Where are the NEC requirements for the various types of cables you might use? You’ll find those in Chapter 3 — Articles 320 through 340. These sections of the Code focus on armored cable, flat cable assemblies, flat conductor cable, integrated gas spacer cable, medium-voltage cable, metal-clad cable, mineral-insulated metal-sheathed cable, nonmetallic-sheathed cable, power and control cable, service-entrance cable, and underground feeder and branch circuit cable. We’ll focus this discussion on the most widely used types of cable.
Armored cable (Type AC) is “a fabricated assembly of insulated conductors in a flexible interlocked metal armor” [320.2]. This product is commonly referred to as “BX” in the field (click here to see Fig. 1).
The conductors and an internal aluminum bonding strip are contained within a flexible metal sheath that interlocks at the edges. This gives the cable an outside appearance similar to that of flexible metal conduit (FMC). However, FMC is a type of raceway; armored cable is classified as a type of cable. Article 320 provides the installation requirements for Type AC cable.
The advantages of any flexible cables, as compared to raceway wiring methods, are:
- There’s no limit to the number of bends between terminations.
- Installation is quick.
These advantages make armored cable very useful. However, as with any wiring method, there are some locations where it can’t be used. It must not be installed [320.12]:
- If subject to physical damage.
- In damp or wet locations.
- In air voids of masonry block or tile walls where such walls are exposed or subject to excessive moisture.
- Where exposed to corrosive conditions.
Previous NEC editions used the word “enclosure” to describe armored cable jackets, but the Art. 100 definition says an enclosure is a case or housing of an apparatus, or fence or wall surrounding an installation. Obviously, no jacket of any cable is an “enclosure.” Therefore, the 2011 revision dropped this misuse — now referring to a “flexible interlocked metal armor.”
A new restriction appears in the 2011 NEC: You can’t use Type AC where exposed to corrosive fumes and vapors. Why was this added? Such exposure was already a violation of 300.6. The problem is that the restriction didn’t appear in Art. 320.
A general rule regarding this type of protection is in 300.6. With the 2011 revision, this requirement is included in Art. 320 for Type AC requirements so it is not as likely to be overlooked.
Nonmetallic Sheathed Cable
Nonmetallic-sheathed (Types NM, NMC, and NMS) cable is flexible, inexpensive, and easy to install. Consequently, it’s a common wiring method for residential and commercial branch circuits. Because it provides minimal physical protection for the conductors, the installation restrictions are strict.
The 2011 Code answers a common question on the use of NM cable in dwelling unit accessory structures, such as detached garages. A detached residential garage isn’t a dwelling unit as defined in Art. 100. Therefore, a literal reading of 334.10(3) would require types NM, NMC, and NMS cables to be concealed within walls, floors, or ceilings using a material with a 15-minute finish rating, such as drywall. The 2011 NEC clarifies that the use of NM cable in all dwelling unit garages and accessory storage buildings without the 15-minute finish rating is allowed, if the cables aren’t subject to physical damage [334.15(B)].
There was yet another change to 334.10. The exception for raceways in buildings permitted to be of Types III, IV, or V construction was formerly in 334.12(A). It’s now in 334.10(5) — no longer an exception and written in positive text. Why the move? The title of 334.12 is “Uses Not Permitted.” If a particular use is an exception to a “disallowed” list, then it is actually allowed, which can become confusing. So this text belongs under 334.10, which is titled, “Uses Permitted.” This is just one of many examples in which the various Code Making Panels have been looking at the exceptions in the Code and converting them into positive text.
The 2011 NEC also clarifies the requirements for ampacity adjustment for NM cable in bored holes of wooden framing members. A confusion factor was how these requirements meshed with sealing foam, caulk, or similar agents. Installers often fill the openings around NM cables with sealing agents not listed for fire-blocking or draft-stopping functions. Most of these agents are excellent thermo-insulators. In fact, that’s why many of them are used. Spray foam, for example, can easily fill a cavity with high-quality thermal insulation.
This may help with energy conservation, but what is it doing to the wiring? Thermally insulating it, of course. Yet, because the wording of 334.10 specifically mentioned some agents and not others, it appeared to exclude many thermally insulating materials from its ampacity adjustment requirements. Given the physics involved, that exclusion wasn’t intentional. The 2011 revision removes the reference to fire- or draft-stopping to clarify that, regardless of the purpose of whatever is sprayed into the hole surrounding the cable, the ampacity of the conductor must be adjusted [334.80].
