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NEC Chapter 5 Review

By Mike Holt


Chapter 5, which covers special occupancies, is the first of four chapters that deal with special topics. Chapters 6, 7 and 8 cover special equipment, special conditions, and communications systems, respectively. Remember, the first four Chapters of the NEC are sequential and form a foundation for each of the subsequent four Chapters.

What exactly is a "special occupancy"? It's a location where the physical facility or use of the physical facility creates specific conditions that require additional measures to ensure the "safeguarding of people and property" mission of the NEC put forth in Article 90.

The NEC groups these logically, as you might expect. Here are the general groupings:

  • Environments that pose additional hazards. Articles 500 - 510. Examples include Class II Locations and Class III Locations.
  • Specific types of facilities that pose additional hazards. Articles 511 - 516. Examples include motor fuel dispensing facilities, aircraft hangars and bulk storage plants.
  • Facilities that pose evacuation difficulties. Articles 517 - 525. Examples include hospitals, theaters, and carnivals.
  • Temporary installations. Article 527.
  • Motion picture-related. Articles 530 and 540.
  • Specific types of buildings. Articles 545 - 553. Examples include park trailers and floating buildings.
  • Marinas and boatyards. Article 555.

Most people struggle to understand the requirements for Special Occupancies, mostly because of the narrowness of application. However, if you study the illustrations and explanations here, you will clearly understand them.

Article 500. Hazardous (Classified) Locations

A hazardous location is an area where the possibility of fire or explosion can be created by the presence of flammable liquids or gases, combustible dusts or ignitable fibers or flyings. Sparks and/or heated surfaces can serve as a source of ignition in such environments.

Article 500 provides a foundation for applying Articles 501 (Class I Locations), 502 (Class II Locations), 503 (Class III Locations), and 504 (Intrinsically Safe Systems)-all of which immediately follow Article 500. It also provides a foundation for applying Articles 510 - 516.

Before you apply any of the Articles just mentioned, you must understand and apply Article 500. It's a fairly long Article, and it's detailed. But don't worry. We will help you master the concepts.

Article 501. Class I Locations

If enough flammable liquids or gases are 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.

One reason so many people have difficulty with Article 501 is they don't understand how it's organized. So, before studying it, let's look at that. After the very short 501.1 (General), Article 501 has these groupings:

  • General power distribution. 501.2 covers transformers and capacitors. 501.3 covers meters, instruments and relays. 501.4 covers wiring methods, and 501.5 covers sealing and drainage.
  • Controls. 501.6 covers switches, circuit breakers, motor controllers, and fuses. 501.7 covers control transformers and resistors.
  • Loads. 501.8 - 501.13 cover motors and generators, luminaires, utilization equipment, flexible cords, receptacles and attachment plugs and conductor insulation. 501.14 covers signaling, alarm, remote control and communications systems. 501.17 covers surge protection.

Article 502. Class II Locations

If an area may have enough combustible dust suspended in the air to ignite or explode, it is classified as a Class II location. Examples of such locations include grain silos, coal bins, wood pulp storage areas and munitions plants. In Article 501, flammable liquids or gases need merely be present.

Article 502 follows a logical arrangement similar to that of Article 501. But Article 520 contains Section 502.9, which addresses ventilation piping. And the content differs. Article 502 pertains strictly to locations that contain combustible dust in the air, rather than simply the presence of such material.

Article 503. Class III Locations

The Class III location definition is cumbersome, and many people have a hard time grasping what it means. If you have easily ignitable fibers or flyings present, you may have a Class III location. The key is you can't have enough of them in the air to produce ignitable mixtures. Put another way, if you don't have enough fibers or flyings in the air to produce an ignitable mixture but they are present, then you have a Class III area. Examples of such locations include saw mills, textile mills, and fiber processing plants.

Article 503 follows a logical arrangement similar to that of Article 502. Interestingly, 503.9 contains the requirements for ventilation piping-as opposed to 502.7 in Article 502.

