If an area contains ignitable gases, dusts, or other ignitable fibers or flyings, how do you install the wiring to avoid an explosion? Chapter 5, which covers special occupancies, is the first of three NEC chapters that deals with special topics. What exactly is a “Special Occupancy?” It’s a location where the facility or use of the physical facility creates specific conditions that require additional measures to ensure the “practical safeguarding of people and property” purpose of the NEC, as put forth in Art. 90 (see SIDEBAR: How Hazardous Is It?).

Word Changes

Electrical tradespeople and other professionals in the electrical industry seldom say “explosionproof apparatus,” so the 2011 revision changed “apparatus” to “equipment.” The term “equipment” is a term defined by the NEC and is therefore more usable than the undefined term “apparatus.”

The 2011 NEC includes a new definition for combustible dust [500.2]. It’s finely divided solid material that’s 420 microns or smaller in diameter and presents a fire or explosion hazard when dispersed and ignited in air. This new definition may help us understand the difference between Class II and Class III locations. Class II locations are where fire or explosion hazards may exist due to combustible dusts and the necessary wiring methods are covered in Art. 502. In Class III locations, the hazard may exist due to ignitable fibers/flyings, which are larger particles than those that the Code defines as being dust, and the pertinent wiring methods are covered in Art. 503.

Previous editions of the NEC used the term “fiber optic” cables in 500.8(F) instead of the more accurate term “optical fiber cable.” The 2011 revision uses the more accurate term (Fig. 1).

Additionally, the term “composite optical fiber cable” was added to help the Code user understand which types of optical fiber cable are required to comply with this section. While this change isn’t a technical one, it does add usability of the NEC for those users that struggle with the differences between conductive, composite, and nonconductive optical fiber cables (see SIDEBAR: Optical Fiber Cable is Not Created Equal).

Conduit Threads and Fittings

The 2011 NEC includes extensive changes to the provisions addressing conduit threads and fittings.

All threaded conduits must be made wrenchtight to prevent arcing when ground-fault current flows through the raceway system and to ensure the explosionproof or dust-ignitionproof integrity of the raceway system [500.8(E)] (see SIDEBAR: The Rules for Threaded Conduit [500.8(E)]). Threaded entries into explosionproof equipment must be made up with at least five threads fully engaged [500.8(E)(1)]. An Exception to this rule notes: For listed explosionproof equipment, factory threaded entries must be made up with at least 4½ threads fully engaged.

The 2008 NEC did a lackluster job of dealing with metric threaded conduits and fittings. Manufacturers had little guidance on what the requirements were for metric threads, unlike NPT (National Pipe Taper, American Standard) threads that had more information in this subsection. Of particular interest is a requirement for metric threads to have eight threads fully engaged for Class I, groups A and B. This requirement doesn’t apply to NPT threaded raceways.

A new subsection, 500.8.E(3), addresses unused openings. While it’s a repetition of 110.12(A), it’s here to emphasize that these fittings must have five threads fully engaged (unless metric in a group A or B), as shown in Fig. 2.

Class I

If sufficient flammable or combustible gases, vapors, or liquids are (or may be) present to produce an explosive or ignitable mixture, you have a Class I location. Examples of such locations include fuel storage areas, certain solvent storage areas, grain processing (where hexane is used), plastic extrusion where oil removal is part of the process, refineries, and paint storage areas.

Article 500 contains a general background on hazardous locations, and it describes the differences between Class I, II, and III locations plus the differences between Division 1 and Division 2 in each of the three classifications.

Article 501 contains the actual Class I, Division 1 and Division 2 installation requirements, including wiring methods, seals, and specific equipment requirements.

New Cross References for Class I

The 2011 revision adds cross references for types MC cable [501.10(A)(1)(c)] and ITC-ER (ITC was previously included) cable [501.10(B)(1)(4)]. Note that 501.10(A) is for Class I, Division 1 and 501.10(B) is for Class I, Division 2. Why the new cross references?

Installations consisting of Types MC or ITC cable in a hazardous location must comply with Art. 330 and Art. 727, respectively. While 90.3 already tells us this quite plainly for MC cable (Art. 330), it doesn’t give the same guidance for a wiring method that’s described in Chapter 7, such as ITC cable (Art. 727). Section 727.4 specifically allows ITC cables to be used in these locations, so this change is a good way to make the Code more user-friendly.

The 2011 revision now includes a listing requirement for flexible cord connectors and fittings [501.10(A)(2)] and 501.10(B)(2)]

Unlike most hazardous location equipment, flexible fittings were not previously required to be listed for the location. The 2011 NEC now specifically states flexible fittings must be listed for the location in Class I, Division 1 locations [501.10(A)(2)] and must be listed for use in Class I, Division 2 locations [501.10(B)(2)]

Bonding of Flexible Raceway

Previous editions of the NEC stated that, in a hazardous location, “liquidtight flexible metal conduit shall not be used as the sole ground-fault current path.” While this sounds like a great rule, it really doesn’t work. Why is that? Because of the definition of “ground-fault current path” in 250.2.

