-limiting device is applied to the circuit serving the heater to limit the current in the heater to a value less than that required to raise the heater surface temperature to 80 percent of the autoignition temperature.
The heater shall be identified for Class I, Division 1 loca‐ tions.

Exception to (2): Electrical resistance heat tracing identified for Class I, Division 2 locations.

Motors. Motors of motor-driven utilization equipment shall comply with 501.125(B).
Switches, Circuit Breakers, and Fuses. Switches, circuit breakers, and fuses shall comply with 501.115(B).

501.140 Flexible Cords, Class I, Divisions 1 and 2.

Permitted Uses. Flexible cord shall be permitted:
1. For connection between portable lighting equipment or other portable utilization equipment and the fixed portion of their supply circuit. The flexible cord shall be attached to the utilization equipment with a cord connec‐ tor listed for the protection technique of the equipment wiring compartment. An attachment plug in accordance with 501.140(B)(4) shall be employed.
2. For that portion of the circuit where the fixed wiring methods of 501.10(A) cannot provide the necessary degree of movement for fixed and mobile electrical utilization equipment, and the flexible cord is protected by location or by a suitable guard from damage and only in an industrial establishment where conditions of main‐ tenance and engineering supervision ensure that only qualified persons install and service the installation.
3. For electric submersible pumps with means for removal without entering the wet-pit. The extension of the flexible cord within a suitable raceway between the wet-pit and the power source shall be permitted.
4. For electric mixers intended for travel into and out of open-type mixing tanks or vats.
5. For temporary portable assemblies consisting of recepta‐ cles, switches, and other devices that are not considered portable utilization equipment but are individually listed for the location.
Installation. Where flexible cords are used, the cords shall comply with all of the following:

Be of a type listed for extra-hard usage
Contain, in addition to the conductors of the circuit, an equipment grounding conductor complying with 400.23
Be supported by clamps or by other suitable means in such a manner that there is no tension on the terminal connections
In Division 1 locations or in Division 2 locations where the boxes, fittings, or enclosures are required to be explo‐ sionproof, the cord shall be terminated with a cord connector or attachment plug listed for the location or a listed cord connector installed with a seal listed for the location. In Division 2 locations where explosionproof equipment is not required, the cord shall be terminated with a listed cord connector or listed attachment plug.
Be of continuous length. Where 501.140(A)(5) is applied, cords shall be of continuous length from the power source to the temporary portable assembly and from the temporary portable assembly to the utilization equip‐ ment.

Informational Note: See 501.20 for flexible cords exposed to liquids having a deleterious effect on the conductor insulation.

501.145 Receptacles and Attachment Plugs, Class I, Divisions 1 and 2.

Receptacles. Receptacles shall be part of the premises wiring, except as permitted by 501.140(A).
Attachment Plugs. Attachment plugs shall be of the type providing for connection to the equipment grounding conduc‐ tor of a permitted flexible cord and shall be identified for the location.

•

501.150 Signaling, Alarm, Remote-Control, and Communica‐

tions Systems.

Class I, Division 1. In Class I, Division 1 locations, all apparatus and equipment of signaling, alarm, remote-control, and communications systems, regardless of voltage, shall be identified for Class I, Division 1 locations, and all wiring shall comply with 501.10(A), 501.15(A), and 501.15(C).
Class I, Division 2. In Class I, Division 2 locations, signal‐ ing, alarm, remote-control, and communications systems shall comply with 501.150(B)(1) through (B)(4).
1. Contacts. Switches, circuit breakers, and make-and-break contacts of pushbuttons, relays, alarm bells, and horns shall have enclosures identified for Class I, Division 1 locations in accordance with 501.105(A).

Exception: General-purpose enclosures shall be permitted if current- interrupting contacts are one of the following:

Immersed in oil
Enclosed within a chamber hermetically sealed against the entrance of gases or vapors
In nonincendive circuits
Part of a listed nonincendive component

Resistors and Similar Equipment. Resistors, resistance devices, thermionic tubes, rectifiers, and similar equipment shall comply with 501.105(B)(3) .
Protectors. Enclosures shall be provided for lightning protective devices and for fuses. Such enclosures shall be permitted to be of the general-purpose type.
Wiring and Sealing. All wiring shall comply with 501.10(B), 501.15(B), and 501.15(C).

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