Article 504. Intrinsically Safe Systems

If you're really concerned about ignition in a given location, you have to go back to basic fire prevention theory. Fire requires three elements: fuel, oxygen and an ignition source. Intrinsically safe systems remove the ignition source part of the equation, due to their low energy levels. Of course, if you have fuel and oxygen, you can still have a fire because circuitry isn't the only source or potential source of ignition. However, the installation of a 504-compliant system greatly reduces the hazards.

You can use Article 504 in conjunction with any of the immediately preceding Articles. That is, you can apply Article 504 in a Class I, Class II or Class III location. The decision to apply Article 504 may come out of a formal risk assessment or it may come out of standard industry practice for a given chemical or industrial process. Prime candidates for intrinsically safe systems include grain silos, grain milling operations, coal pulverizing systems and oil refineries. Before doing any work in a hazardous location, ask about the Article 504 requirements for that location.

Article 511. Commercial Garages, Repair and Storage

To avoid misunderstandings about what a garage is, refer to the definition in Article 100. Essentially, a commercial garage is a place where people store or repair vehicles that burn volatile liquids. Examples of these liquids are gasoline, liquid propane, and alcohol. The requirement is a bit more detailed, but this is the general idea. Article 511 also draws a distinction between a parking garage and a garage used for repair or storage.

Article 511 can be confusing because it provides different rules for five different kinds of classified areas that can potentially be in the same room. This sounds more complicated than it really is. In truth, it's not so hard to keep straight. Just remember:

  • 18 inches. The area 18 inches above grade and the area 18 inches below the ceiling require special attention. In either case, the potential problem is the accumulation of vapors.
  • A pit. Flammable liquids or vapors can accumulate in any depression below floor grade.
  • Adjacent areas. If you ensure an area adjacent to a classified area meets certain ventilation requirements or you can satisfy the AHJ that the area does not present an ignition hazard, it is possible for that to be an unclassified area.

Article 513. Aircraft Hangars

This article is similar in concept to Article 511, and for good reason. Aircraft burn fuel. But the fuel they burn is very high grade. Therefore, aircraft hangars have their own Article.

As with Article 511, Article 513 can be confusing, because it provides different rules for different kinds of classified areas that can potentially be in the same room. In the case of Article 513, there are only four such areas rather than five. You can think of them as these three, because 513.3(D) is just the flip side of 513.3(B):

  • A pit. Flammable liquids or vapors can accumulate in any depression below floor grade.
  • Adjacent areas. An area adjacent to a hangar also falls under Article 513 unless certain conditions of isolation and separation are met.
  • Next to the aircraft. If it is within 5 ft of aircraft or an aircraft fuel tank, you must classify it as a Class I, Division 2 location or as a Zone 2 location.

Article 514. Motor Fuel Dispensing Facilities

If a facility where fuel is dispensed from storage tanks into the fuel tanks of vehicles, you need to look at Article 514. That facility probably must conform to Article 514 requirements.

What is most striking about Article 514 is the large table that makes up about half of it. This table doesn't provide any electrical requirements, list any electrical specifications or address any electrical equipment. What it does tell you is how to classify a motor fuel dispensing area based on the equipment contained therein. The rest of Article 514 contains specific provisions, but also refers you to other Articles that you must also apply.

Article 517. Health Care Facilities

Health care facilities differ from other facilities in many important ways. Article 517 is primarily concerned with those parts of health care facilities where patients are examined and treated. Whether those facilities are permanent or movable, they still fall under Article 517. On the other hand, Article 517 wiring and protection requirements do not apply to business offices, patient sleeping areas, and waiting rooms.

Just as Article 100 contains many definitions, so does Article 517. While you don't need to be able to quote these definitions, you should have a clear understanding of what these words mean.

As you study Part II and Part III, keep in mind the special requirements of hospitals and why these requirements exist. The requirements in Part II and Part III are highly detailed and not intuitively obvious. But we will show you the key concepts so you can easily understand and apply the requirements. These are three of the main objectives of Article 517, Part II and Part III:

  • Maximize the physical and electromagnetic protection or wiring by requiring metal raceway.
  • Minimize electrical hazards by keeping the voltage potential between patients' bodies and medical equipment low. This involves many specific steps, beginning with 517.11.
  • Minimize the negative effects of power interruptions by defining specific requirements for essential electrical systems.

