Chapter 3 of the Code is titled “Wiring Methods and Materials.” The first article in Chapter 3 is Art. 300, “Wiring Methods.” This article covers the general wiring methods for all wiring installations unless modified by the other articles. It doesn’t apply to communications systems (covered in Chapter 8), except when Art. 300 is specifically referenced in Chapter 8.
A good understanding of Art. 300 will help you correctly install the wiring methods included in Chapter 3. For example, 300.2 includes voltage and temperature limitations that apply to all Chapter 3 wiring methods, and 300.3 provides some general rules for conductor installation for all wiring methods.
Previous NEC editions have required protection of conductors where subject to physical damage. While most Code users understand this rule is intended to apply to all conductors in all wiring methods, it didn’t clearly state that. The 2011 revision clarifies that all conductors in all wiring methods must be protected from physical damage [300.4].
The rule in 300.4(E) regarding protection for raceways installed under metal-corrugated sheet roof decking has been expanded. The 2008 NEC introduced a requirement for the protection of most raceways when installed within 1½ in. of the underside of the roof deck. Although this 2008 rule change went a long way toward protecting wiring systems from damaging roofing screws that can penetrate the raceways, it left out one critical part of the installation — boxes. The 2011 revision corrects that and also clarifies that the measurement is to be made to the top of the cable, raceway, or box from the lowest surface of the roof decking (Fig. 1).
Fig. 1. A 2011 revision clarifies that the measurement is to be made to the top of the cable, raceway, or box from the lowest surface of roof decking.
This rule was also changed to prohibit wiring methods from being installed in concealed locations above the roof decking. In some instances, installers placed raceways above the roof deck prior to the insulation being installed. This resulted in the same potential for damage from roofing screws.
In 300.4(G), the term “substantial fitting” has been replaced with the term “identified” so inspectors won’t have to interpret the Code unnecessarily. The term “identified” is clearly defined in Art. 100, is used throughout the NEC, and takes interpretation out of the requirement.
Although fittings that are designed to provide this protection are typically used to achieve compliance with this requirement, it could have been argued that fittings designed for another application could satisfy this rule. This change removes that argument.
If raceways contain insulated circuit conductors 4 AWG (or larger) that enter an enclosure, the conductors must be protected from abrasion during and after installation by a fitting identified to provide a smooth, rounded insulating surface (such as an insulating bushing), as shown in Fig. 2.
Fig. 2. If raceways contain insulated circuit conductors 4 AWG (or larger) that enter an enclosure, the conductors must be protected from abrasion during and after installation by a fitting identified to provide a smooth, rounded insulating surface (such as an insulating bushing).
- If IMC or RMC conduit enters an enclosure without a connector, you must provide a bushing (regardless of the conductor size) [342.46 and 344.46].
- A grounding electrode conductor doesn’t require an insulated fitting.
- Insulating bushings aren’t required if a raceway terminates in a threaded raceway entry that provides a smooth, rounded, or flared surface for the conductors. An example would be a meter hub fitting or a Meyer’s hub-type fitting [300.4(G), Ex].
Larger commercial/industrial buildings commonly have expansion joints. Previously, the NEC offered no guidance on how these affect wiring methods. Now, it does. You must use a fitting or other approved means to allow for expansion or deflection of the wiring method [300.4(H)].
You can now install Type MI and Type MC cables under buildings without a raceway [300.5(C) Ex 2:], as shown in Fig. 3. Although certain types of MC cable are listed for direct burial and concrete encasement, 300.5 formerly prohibited them from being installed underneath buildings. This change now allows cables to be installed under the floor slab of a building. Inspectors have accepted this practice for some time. Interestingly, this change doesn’t recognize other cables that are listed for this application, such as UF cable.
Fig. 3. You can now install new Type MI and MC cables under buildings without a raceway.
Same circuit grouping
Conductors of the same circuit must be grouped in the same raceway or cable to help reduce the inductive reactance of the conductors. A literal reading of previous Code editions didn’t address the use of single-conductor cables, such as many USE cables, in parallel installations. The NEC now recognizes this practice while giving guidance on how to install these cables. The issues of inductive reactance are addressed by requiring these single-conductor cables to be installed within close proximity of each other [300.5(I)].
