|Send to a Friend||View / Add Comments|
by Mike Holt
CHAPTER 3. WIRING METHODS AND MATERIALS
Chapter 2 provided the general rules for wiring and protection of conductors. Chapter 2 is primarily concerned with correct installation of circuits and the means of protecting them. This differs from the Chapter 3 purpose, which is to correctly size and install the conductors that comprise those circuits.
Chapter 2 was a bit of an uphill climb, because many rules had a kind of abstract quality to them. Chapter 3, on the other hand, gets very specific about conductors, cables, boxes, raceways, and fittings. It is also highly detailed about the installation and restrictions involved with wiring methods.
It's because of that detail that many people incorrectly apply Chapter 3 wiring methods. You need to pay careful attention to the details, rather than making the mistake of glossing over something. This is especially true when it comes to applying the tables.
The type of wiring method you will use depends on several factors. Code requirements, environment, need, and cost are among them.
Power quality is a major concern, today. The costs of poor power quality run into the millions of dollars each month in the United States alone. Grounding deficiencies (refer back to Article 250) constitute the number one cause of power quality problems. Violations of Chapter 3 wiring methods constitute the number two cause of power quality problems. Code violations in general lead to fire and other hazards. This is particularly true of Chapter 3 violations.
Chapter 3 is really a modular assembly of Articles, each detailing a specific area of electrical installation. It starts with wiring methods (300), covers conductors (310), then covers enclosures (312 and 314). The next string of Articles (320 - 376) address specific types of cables. The remaining Articles (378 - 392) address specific types of raceways.
Article 300. Wiring Methods
Article 300 contains the general requirements for all wiring methods included in the NEC. However, this Article does not apply to signal and communications systems as covered in Chapters 7 and 8.
This Article is primarily concerned about how you install, route, splice, protect, and secure conductors and raceways. How well you conform to the requirements of Article 300 will generally be evident in the finished work, because many of the requirements tend to determine the appearance of the installation.
Because of this, it's often easy to spot Article 300 violations if you are looking for Code violations. For example, you can easily spot when someone runs a ground wire outside a raceway instead of grouping all conductors of a circuit together as required by Section 300.3(B). But, don't confuse that ground wire with an external bonding jumper. Something you'll learn from your study of Article 300 is there's a difference between running a circuit grounding conductor outside a raceway (illegal) and running an external bonding jumper outside the same raceway.
This is just one of the common points of confusion your studies here will clear up for you. To help achieve that end, be sure to carefully consider the accompanying illustrations and also refer to Article 100 as needed.
Article 310. Conductors for General Wiring
This Article contains the general requirements for conductors, such as insulation markings, ampacity ratings, and use. Article 310 does not apply to conductors that are part of cable assemblies, flexible cords, or fixture wires, or to conductors that are an integral part of equipment [90.6, 300.1(B)].
Where do most people make errors in applying Article 310? In the ampacity tables. If you'll notice, we don't reproduce the ampacity tables in this text. Instead, we explain what those tables mean. If you study the explanations carefully, you'll avoid common errors such as applying Table 310.16 when you should be applying Table 310.17.
But, why so many tables? Why does 6 AWG THHN carry 105A in Table 310.17 and only 75A in Table 310.16? To answer that, go back to Article 100 and review the definition of ampacity. Notice the phrase "conditions of use."
What these tables do is set a maximum current value at which you can ensure the installation won't undergo premature failure of the conductor insulation in normal use. The designations THHN, THHW, RHH, and so on are designators for the insulation. Every type of insulation has a limit to how much heat it can withstand. Current flowing through a conductor creates heat. How well the insulation around a conductor can dissipate heat depends on factors such as whether that conductor is in free air or not. Think what happens to you if you put on a sweater, a jacket, and then a coat-all at the same time. You heat up. Your skin (insulation) cannot dissipate heat with all this clothing on nearly as well as it dissipates heat in free air.
Conductors do fail with age. That's why we do cable testing and take other measures to predict failure and replace certain conductors (e.g., feeders or critical equipment conductors) while they are still within design specifications. But, such failure takes decades under normal use and occurs slowly-and it's a maintenance issue. If the conductor is forced to exceed the ampacity listed in the appropriate table, failure happens much more rapidly-often catastrophically. Exceeding the allowable ampacity is a safety issue.
Article 312. Cabinets, Cutout Boxes, and Meter Socket Enclosures
This article addresses the installation and construction specifications for the items mentioned in its title. Why do we need a Code article telling us how to install these things? What can you possibly do wrong? In Article 310, we observed that you need different ampacities for the same conductor, depending on conditions of use. The same thing applies to these items-just in a different way. For example, you can't use just any enclosure in a wet area or in a hazardous location. The conditions of use impose special requirements for these situations.
For all such enclosures, certain requirements apply-regardless of the use. For example, you must cover any openings, protect conductors from abrasion, and allow sufficient bending room.
Part I is where you'll find the requirements most useful to the electrician in the field. Part II applies to manufacturers. If you use name brand components-which are listed, labeled, or approved-you don't need to be concerned with Part II. If you are specifying custom enclosures, you may need to be familiar with these requirements.