Many things are “ground-fault current paths,” including the earth itself, although paths such as the earth certainly don’t achieve the goal of this requirement. While the term “effective ground-fault current path” is better than the previous NEC language, the 2011 revision makes it even simpler than that. It now tells you exactly what to do: Use a green or bare wire in (or next to) these flexible wiring methods. Section 501.30(B) has been revised to add clarity to the requirement.

Because of the explosive conditions associated with electrical installations in hazardous locations [500.5], electrical continuity of metal parts of equipment and raceways must be ensured by one of the following methods, whether or not you install an equipment grounding conductor of the wire type within the metal raceway [250.100]:

  • Threaded couplings or threaded entries on enclosures [250.92(B)(2)].
  • Threadless raceway couplings and connectors [250.92(B)(3)].
  • Bonding the raceway to an enclosure with listed devices, such as a bonding-type locknut, bonding wedge, or bushing with a bonding jumper [250.92(B)(4)].

Standard locknuts alone aren’t suitable for this purpose. 501.30(B) Bonding — Flexible Raceway. Liquidtight flexible metal conduit must have an equipment bonding jumper of the wire type in accordance with 250.102 (Fig. 3).

Note that you need to size bonding jumpers per Table 250.122, based on the rating of the overcurrent device [250.102(D)]. Where installed outside of a raceway, the length of bonding jumpers must not exceed 6 ft — and they must be routed with the raceway [250.102(E)(2)].

Flexible Cords

Manufacturers are often asked to provide portable power carts (type of temporary portable assembly), consisting of such items as receptacles, switches, and similar devices, for Class I locations. These carts are used during maintenance activities in areas such as refineries. The previous Code language didn’t include a provision for this type of equipment to be supplied by a cord and plug connection, due to the limitations placed on flexible cord usage. Equipment such as this is now allowed, provided the individual components of the cart (or similar equipment) are listed for the application [501.140(A)(5)].

While previously the NEC was clear on the topic of requiring seals on flexible cords in Class I locations, it didn’t address cord connectors and attachment plugs. Even though it seems obvious that a cord connector/attachment plug in a Class I location would have to be listed for the application, the Code didn’t clearly state this requirement. Now 501.140(B)(4) makes it clear that such a cord connector or attachment plug must be listed for the location in which it’s installed. The change to 501.140(B)(5) is a correlating change to the one that occurred in 501.140(A)(5). When the temporary portable assemblies discussed in that section are used, the cord must be a single, continuous length.

Classes I, II, and III
Changes in Art. 501 have similar counterparts in Art. 502 (Class II) and Art. 503 (Class III). For example, the cross reference changes in 501.10 also show up in 502.10 and 503.10. But don’t just jump into any of these three articles. First, take the time to understand the relevant definitions and concepts in Art. 500.
Article 500 is fairly long and detailed, and it contains many Informational Notes. Thus, it can seem a bit much to digest. However, not all of what’s in there is relevant to the Class you’re working in. If, for example, you’re working in a Class I location, you’ll be referring to Art. 501 and only the relevant parts of Art. 500.   

Holt is the owner of Mike Holt Enterprises, Inc., Leesburg, Fla. He can be reached at www.mikeholt.com.

SIDEBAR: How Hazardous is It?

A hazardous location is an area where the possibility of fire or explosion can exist due to the presence of flammable or combustible gases or vapors, combustible dusts, or ignitable fibers/flyings. Electric arcs, sparks, and/or heated surfaces can serve as a source of ignition in such environments.

Article 500 is the first of seven articles that address hazardous locations. It’s the basis for the other six. Articles 501, 502, and 503 provide the requirements for Class I, Class II, and Class III locations, respectively. The class designations are defined by 500.5. Article 504 provides the requirements for “intrinsically safe” equipment used in Class I, II, and III locations. Articles 510 through 517 cover specific occupancies, garages, aircraft hangers, motor fuel dispensing facilities, bulk storage plants, spray application, dipping and coating processes, and health care facilities [500.9].

SIDEBAR: The Rules for Threaded Conduit [500.8(E)]

This requirement ensures that if an explosion occurs within a raceway or enclosure, the expanding gas will sufficiently cool as it dissipates through the threads. This prevents hot flaming gases from igniting the surrounding atmosphere of a hazardous location.

Keep in mind — it’s assumed that the flammable atmosphere outside the raceway will seep into the raceway system over time. The goal of the Code is to contain any explosion that occurs inside the raceway so the event won’t ignite the flammable mixture outside the raceway system.

SIDEBAR: Optical Fiber Cable is Not Created Equal

Composite optical fiber cable contains current-carrying conductors [770.2], and they’re capable of causing a dangerous spark that can ignite flammable atmospheres. In a hazardous location, you must install them the same way you install other conductors.

On the other hand, nonconductive optical fiber cable [770.2] has no current-carrying conductors or metallic strength members, so it can’t ignite flammable atmospheres. Consequently, you don’t have to install it in accordance with Articles 500, 501, 502, or 503.