Part IV addresses gas anesthesia stations. The main objective of Part IV is to prevent ignition. Part V addresses X-ray installations. Part V really has two main objectives. The first is to provide adequate ampacity and protection for the branch circuits. The second is to address the safety issues inherent in high-voltage equipment installations.

Part VI provides requirements for low-voltage communications systems, such as fire alarms and intercoms. The main objective there is to prevent compromising those systems from inductive coupling or other sources of interference.

Part VII provides requirements for isolated power systems. The main objective is to keep them actually isolated.

Article 518. Assembly Occupancies

If a building or portion of a building is specifically designed or intended for the assembly of 100 or more people, it falls under Article 518. Article 518 goes out of its way to eliminate confusion. See 518.2 for a list of examples of what 581.1 means. Article 518 recognizes that it's much harder to evacuate 100 or more people from a burning building than it is to evacuate just a few people. That concept underlies much of the reasoning behind Article 518 requirements.

While Article 518 appears to mostly reference requirements in other Articles, it does have several of its own. For example, if you want to install nonmetallic tubing or conduit in spaces with a finish rating, you must satisfy one of two conditions-which we will explain to you.

Article 525. Carnivals, Circuses, Fairs and Similar Events

These locations are similar to places of assembly, but there's a big difference. The places of assembly covered by Article 518 aren't temporary-those covered by Article 520 are temporary, and they consequently use portable wiring. For such events and activities held in permanent structures, though, Articles 518 and 520 apply.

Another big difference is that the items covered by Article 525 include such things as amusement rides and attractions. These must be located no less than 15 feet in any direction from overhead power lines. If those lines are over 600V, the attractions cannot be located under them.

As with Article 518, portable cords must be listed for extra-hard usage. See if you can spot other similarities, as well as differences, between Article 518 and Article 525 as you study. Being aware of these will help you understand both Articles better.

Article 527. Temporary Installations

It's a common enough misconception that temporary wiring meets a lower standard than that of other wiring. In truth, it merely meets a different standard. The same rules of workmanship, ampacity and circuit protection apply to temporary installations as they do to other installations. In fact, services for temporary installations must meet the same requirements as services for other installations.

So how is a temporary installation different? In one sense, it does meet a lower standard. For example, you can use NMC rather than the normally required raceway-enclosed wiring-without height limitation. And you don't have to put splices in boxes. But there are trade-offs to these freedoms. For example, all receptacles must be of the grounding type.

In addition, there are time limits to temporary installations. You must remove a temporary installation upon completion of the purpose for which it was installed. If the temporary installation is for holiday displays, it can't last more than 90 days.

Temporary installations change the rules a bit, with the goal of increased cost-efficiency. Instead of making a temporary installation safe by "method A," Article 527 makes it safe with "method B." What Article 527 does not do is provide a license for unsafe work. Unsafe work is unsafe, no matter how long or short it lasts. The time factor comes into play when considering the lifespan of the insulators used in temporary wiring-it has nothing to do with "playing the odds." The NEC does not permit unsafe "lower standard" work, regardless of how long it will be in use or how little of a chance there is for something bad to happen.

The NEC requirements for permanent installations are very cost-effective, given enough time. But they are less cost-effective when applied to a temporary situation. Therefore, another approach-that provided by Article 527-provides a cost-savings. It also addresses some practicality and execution issues inherent in temporary installations, thereby making the installation less time consuming.

Article 547. Agricultural Buildings

Two factors have a tremendous influence on the lifespan of agricultural equipment: dust and corrosion. Dust gets into mechanisms and causes premature wear. But with electricity on the scene, dust adds two other dangers: fire and explosion.

Dust from hay, grain and fertilizer is highly flammable. Litter materials, such as straw, are also highly flammable. The excrement from farm animals may cause corrosive vapors that eat at mechanical equipment but can also cause electrical equipment to fail. For these reasons, Article 547 includes requirements for dealing with dust and corrosion.