Article 314 Outlet, Device, Pull and Junction Boxes, Conduit Bodies, Fittings, and Handhole Enclosures
Article 314 contains installation requirements for outlet boxes, pull and junction boxes, conduit bodies and manholes.
This is another article that can make you pause and ask, "What do we need this for?" It might seem there's no secret to installing a conduit body, fitting, or anything else mentioned in the title. But, as with Article 312, conditions of use apply. If you're running raceway in a hazardous location, for example, you must use the correct fittings and the proper installation methods. But, take something as simple as a splice. It makes sense that you wouldn't put a splice in the middle of a raceway-doing so means you can't get to it. But, if you put a splice in a conduit body you are fine, right? Not necessarily. Suppose that conduit body is a "short radius" version. Think of it as an elbow with the bend chopped off. You don't have much room in side that fitting. For that reason, you can't put a splice inside a short radius conduit body.
Properly applying Article 314 means you will need to account for the internal volume of all boxes and fittings, and then determine the maximum wire fill. You'll also need to understand many other requirements, which we'll cover. If you start to get confused, take a break. Look carefully at the illustrations, and you'll learn more quickly and with greater retention.
Articles 320 through 340 address specific types of cable. If you take the time to become familiar with the various types of cable, you will:
Here's a brief overview of each one:
Article 320. Armored Cable (Type AC)
Armored cable is an assembly of insulated conductors, 14 AWG through 1 AWG, that are individually wrapped within waxed paper. The conductors are contained within a flexible spiral metal (steel or aluminum) sheath that interlocks at the edges. Armored cable has an outside appearance like flexible metal conduit. Key point: There are no limitations to the number of bends between terminations.
Article 330. Metal-Clad Cable (Type MC)
Metal-clad cable encloses one or more insulated conductors in a metal sheath of either corrugated or smooth copper or aluminum tubing, or spiral interlocked steel or aluminum. The physical characteristics of MC Cable make it a versatile wiring method you can use in almost any location and for almost any application. The most common type of MC Cable is interlocking type, which has an appearance similar to armored cable or flexible metal conduit. Key point: Limited as to installation environments, unless specifically identified for the purpose.
Article 334. Nonmetallic-Sheathed Cable (Type NM)
Nonmetallic-sheathed cable is a wiring type enclosing two or three insulated conductors, 14 AWG through 2 AWG, within a nonmetallic outer cover. Because this cable is nonmetallic, it contains a separate equipment grounding conductor. Nonmetallic-sheathed cable is a common wiring method used for residential and commercial branch circuits. Key point: Not to be used outside unless specifically identified for the purpose; cannot be used for service wiring.
Article 336. Power and Control Tray Cable (Type TC)
TC is a factory assembly of two or more insulated conductors under a nonmetallic sheath, for installation in cable trays, in raceways, or where supported by a messenger wire. Key point: Cannot be used as open cable on brackets or cleats.
Article 338. Service-Entrance Cable (Type SE and USE)
Service-entrance cable can be a single conductor or multiconductor assembly with an overall covering. This cable is used primarily for services not over 600V, but can also be used for feeders and branch circuits. Key point: Must be installed per the requirements of Article 230.
Article 340. Underground Feeder Cable (Type UF)
UF cable is a moisture-, fungus-, and corrosion-resistant cable system suitable for direct burial in the earth and comes in sizes 14 AWG through 4/0 AWG [340.104]. The covering of multiconductor UF cable is molded plastic that encapsulates the insulated conductors. Key point: Cannot be used as service wiring.
Articles 342 through 392 address specific types of raceway. Refer to Article 100 for the definition of raceway. If you take the time to become familiar with the various types of cable, you will:
Here's a brief overview of each one:
Article 342. Intermediate Metal Conduit (Type IMC)
Intermediate metal conduit is a circular metal raceway with an outside diameter the same as rigid metal conduit (RMC). The wall thickness of IMC is less than that of RMC, so it has a greater interior cross-sectional area. IMC is lighter and less expensive than RMC, but it can be used in all of the same locations as RMC. IMC also uses a different steel alloy, which results in its being more rigid than RMC, even though the walls are thinner.
Article 344. Rigid Metal Conduit (Type RMC)
Rigid metal conduit (RMC) is a circular metal raceway with an outside diameter the same as that of IMC. The wall thickness of RMC is greater than that of MC, so it has a smaller interior cross-sectional area. RMC is heavier and more expensive than IMC, and it can be used in any location.
Article 348. Flexible Metal Conduit (Type FMC)
FMC is a raceway of circular cross section made of helically wound, formed, interlocked metal strip of either steel or aluminum. It is commonly called Greenfield or simply "flex." The primary use of this is to provide a final 6 feet or less of raceway between a more rigid raceway system and equipment that moves, shakes, or vibrates. Examples of such equipment include pump motors and industrial machinery.
