Article 501 Class I Locations

I. General

501.1	Scope
501.5	General

This article refers to those areas classified as Class I in Section 500.5.

II. Wiring

501.10

Wiring Methods

(A) Class I, Division 1. Only threaded rigid metal conduit, threaded steel intermediate conduit, or Type MI cable can be used. Boxes and fittings must be approved for Class I, Division 1. At least five threads must be engaged. MI cable has to be installed to prevent stress at the terminations. There is one exception. There are also requirements for industrial establishments with the use of MC-HC and ITC-HC cable.

(B) Class I, Division 2. Methods permitted are threaded rigid metal conduit; threaded steel intermediate metal conduit; enclosed gasketed busways; enclosed gasketed wire-ways; Type MI, MC, MV, or TC cable with approved terminations; Type PLTC cable; type ITC cable as permitted in 727.4 and all methods permitted in 501.10(A). Explosion-proof boxes or fittings are not required. Additional requirements and exceptions are noted.

501.15

Sealing and Drainage

Seals must be provided and approved items must be used.

(A) Conduit Seals, Class I, Division 1

1. A seal must be placed in each conduit run which enters the enclosure for a switch, circuit breaker, relay, resistor, fuse, or any item that can cause arcing, sparking, or high temperatures or the entry is metric designator 53 (trade size 2) or larger and the enclosure contains terminals, splices, or taps. The seal must be within 450 mm (18 in.) of the enclosure. There is a listing of the enclosures or fittings that are allowed between the seal fitting and enclosure. The conduit bodies cannot be larger than the largest trade size of the conduits.
2. If the enclosure is pressurized and the conduit is not, the seals must be installed within 450 mm (18 in.) of the pressurized enclosure.
3. If a run of conduit between two enclosures is not more than 914 mm (36 in.), one seal not more than 450 mm (18 in.) from an enclosure is acceptable.
4. A seal is required where a conduit run leaves a Class I, Division 1 location. The sealing fitting be within 3.05 m (10 ft.) of the boundary on either side. The conduit must not have a fitting, union, coupling, or box between the seal and point of leaving the Class I, Division 1 location. There are exceptions.

(B) Conduit Seals, Class I, Division 2

1. Seals are required for connection to explosion-proof enclosures and must comply with 501.15(A)(1) through (A)(3) above.
2. A seal is required when the conduit run leaves this area to an unclassified area. This paragraph describes additional requirements. There are four exceptions.

(C) Class I, Divisions 1 and 2. These are general rules.

1. The fittings must be listed for the location used.
2. The compound must provide a proper seal.
3. The thickness of the compound must be at least the trade size of the conduit and not less than 16 mm (5/8 in.).
4. Splices and taps cannot be made in fittings that are used for sealing.
5. Assemblies must be identified for the locations.
6. Conductor cross-sectional area in a seal cannot be more than 25% of a conduit of the same size unless identified for a higher fill.

(D) Cable Seals, Class I, Division 1

(E) Cable Seals, Class I, Division 2

(F) Drainage. In certain cases drainage of accumulated liquid is permitted and even required. Special requirements must be followed.

501.20

Conductor Insulation, Class I, Divisions 1 and 2

If the conductors come in contact with condensed vapors or liquids, the insulation must be identified for this use or have protection provided by a lead sheath or another approved sheath.

501.25

Uninsulated Exposed Parts, Class I, Divisions 1 and 2

There can be no uninsulated exposed parts such as conductors, buses, etc., that operate at more than 30 volts. This limit is reduced to 15 volts in wet locations. The protection must be in accordance with 500.7(E), 500.7(F), or 500.7(G).

501.30

Grounding and Bonding, Class I, Divisions 1 and 2

Wiring and equipment must be grounded in accordance with Article 250. Bonding jumpers with proper fitting or other approved means of bonding must be used. Locknut bushings and double locknut types of contacts are not acceptable. The means of bonding applies to all raceways, fittings, boxes, enclosures, etc., between Class I locations and the grounding point for service equipment or separately derived systems. If flexible conduit is used it must have internal or external bonding jumpers in parallel with each conduit. It must also comply with the requirements of Section 250.120 where flexible metal or liquidtight flexible metal conduit is used. There are exceptions.

501.35	Surge Protection
501.40	Multiwire Branch Circuits

 

III. Equipment

501.100

Transformers and Capacitors

(A) Class I, Division 1

1. Containing liquid that will burn. They must be installed in a vault complying with Sections 450.41 through 450.48 with no door or opening to the Class I, Division 1 locations, with proper ventilation and the vents going outside the buildings and being large enough to relieve an explosion. The ducts for the vents must be constructed of reinforced concrete.
2. Not containing liquid that will burn. These must be installed in a vault as noted in (a)(1) or be approved for a Class I location.

(B) Class I, Division 2. In these locations they must be in accordance with Sections 450.21 through 450.27.

501.105

Meters, Instruments, and Relays

(A) Class I, Division 1. They must be in Class I, Division 1 enclosures.