Another factor to consider in agricultural buildings is water. Water is present for many reasons, including washdown. Thus, Article 547 has requirements for dealing with wet and damp environments. Article 547 also includes other requirements. For example, it requires you to install equipotential planes in all concrete floor confinement areas of livestock buildings that contain metallic equipment accessible to animals and likely to become energized.

Article 550. Mobile Homes, Manufactured Homes and Mobile Home Parks

Among dwelling types, mobile homes have the highest rate of fire. Article 550 addresses some of the causes of those fires with the intent of reducing these statistics.

Article 550 recognizes that the same structures used for mobile or manufactured homes are also used for non-dwelling purposes, such as construction offices. Thus, it excludes those structures from the 100A minimum service requirement when so used.

Many people do not know there is a difference between a mobile home and a manufactured home, but there is. And Article 550 has different requirements for each. For example, you cannot locate service equipment on a mobile home. However, you can install service equipment on a manufactured home (provided you meet seven conditions). Pay close attention to the definitions in 550.2 so you don't get confused.

When grounding mobile homes or manufactured homes, pay attention to the special requirements of 550.16. Simply applying Article 250 will not be sufficient because these structures pose additional challenges to effective grounding.

Article 551 Recreational Vehicles and Recreational Vehicle Parks

Article 551 is similar to Article 550. After all, RVs are similar to mobile homes. While mobile homes are essentially trailers, RVs are essentially vehicles. Thus, RVs have their own Article. RVs also have voltage converters, which mobile homes do not have. And other differences emerge as you look more closely.

While most of the requirements in Part II and Part III apply to the RV manufacturer, an electrician doing work in an RV must also comply with these requirements. For example, the low-voltage wiring in RVs must be made of copper. If you're used to wiring with THHN, you'll have to switch to some other kind of wire to install low-voltage wiring in an RV. Your choices are Type GXK, HDT, SGT, SGR and SXL.

If you're going to install an engine generator for an RV, you'll need to understand Part IV. Part V and Part VI are primarily for RV manufacturers.
The typical electrician starts to get involved in Article 551 with Part VII, which provides the requirements for RV parks. Perhaps the most important aspect of Part VII is understanding how to calculate loads and apply the demand factor, based on the number of RV sites in the park. It's also very important item to not connect a grounding electrode to the load side.

Article 555. Marinas and Boatyards

Water level isn't constant. As the earth and the moon play their eons-old game of tug and war, oceans, lakes, rivers, and other bodies of water rise and fall at the shoreline. Other forces also cause the water level to change. For example, lakes and rivers vary in depth in response to rain. The variations can sometimes be dramatic.

To provide power to a marina or boatyard, you must allow for the variations in height between the point of use and the power source. Article 555 addresses this issue.
But that's not the only issue involved with marinas and boatyards. As you might expect, Article 555 also presents requirements for accommodating the high levels of moisture inherent in these installations. Boatyard and marina installations pose further challenges as well. For example, sunlight off the water is much more intense than it would otherwise be-and this has implications for insulation. Other factors to consider include temperature extremes, increased abrasion, oil, gasoline, diesel fuel, ozone, acids and chemicals.

Then, of course, docking a boat is not as easy as pulling into a shopping center parking spot with your automobile. Electrical equipment must meet certain spatial requirements, such as not interfering with mooring lines or masts.

Article 555 begins with the concept of the electrical datum plane. You might think of it as the border of a "demilitarized zone" for electrical equipment. Or you can think of it as a line that marks the beginning of a "no man's land" where you simply do not place electrical equipment. Once you determine where this plane is, don't locate transformers, connections or receptacles below that line.

Understanding the NEC, Volume 2 Workbook — 2002
This workbook contains 350 NEC Code questions which follow along with the 2002 Understanding the NEC Volume II. It also includes a recommended customizing list for your NEC Code Book.

Product Code: 02UNWB2
Pages: 29
Illustrations: N/A

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