Article 350. Liquidtight Flexible Metal Conduit (Type LFMC)
LFMC is a listed raceway of circular cross section having an outer liquidtight, nonmetallic, sunlight-resistant jacket over an inner flexible metal core with associated couplings, connectors, and fittings and approved for the installation of electric conductors. LFMC is commonly called Sealtight® or simply "liquidtight." LFMC is of similar construction to FMC, but has an outer liquidtight thermoplastic covering. This has the same primary purpose as FMC.
Article 352. Rigid Nonmetallic Conduit (Type RNC)
RNC conduit is a listed nonmetallic raceway of circular cross section with integral or associated couplings, approved for the installation of electrical conductors. This type of conduit gives you much of the advantages of rigid conduit, while allowing installation in areas that are wet or corrosive.
Article 353 High Density Polyethylene Conduit: Type HDPE Conduit
This is a new article in the 2005 NEC. Check with your AHJ before attempting to use HDPE as your AHJ may not yet recognize or approve of it. This type of conduit has been in use in the U.K. for a long time. It is lightweight and durable. It's resistant to decomposition, oxidation, and hostile elements that cause damage to other materials. It's mechanically and chemically resistant to a host of environmental conditions. Uses include communication, data, cable television, and general purpose raceways.
Article 354. Nonmetallic Underground Conduit with Conductors (Type NUCC)
Nonmetallic underground conduit with conductors is a factory assembly of conductors or cables inside a nonmetallic, smooth-wall conduit with a circular cross section. The nonmetallic conduit is manufactured from a material that is resistant to moisture and corrosive agents. It is also capable of being supplied on reels without damage or distortion and is of sufficient strength to withstand abuse, such as impact or crushing, in handling and during installation without damage to conduit or conductors.
Article 356. Liquidtight Flexible Nonmetallic Conduit (Type LFNC)
LFNC is a listed raceway of circular cross section having an outer liquidtight, nonmetallic, sunlight-resistant jacket over an inner flexible core with associated couplings, connectors, and fittings and approved for the installation of electric conductors. There are three types:
Article 358. Electrical Metallic Tubing (Type EMT)
EMT is a listed metallic tubing of circular cross section approved for the installation of electrical conductors when joined together with listed fittings. Compared to RMC and IMC, EMT is relatively easy to bend, cut, and ream. This makes it very popular. Because it is not threaded, all connectors and couplings are of the threadless type.
Article 362. Electrical Nonmetallic Tubing (Type ENT)
Electrical nonmetallic tubing is a pliable, corrugated, circular raceway made of polyvinyl chloride (PVC). In some parts of the country, this raceway is called "Smurf pipe," because when it originally came out, at the height of popularity of the children's characters the Smurfs, it was available only in blue.
Article 376. Metal Wireways
A metal wireway is a sheet metal trough with hinged or removable covers for housing and protecting electric wires and cable and in which conductors are placed after the wireway has been installed.
Article 378. Nonmetallic Wireways
A nonmetallic wireway is a flame-retardant trough with hinged or removable covers for housing and protecting electric wires and cable and in which conductors are placed after the wireway has been installed as a complete system.
Article 380. Multioutlet Assembly
A multi-outlet assembly is a surface, flush, or freestanding raceway designed to hold conductors and receptacles, assembled in the field or at the factory.
Article 384. Strut-Type Channel Raceway
Strut-type channel raceway is a metallic raceway intended to be mounted to the surface or suspended, with associated accessories, in which conductors are placed after the raceway has been installed as a complete system.
Article 386. Surface Metal Raceway
A surface metal raceway is a metallic raceway intended to be mounted to the surface, with associated accessories, in which conductors are placed after the raceway has been installed as a complete system.
Article 388. Surface Nonmetallic Raceways
A surface nonmetallic raceway is a nonmetallic raceway intended to be mounted to the surface, with associated accessories, in which conductors are placed after the raceway has been installed as a complete system.
Article 392. Cable Trays
This Article covers cable tray systems. These include ladder, ventilated trough, ventilated channel, solid bottom, and other similar structures. A cable tray system is a unit or assembly of units or sections and associated fittings forming a structural system used to securely fasten or support cables and raceways.
CHAPTER 4. EQUIPMENT FOR GENERAL USE
With the first three Chapters behind you, the final Chapter in building a solid foundation in the NEC for general work is Chapter 4. With this Chapter, you will be applying the first three Chapters to general equipment. These first four Chapters follow a natural sequential progression. The next four Chapters-5, 6, 7, and 8-each build from the first four, but in no particular order. You do not need to understand any of the other "next Chapters" to work with any one of them. But, you do need to understand all of the first four Chapters to properly apply any of the next four.
Chapter 4 has some logical arrangement of its own. Here are the groupings:
These groupings make sense. For example, motors, refrigeration equipment, generators, and transformers are all wound devices and articles concerning them appear one after the other in order.
This logical arrangement of the NEC is something to keep in mind when you "can't find something." You know, for example, that transformers are general equipment. So, you would find the Code requirements in Chapter 4. You know they are wound devices. So, you would find transformer requirements located somewhere near motor requirements. Refrigeration equipment in the NEC means hermetically sealed motors. So, the requirements should logically be located right next to motor requirements. And that's exactly where they are.
|[ Back to Top ]|