(B) Class I, Division 2

1. Contacts. Make-or-break contacts, switches, circuit breakers, etc., must have identified Class I, Division 1, enclosure in accordance with 501.105(A). Four exceptions are: immersed in oil, in a hermetically sealed chamber, in nonincendive circuits, and where part of a listed nonincendive component.
2. Resistors and similar equipment. These must comply with Section 501.105(A). An exception is made if the exposed surface does not exceed 80% of the ignition temperature and there are no make-or-break or sliding contacts. The exception does not apply to thermionic tubes.
3. Without make-or-break contacts. General-purpose enclosures can be used for this category for transformer windings, impedence coils, solenoids, and other windings.
4. General-purpose assemblies. If an assembly has items that can use general-purpose enclosures [noted in 501.105(B)(1), (B)(2), and (B)(3)], a single, general-purpose enclosure can be used. If equipment noted in 501.105(B)(2) is used, the maximum surface temperature must be noted.
5. Fuses. Fuses for instrument circuits not subject to overloading can be mounted in general-purpose enclosures if approved in 501.105(B)(1) through (B)(4), and if they are preceded by a switch in accordance with 501.105(B)(1).
6. Connections. Process control can use a cord, plug, and receptacle if (a) a switch is complying with 501.105(B)(1) is used so that the plug is not depended upon to interrupt current; (b) current is not greater than 3 A at 120 V; (c) cord is not longer than 900 mm (3 ft.), is rated for extra hard usage or hard usage depending on location, and uses locking and grounding-type plug and receptacle; only necessary receptacles are provided; and (e) a warning is posted against unplugging under load.

501.115

Switches, Circuit Breakers, Motor Controllers, and Fuses

(A) Class I, Division 1. Must be in an enclosure and the entire assembly must be identified for Class I locations.

(B) Class I, Division 2

1. Must be in enclosures identified for Class I, Division 1, locations unless a general-purpose enclosure is used and it is hermetically sealed, or has oil-immersed make-or-break contacts with special requirements, the device is solid state without contacts, or current interruption occurs in a factory-sealed explosion-proof chamber approved for the location.
2. If not intended to interrupt current, a general-purpose enclosure can be used with a disconnecting or isolating switch for transformers or capacitor banks.
3. Standard plug or cartridge fuses can be used in certain cases, but they must have enclosures identified for the location. If they are oil immersed, they can be placed in general-purpose enclosures.
4. Within luminaires (lighting fixtures), listed cartridge fuses are permitted as supplementary protection.

501.120

Control Transformers and Resistors

Transformers, impedance coils, and resistors are covered by this section.

(A) Class I, Division 1. In this location they must have enclosures identified for Class I, Division 1.

(B) Class I, Division 2. In this location switching mechanisms must be in accordance with Section 501.6(B). General-purpose enclosures can be used for coils and windings. Resistors must have enclosures identified for Class I locations. If the resistor is fixed and the maximum operating temperature is not more than 80% of the ignition temperature of the gas or vapor, a general-purpose enclosure can be used.

501.125

Motors and Generators

(A) Class I, Division 1. In this location they must be either identified for Class I, Division 1 locations or be totally enclosed with a positive pressure ventilation (the arrangement being that the device cannot start until the ventilation has started and 10 volumes of air have been purged from the enclosure) or be totally enclosed filled with inert gas. There is also a requirement for the type submerged in a liquid.

(B) Class I, Division 2. In this location you can have open or nonexplosion-proof enclosed motors if they do not have brushes, switching mechanisms, or arc-producing devices. If they do have brushes, switching mechanisms, or arc-producing devices, the motor or generator must be identified for Class I, Division 1, locations or the devices must be in Class I, Division 1 identified enclosures as per Section 501.105(B). A requirement is noted for the surface temperature of space heaters used to prevent condensation. There are other specific requirements.

501.130

Luminaires (Lighting Fixtures)

(A) Class I, Division 1. The entire fixture assembly must be identified for a Class I, Division 1 location and be protected against physical damage. Pendant luminaires (fixtures) must be wired and hung through rigid metal conduit or threaded steel intermediate conduit. Stems longer than 300 mm (12 in.) must have additional lateral bracing not more than 300 mm (12 in.) from the end of the stem or a flexible fitting approved for a Class I, Division 1 location. All boxes, fittings, etc., must be identified for Class I, Division 1 locations.

(B) Class I, Division 2. Portable lighting equipment must comply with Class I, Division 1 requirements. There is an exception when it is mounted on a moveable stand and connected by flexible cord. Fixed lighting must be protected against physical damage. If there is danger of falling sparks or hot metal igniting vapors or gases, proper enclosures must be provided. If the operating temperature will exceed 80% of the ignition temperature of the gases or vapors, they must comply with Section 501.130(A)(1) or be of the type that has been tested to determine the temperature range or operating temperature marked. Pendant luminaires (fixtures) have the same rules as Class I, Division 1 locations. Switches must be in accordance with Section 501.6(B)(1). Starting equipment for electric-discharge lamps must be in accordance with Section 501.7(B) unless it is part of a "thermally protected fluorescent lamp ballast" and the lighting fixture has been identified for this location.

501.135	Utilization Equipment
501.140	Flexible Cords, Class I, Divisions 1 and 2

These are permitted only to connect the portable equipment to the fixed part of the system. They can also be used under certain conditions where a greater degree of movement is needed. As such, they must be identified for extra hard usage, contain a grounding conductor, be connected to the supply conductors or terminals in an approved way, ensure that there is no tension applied, and be provided with seals where necessary. Exceptions are made in Sections 501.10(B) and 501.105(B)(6). There is additional information concerning electrical submersible pumps and electric mixers.

501.145

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

Must be of the grounding type and approved for Class I locations except as provided in Section 501.105(B)(6).

501.150

Signaling, Alarm, Remote-Control, and Communication Systems