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Article 503: Class III Hazardous Locations

Article 503: Class III Hazardous Locations

If there’s risk of fire or explosion due to the presence of combustible fibers, apply Art. 503.

Article 503 gives you the option of using the Zone system (Art. 506) instead of the Division system [503.6]. Article 506 applies to Class II and Class III locations; we’ll discuss it in a future installment.

Article 501 and Art. 502 present many requirements with a Division-based pattern, but Art. 503 does this only once (for the wiring methods). Mostly, the requirements are the same, regardless of Division.

Wiring methods

The requirements for Class III wiring methods are the same whether it is Div. 1 or Div. 2 [503.10(B)], but there is an exception. In Div. 2, you can run open wiring on insulators if the wiring is:

• Only in sections, compartments, or areas used solely for storage and containing no machinery.

• Installed per Art. 398.

• Protected per 398.15(C) where conductors are not run in roof spaces.

• Well out of reach of sources of physical damage.

Photo. This paper machine is a prime example of a Class III location, due to the presence of pulp fibers.

Other than this exception, the wiring methods for Class III locations must comply with the following.

In Class III locations, you can use:

• These types of raceway: rigid metal conduit (RMC), intermediate metal conduit (IMC), electrical metallic tubing (EMT), PVC, Type RTRC, and dusttight wireways. You can also use Types MC and MI cables [503.10(A)(1)].

• Type PLTC and Type PLTC-ER cable (installed per Art. 725).

• Type ITC and Type ITC-ER cable (as permitted by Art. 727.4).

• Type MC, MI, or TC cable in cable trays (under specific conditions).

• Boxes and fittings, but only if they are dusttight.

• Flexible connections that comply with 503.10(A)(3).

• Nonincendive field wiring installed per the control drawings and complying with the requirements for unclassified locations.

Sealing requirements

Nearly half of Art. 501 is devoted to sealing requirements, and Art. 502 contains about a quarter page of those requirements. There are no such requirements for Class III areas. However, from an engineering standpoint, strategic use of seals often makes sense and can lower total cost of ownership.

Spark prevention

Like Art. 501 and Art. 502, Art. 503 provides requirements to prevent ignition, and the first requirement is “no uninsulated exposed parts operating at over 30V” (15V for wet locations) [503.25].

As in Art. 501 and Art. 502, Sec. 25 mixes grounding in with bonding. In all three cases, a quick visit to Art. 100 helps clear this up. It’s critical you don’t ground where you should bond.

Unlike Art. 501 and Art. 502, Art. 503 makes no mention of surge protection, nor does it impose limits on multiwire branch circuits.


As with Art. 501, Art. 502 provides extensive requirements for transformers, but Art. 502 requirements are less rigorous, because you’re dealing with dust rather than gases.

Transformers in Class III locations must be treated as if they are in Class II, Div. 1 locations. That is, whether Div. 1 or Div. 2, they must comply with 502.100(B):

• If a transformer contains a liquid that will burn, then its vault must comply with 450.41 through 450.48.

• If a transformer rated over 25kVA contains askarel, it must have pressure-relief vents. It must also have a means of absorbing gases created by arcing — or be pressure-relief vented to a chimney or flue that carries the gases outdoors. Finally, the airspace between the transformer case and any combustible material must be at least 6 in.

• Dry-type transformers must be in vaults, unless their windings and terminal connections are enclosed in tight metal housings with no vents or other openings, and they can’t operate at more than 600V [502.100(A)].


You must provide dusttight enclosures for switches, circuit breakers, motor controllers, and fuses. This rule also applies to pushbuttons, relays, and similar devices [503.115].

Control transformers and resistors must also be in dusttight enclosures, and the enclosures must comply with the temperature limitations of 503.5. Basically, if the equipment:

• Is not subject to overload, the maximum surface temperature cannot exceed 329°F.

• May be overloaded (e.g., it’s a motor or power transformer), the maximum surface temperature cannot exceed 248°F.


Motors and generators used in Class III locations must be totally enclosed pipe-ventilated [503.125]. They don’t have to meet temperature limitations, as do those in Class II locations. Further, Art. 503 provides three exceptions to this requirement:

1) Self-cleaning textile motors of the squirrel cage type.

