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Chapter 3 Wiring Methods and Materials
Chapter 2 provided the general rules for wiring and protection of conductors and is primarily concerned with correct installation of circuits and the means of protecting them. This differs from the purpose of Chapter 3, which is to correctly size and install the conductors that make up 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's also highly detailed about the installation and restrictions involved with wiring methods.
It is 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 cost of poor power quality runs 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 causes 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 [Article 300], covers conductors [Article 310], then covers enclosures [Articles 312 and 314]. The next string [Articles 320 - 376] addresses specific types of cables. The remaining [Articles 378 - 392] addresses 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 doesn't apply to signaling and communications systems, which are covered in Chapters 7 and 8, except as specifically referenced in those chapters.
This article is primarily concerned with 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're looking for Code violations. For example, you can easily spot when someone runs an equipment grounding (bonding) conductor outside a raceway instead of grouping all conductors of a circuit together as required by 300.3(B). From your study of Article 300 you'll learn there's a difference between running a circuit grounding conductor outside a raceway, which is 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.
Part I. General Requirements
ARTICLE 310 Conductors for General Wiring
This article contains the general requirements for conductors, such as insulation markings, ampacity ratings, and conditions of use. Article 310 doesn't apply to conductors that are part of cable assemblies, flexible cords, fixture wires, or to conductors that are an integral part of equipment [90.6, 300.1(B)].
People most often make errors in applying the ampacity tables contained in Article 310. If you study the explanations carefully, you'll avoid common errors such as applying Table 310.17 when you should be applying Table 310.16.
But why so many tables? Why does table 310.17 list the ampacity of 6 THHN as 105A, yet Table 310.16 only lists the same conductor as having an ampacity of only 75A? 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 heat withstand limit. When current flows through a conductor it creates heat. How well the insulation around a conductor can dissipate that 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 cannot dissipate heat with all this clothing on nearly as well as it dissipates heat in free air.
Conductors fail with age. That's why we conduct 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 conductor failure takes decades under normal use and occurs slowly-and it's a maintenance issue. However, if a 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.
Mike Holt's Comment: If you desire more information about any of the above changes, be sure to order my Changes book and/or library (Video/DVD).
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