While some installers didn’t read this as an exclusion, some did. Those who saw a possible exclusion had to choose between spending the time to determine the role of the sealing foam and just doing the ampacity adjustments. Now there’s no question as to the role of the sealing foam or caulk — any bored hole with foam or caulk in it will have to comply with this section (see Sidebar: Adjust the Ampacity below).
This requirement has no effect on conductor sizing if you bundle no more than nine current-carrying 14 AWG or 12 AWG conductors. For example, three 14/2 cables and one 14/3 cable (nine current-carrying 14 THHN conductors) are bundled together in a dry location. You must adjust the ampacity for each conductor [25A at 90°C, Table 310.15(B)(16)] by a 70% adjustment factor [Table 310.15(B)(3)(a)].
Adjusted conductor ampacity = 25A × 0.70 = 17.5A
If you install multiple Type NM cables in contact with thermal insulation without maintaining spacing between cables, then you must adjust the allowable ampacity of each conductor per Table 310.15(B)(3)(a), as shown in Fig. 2 (click here to see Fig. 2).
Service-entrance cable (Types SE and USE) is a single conductor or multiconductor assembly used primarily for services not over 600V. You can also use it for feeders and branch circuits when you observe the limitations of Art. 338.
As you might have guessed, “SE” stands for “Service Entrance.” This cable has a flame-retardant, moisture-resistant covering. You might also have guessed that USE cable is SE cable identified for underground use. It has a moisture-resistant covering. This listing doesn’t require it to have a flame-retardant covering, although USE may be offered with that feature. A flame-retardant covering is a requirement for indoor use.
The 2011 revision to the NEC brought changes to 338.10, which is titled “Uses Permitted.” The first change is to the title of a subsection, 338.10(B)(2), which now reads “Use of Uninsulated Conductor.” Previously, it read “Grounded Conductor Not Insulated.” Why the change?
When an installer uses an SE cable as a feeder or branch circuit, the uninsulated conductor is allowed to be used only as an equipment grounding conductor. The change to the title of this section reflects that fact. The exception allowing the uninsulated conductor to be used as the neutral in pre-existing installations has also been revised to clarify that this practice is allowed only in certain existing applications.
The 2011 revision drops the ampacity adjustment formerly required in 338.10(B)(4), which, in effect, required SE cables used for indoor wiring to comply with Part II of Art. 334, including the ampacity requirements of 334.80. Because there was no real technical substantiation for the ampacity limits thus imposed, this requirement probably shouldn’t have been there in the first place. No fires or incidents were reported to warrant this change, yet it still became a requirement in the 2008 Code. In the 2011 NEC, 338.10(B)(4) still requires the use of the installation methods of Part II of Art. 334 with the exception of 334.80 (click here to see Fig. 3).
A new paragraph in 338.10(B)(4)(a) requires that SE cable installed in thermal insulation have its ampacity determined per the 60°C column of 310.15(B)(16). The column that corresponds to the actual maximum conductor temperature rating can be used for ampacity adjustment and correction if the final derated ampacity does not exceed that for a 60°C rated conductor.
A new Informational Note in 338.10(B)(4)(a) alerts Code users to Table 310.15(B)(7) for SE cables used as a main power feeder. This is a common application in residential installations.
Name that Cable
Use this three-step process to ensure you’re using the right cable:
- Identify the type of cable under consideration by its Chapter 3 Article name (not its brand name).
- Go to that Article and review the permitted uses and uses not permitted.
- Compare those uses to the intended use.
Now that you’ve identified the correct cabling for the installation, be sure you consider the ampacity adjustments so what’s specified in the bill of materials is the right size. You may need to adjust other components of the installation in order to accommodate any necessary cable size changes.
Many cables have a specified bending radius. Exceeding it during installation damages the cable. Following the proper bending radius may change the enclosure sizing or layout for the project, so be sure to read the entire cable article.
We have not covered metal-clad cable (Type MC) in this article, as we are viewing primarily 2011 Code changes. There were only minor changes made to Art. 330; however, MC cable should not be overlooked as a significant wiring method as far as cables are concerned. MC is used extensively in commercial wiring, and some specialized MC cables are available for purposes as diverse as underground installations, patient care locations, and hazardous classified locations.
Sidebar: Adjust the Ampacity
If multiple Type NM cables pass through the same wood framing opening that’s to be sealed with thermal insulation, caulking, or sealing foam, adjust the allowable ampacity of each conductor per Table 310.15(B)(3)(a).