2) Standard open-type machines without switching mechanisms.

3) Standard open-type machines with switching mechanisms, if those are enclosed in tight housings without openings.

The ventilating piping requirements of Art. 503 are similar to those for Art. 502. But in Class III locations, the requirements are the same, regardless of Division. As with Class II locations, the vent pipes for enclosures and rotating electrical machinery in Class III locations must be metal and lead directly to outside air [502.128]. Like Class II, this pipe needs a bug screen.

The pipes must be sufficiently tight to keep appreciable amounts of flying fibers from entering. They should also prevent the escape of ignition sources, such as sparks.

Miscellaneous equipment

Heaters must be identified for Class III locations [503.135].

The rules for Class III luminaires, receptacles, and cranes are found in 503.130, 503.145, and 503.155, respectively. These sections require you to minimize the entry of combustible fibers/flyings and prevent the escape of combustibles. For each type of equipment, you’ll find additional requirements.

Flexible cords used in Class III locations must be listed for extra-hard usage [503.140]. These cords must also contain an equipment grounding conductor, be supported to eliminate tension on the terminal connections, and be terminated with a listed dusttight cord connector. Yes, that connector must be dusttight even though this isn’t a Class II location.

If you install a battery backup system, locate the batteries in separate rooms built with (or lined with) substantial noncombustible materials. This would, for example, preclude a woodframe room with drywall panels. But a cinderblock room is acceptable.

Ensure the battery room is constructed to prevent the entrance of ignitable amounts of flyings. It also must be well ventilated [503.16], whether in a Class III location or not [480.9]. Complying with both the ventilation and “keep the fibers out” requirements may call for some extensive ducting. Also, keep in mind that you must locate the intakes such that fibers won’t continually clog up their filters.

The trio compared

The three NEC articles providing requirements for Class I, II, and III locations have similarities and differences. One way they differ is the requirements decrease in size and number as you progress from Art. 501 to Art. 502 to Art. 503 (see the Table).

Table. This table helps show how Art. 501, 502, and 503 requirements compare. The requirements decrease as you go from Art. 501 to 502 to 503. Art. 501 and 502 don’t have Sec. 155 or 160, thus these are “n/a” in the table. See 610.3 for crane requirements. Storage batteries can’t conform to 480.9 and be in Class I and Class II locations, nor do you want such ignition sources in such locations; the solution is a battery room.

But why aren’t the requirements just different, rather than become dramatically less as you go from 501 to 502 to 503? It’s because of the physics involved. Gases (Art. 501) are very tiny particles compared to dust (Art. 502), and dust particles are smaller than the fibers/flyings of Art. 503. The smaller the particle size, the harder the particles are to control, contain, or keep out.

This particle size difference is why, for example, Art. 502 has you using fittings, and Art. 503 does not. It’s why Art. 501 and Art. 502 have transformer requirements, but Art. 503 doesn’t.

These Articles are similar in that they have the same structure, and a given subsection in one Article covers the same topic as its counterpart in the other two. Another similarity is before you apply one of these Articles, you first apply Art. 500.

In addition to providing requirements for controlling ignitable particles, these Articles provide requirements for controlling or eliminating ignition sources. For example, the bonding requirements are the same for all three Articles. What they prescribe is a practice you should follow for all equipment, whether in a classified location or not. Eliminate dangerous differences of potential by bonding all non-current carrying metallic objects.

Class III caution

Article 503 differs from the other two in a very important way. It often makes financial sense to classify a location based on likely use, not just intended use. Instead of trying to minimize the hazardous location size to save money on construction and maintenance, you expand it to save money on operations.

Suppose a company stores its fibrous product in bales. You’re doing the electrical work on a new warehouse. The client tells you they’ll be storing this product on the west end only. But the warehouse is a simple structure, and moving the product to the east end is easy. There’s no physical reason lift truck operators can’t routinely take product across the warehouse to trucks waiting on the east end.

If you classify the whole warehouse as Class III, rather than classifying only the west end, this small investment frees operations managers to use the building as needed. They don’t incur operational hassles and costs simply because receptacles specified for the west end weren’t also specified for the east end.            

Lamendola is an electrical consultant located in Merriam, Kan. He can be reached at [email protected].


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