Four NEC Articles form the basis for correctly applying Chapter 3 wiring methods. You can’t use just Art. 344 and hope your rigid metal conduit (RMC) installation won’t have Code violations. At least three of these four Articles will also apply. Last month, we looked at one of those four (Art. 300). Now we’ll look at the other three:
• Article 310
• Article 312
• Article 314
When you run RMC, of course the raceway must conform to Art. 344. However, the conductors you pull through it must also conform to Art. 310. This Article contains the general requirements for conductors. The requirements cover such things as insulation markings, ampacity ratings, and conditions of use. Article 310 doesn’t apply to conductors that are part of flexible cords, fixture wires, or that are an integral part of equipment [90.6 and 300.1(B)].
One common error is that of selecting the wrong ampacity table (from the several that exist). If you look at the tables, you will see that Table 310.15(B)(17) lists the ampacity of No. 6 THHN as 105A, while Table 310.15(B)(16) lists the same conductor as having an ampacity of only 75A. Which value is correct? That depends on which table is correct for your application.
To be able to arrive at the right answer, look at the Art. 100 definition of ampacity. It’s the maximum current (in amperes) a conductor can carry continuously under the conditions of use, without raising the temperature of the conductor in excess of its insulation temperature rating. Notice the phrase “conditions of use.” The ampacity tables set a maximum current value at which premature failure of the conductor insulation shouldn’t occur during normal use, under the conditions described in the tables.
What’s the secret to knowing you are using the right ampacity table? Understand the conditions of use. The title of each table states the conditions of use for which that table applies. These titles are long, but rather than let your eyes glaze over as you flip from table to table, carefully read the title on each table.
Even if you do select the correct ampacity table, you can’t just look up the conductor size and assume the maximum ampacity you find is what you will actually use for your application. Why? Notice the table numbers. These begin with the string, “310.15(B).” The reason they are numbered this way rather than, say, Table 1, Table 2, etc., is because they are part of Sec. 310.15(B). If you read that section carefully, you will understand what to do with the information you get from the table.
For example, you must apply temperature correct factors. Preceding the ampacity tables are two tables of ambient temperature correction factors [Table 310.15(B)(2)(a), Table 310.15(B)(2)(b)]. And in 310.15(B), you’ll find an explanation of how to use them.
Keep in mind that working with these tables will give you only the minimum ampacity. It won’t give you the ampacity to account for other factors such as voltage drop. See 210.19(A) Note 4 for branch circuits and 215.2(A)(1) Note 2 for feeders.
Article 312
So now your raceway conforms to its Article (in the case, Art. 344), and your conductors conform to Art. 310. But to what are you connecting the raceway? A cabinet? A cutout box?
Article 312 covers the installation and construction specifications for cabinets, cutout boxes, and meter socket enclosures (Fig. 1). These aren’t the only types of “connection points” for a raceway, but they are commonly used (we’ll cover the other types in a moment).
Just as the conditions of use have an effect on the ampacity of a conductor, so do they have an effect on the selection and application of cabinets, cutout boxes, and meter socket enclosures. For example, you can’t use just any enclosure in a wet location or in a hazardous location. The conditions of use impose special requirements for these situations.
Certain requirements apply to all enclosures, regardless of the use. For example, you must cover any openings, protect conductors from abrasion, and allow sufficient bending room for conductors.
Notice that Art. 408 covers switchboards and panelboards, with primary emphasis on the interior, or “guts,” while Art. 312 covers the cabinet that would be used to enclose a panelboard. Therefore, you’ll find that some important considerations, such as wire-bending space at terminals of panelboards, are included in Art. 312.
Article 314
Your installation has thus far has conformed to Articles 344, 310, and 312, but it still might have Code violations. What about the pullbox and the three conduit bodies you installed? And the receptacles supplied by this branch circuit must be installed in outlet boxes. This is where Art. 314 comes in.
Article 314 contains installation requirements for outlet boxes, pull and junction boxes, conduit bodies, and handhole enclosures (Fig. 2). As with the cabinets, cutout boxes, and meter socket enclosures covered in Art. 312, the conditions of use have a bearing on the type of material and equipment selected for a particular installation. If you install a raceway in a wet location, for example, you must use the correct fittings and the proper installation methods.
Article 314 will help you size an outlet box using the proper cubic-inch capacity. For larger pull boxes, it will help you calculate the minimum dimensions. It provides limits on the amount of weight that can be supported by an outlet box and rules on how to support a device or outlet box to various surfaces.
Code controversies
The requirements of one or more of these foundational Articles change with every Code cycle. Usually, the changes are minor, but sometimes they can really alter how you do things. Thus, you need to be familiar with the latest revisions of these Articles before trying to apply the other Articles in Chapter 3.
Consider this example. The 2011 NEC attempted to clarify the rules for ampacity adjustment as they pertain to spare conductors. The attempt didn’t quite work, because the title of the table, the table note, and the Code text seemed to argue with each other. Three years later, the 2014 NEC cleared up this confusion. It added the note at the bottom of Table 310.15(B)(3)(a) to make it clear that you count spare conductors for ampacity adjustment.
By definition, however, spare conductors aren’t used and therefore don’t carry current. So why must you count them as current-carrying conductors? Because they’ll probably be used eventually, and it would be a difficult to convince a customer that simply terminating these conductors to equipment suddenly makes every wire in the raceway undersized. To avoid that situation, the conductor is counted immediately upon installation.
In addition, this table now addresses conductors that cannot be energized simultaneously. Consider a set of 3-way switches. One switch is fed by the supply conductor while the other switch feeds the luminaire. Between these switches are two conductors, often referred to as “travelers.” Regardless of the position of either switch, one traveler will be energized and the other will not. If you change the position (up or down) of either switch, you will change which traveler is energized. But in no case can you possibly energize both, so there’s no reason to count both conductors in the ampacity adjustment; only one of them will be adding heat.
And then there’s the rule on raceways and cables. Of all the NEC rules that have been added over the last three Code cycles, this is one of the most controversial.
“Section 310.15(B)(3)(c) Raceways and Cables Exposed to Sunlight on Rooftops. When applying ampacity adjustment correction factors, the ambient temperature adjustment contained in Table 310.15(B)(3)(c) is added to the outdoor ambient temperature for conductors installed in raceways or cables exposed to direct sunlight on or above rooftops to determine the applicable ambient temperature for ampacity correction factors in Table 310.15(B)(2)(a) or Table 310.15(B)(2)(b),” as shown in Fig. 3.
It started as an Informational Note (actually a Fine Print Note), and then grew into a requirement that receives dozens of proposals every three years. New with the 2014 NEC, the conductors in any raceway, not just circular raceways, must comply with this requirement. Conductors inside of cable assemblies also must comply.
Don’t forget the roof
You can see that understanding those first four Articles in Chapter 3 is foundational to correctly applying any other Chapter 3 Article. However, a building with a good foundation but no roof doesn’t protect you from the weather. Cover everything with a good review of the requirements in the current Code.
Keep this in mind as well. Trying to apply an outdated understanding of the Code rather than applying the current Code means you’ll get Code violations. It’s like using old shingles on a new roof and not expecting it to leak.
Holt is the owner of Mike Holt Enterprises, Inc. in Leesburg, Fla. He can be reached at www.mikeholt.com.