Note that installing ungrounded and neutral conductors in different PVC conduits makes it easier to terminate larger parallel sets of conductors but results in higher levels of electromagnetic fields (EMF).
Nonfire-rated ceiling assemblies
The rule requiring identification of electrical ceiling support wires has been expanded. Identification of ceiling-support wires that are used for electrical equipment was previously limited to those wires that are in fire-resistance-rated ceiling assemblies.
To distinguish which support wires are installed by the electrician versus those installed by the ceiling contractor, this change requires identification of the independent support wires for all ceiling systems. Previously, that requirement was only for fire-rated assemblies [300.11(A)(1)] but now includes non-fire rated [300.11(A)(2)].
Outlet boxes [314.23(D)] and luminaires can be secured to the suspended-ceiling grid if securely fastened to the ceiling-framing members by mechanical means, such as bolts, screws, or rivets, or by the use of clips or other securing means identified for use with the type of ceiling-framing member(s) used [410.36(B)].
The rule on protecting the integrity of fire-resistance-rated assemblies was clarified. Because the 2008 NEC referred to protecting penetrations through fire-resistance-rated assemblies, such as walls, floors, or ceilings, it was argued that it didn’t address openings into these assemblies. The Code now clarifies that it intends to protect these assemblies for both through penetrations and membrane penetrations [300.21].
This level of protection has long been required in the building codes, so this NEC change doesn’t change installation requirements in areas where those codes are enforced.
There’s long been confusion about the terms “plenum” and “other space(s) used for environmental air.” While nearly all mechanical codes (NFPA 90A Standard for the Installation of Air-Conditioning and Ventilating Systems, International Mechanical Code, and Uniform Mechanical Code) use the term “plenum” for anything that moves environmental air, the NEC referred only to a physically constructed duct as a “plenum.”
The space beneath a raised floor and the space above a suspended ceiling (when used for air handling) were referred to as “other spaces used for environmental air.” Obviously, an electrical code shouldn’t be the document that defines air system components. Yet, manufacturers were forced to have their product literature include provision for these “other spaces.” This created massive confusion for people installing, designing, and inspecting mechanical systems.
Previously, the NEC addressed this confusion by adding an Informational Note that tried to explain what these spaces really were. With the 2011 revision, a component of an air-handling system that’s created for the sole purpose of moving air (e.g., a duct made of tin or drywall) is now called a “duct specifically fabricated for environmental air” [300.22].
The space above a ceiling or below a floor that’s used for air moving isn’t fabricated for that sole purpose; therefore, it isn’t a “duct,” according to mechanical codes. The NEC now calls these spaces “other spaces used for environmental air (plenums).” That correlates with other codes and also has the desired effect of using commonly accepted trade language — most people refer to the space above a suspended ceiling as a “plenum ceiling,” not an “other space used for environmental air ceiling” (Fig. 4).
Fig. 4. Most people refer to the space above a suspended ceiling as a "plenum ceiling," not an "other space used for environmental air ceiling."
Additional changes to this section include a new provision dealing with cable trays in other spaces used for environmental air (plenums), which requires these cable trays to be metallic. Solid metal cable trays with metal covers can now be used to support and enclose wiring methods that previously weren’t allowed in these locations. See Plenum Tips for a couple of other plenum requirements to be aware of.
Seeing Art. 300 violations
Article 300 is primarily concerned with how to install, route, splice, protect, and secure conductors and raceways. How well you conform to its requirements usually shows in the finished work.
If you’re looking for Code violations, Art. 300 deficiencies tend to stick out. For example, you can see when someone runs an equipment grounding conductor outside a raceway instead of grouping all conductors of a circuit together per 300.3(B).
If you understand and follow the rules of Art. 300, you are well on your way to understanding the installation requirements for the wiring methods of Chapter 3 and better prepared to provide an installation that will be safe long after you complete it.
SIDEBAR: PLENUM TIPS
You can’t install nonmetallic or nonmetallic covered wiring methods, such as PVC conduit [Art. 352], electrical nonmetallic tubing [Art. 362], liquidtight flexible conduit, and nonmetallic cables in spaces used for environmental air, because they give off deadly toxic fumes when burned or superheated. You can install equipment with metal enclosure, such as dry-type transformers rated not over 50kVA, above
suspended ceilings used for environmental air [300.22(C)(3) and 450.13(B)].