Article 90 is the introduction to the National Electrical Code (NEC) book, and it includes specifications that are essential to understanding the chapters and informative annexes that follow. For example, Article 90 explains how to recognize mandatory rules versus permissive rules. In accordance with 90.5(A), mandatory rules identify actions that are specifically required or specifically prohibited. Mandatory rules are easy to recognize because they use the terms “shall” or “shall not.” 


On the other hand, 90.5(B) covers permissive rules, which identify actions that are allowed but are not required. Permissive rules are also used to describe options or alternative methods. The terms “shall be permitted” or “shall not be required” are used in permissive rules. 


As stated in 90.5(C), informational notes contain explanatory material, such as references to other standards, references to related sections of the Code, or information related to a Code rule. Before the 2011 edition of the NEC, informational notes were fine print notes (FPN). Informational notes are not enforceable as requirements. Mandatory rules, permissive rules and informational notes are located throughout the Code book. 


While Article 100, Definitions, contains informational notes, it does not contain rules (mandatory or permissive). Article 110, Requirements for Electrical Installations, contains mandatory rules, permissive rules and informational notes.


Article 110 contains general provisions related to electrical installations, including specifications pertaining to electrical equipment mounting and cooling. In accordance with 110.13(A), electrical equipment shall be firmly secured to the surface on which it is mounted. While this is the first requirement about secure mounting, it is not the only one. Provisions containing phrases such as “firmly secured to the surface,” “held firmly in place,” “rigidly secured,” “securely fastened in place,” “rigidly supported,” etc., are located in hundreds of sections throughout the Code. For example, an enclosure mounted on a building or other surface shall be rigidly and securely fastened in place [314.23(A)]. Cables and raceways are also required to be securely fastened in place. The raceway and cable articles go into a little more detail by specifying the maximum distance to a support and between supports. Electrical metallic tubing (EMT) shall be securely fastened in place at least every 3 meters (10 feet). In addition, each EMT run between termination points shall be securely fastened within 900 millimeters (3 feet) of each outlet box, junction box, device box, cabinet, conduit body or other tubing termination [358.30(A)]. This section also contains a provision that specifically prohibits a certain mounting method. As stated in the last sentence of 110.13(A), wooden plugs driven into holes in masonry, concrete, plaster or similar materials shall not be used. This mandatory requirement prohibiting the use of wooden plugs has been in the Code a very long time. Although wooden plugs were a cheap alternative for lead anchors, they were never reliable (see Figure 1).


Securing and mounting equipment is not limited to being secured to a building or structure. If one item of electrical equipment is mounted to another, the item of electrical equipment being mounted shall also be firmly secured to the surface on which it is mounted. In accordance with 406.5(C), receptacles mounted to and supported by a cover shall be held rigidly against the cover by more than one screw or shall be a device assembly or box cover listed and identified for securing by a single screw. As stated in this section, unless the cover’s installation instructions allow the mounting of a receptacle to the cover by a single screw, two screws are required (see Figure 2).


Requirements pertaining to electrical equipment cooling are covered in 110.13(B). Electrical equipment that depends on natural air circulation and convection principles for exposed surfaces cooling shall be installed so that walls or adjacent installed equipment do not prevent room airflow over such surfaces. This section does not instruct users to put air conditioning or ventilation in or on electrical equipment. It instructs users to install the equipment so normal airflow is not prevented. It continues by stating that, if the electrical equipment is designed for floor mounting, there must be clearance between the top surface and adjacent surfaces so rising warm air can dissipate.


Some electrical equipment is provided with ventilated openings. If so, install the equipment so walls or other obstructions do not prevent the free circulation of air through the equipment. Some dry-type transformers have ventilated openings in the front and back near the top of the transformer. This type of transformer also has vents in the bottom to allow air to circulate through it. The installation instructions usually state the minimum distance the transformer can be located to a wall or other surface. A new requirement in the 2014 NEC states a terminal bar in dry-type transformers must be installed, but the Code prohibits installing the terminal bar over any vented portions of the equipment. In accordance with 450.10(A), where separate equipment grounding conductors and supply-side bonding jumpers are installed, a terminal bar for all grounding and bonding conductor connections shall be secured inside the transformer enclosure. This also states the terminal bar shall be bonded to the enclosure in accordance with 250.12 and shall not be installed on or over any vented portion of the enclosure. This section has an exception for dry-type transformers equipped with wire-type connections (leads) (see Figure 3).


The next section in Article 110 contains quite a bit of information pertaining to electrical connections. Because of different characteristics of dissimilar metals, devices such as pressure terminals or pressure splicing connectors and soldering lugs shall be identified for the material of the conductor and shall be properly installed and used [110.14]. If a terminal, connector, lug, etc., is listed for copper only, only copper conductors shall be installed. Likewise, if the termination is listed for aluminum only, only aluminum conductors shall be installed. Terminations are available that are listed for copper or aluminum conductors. A termination listed for copper and aluminum may be marked AL/CU, CU/AL or CU/ALR.


When different conductor types will be installed in one termination, the termination shall be listed as such. Do not mix conductors of dissimilar metals (such as copper and aluminum, copper and copper-clad aluminum, or aluminum and copper-clad aluminum) in a terminal or connector where the conductors physically contact each other unless the terminal or connector is identified for the purpose and conditions of use. Some split bolts are listed for either aluminum conductors or copper conductors but not for both at the same time. Some split bolts are listed for splicing copper and aluminum conductors together, but these split bolts usually have a divider between the conductors. A marking of “AL9CU” or “CU9AL” on a lug indicates the lug is listed for copper and aluminum conductors and has a 90°C conductor temperature rating (see Figure 4).


The last sentence of 110.14 was new in the 2011 NEC. This provision states that, for conductors more finely stranded than Class B and Class C, the connection or terminal shall be identified for the specific conductor class or classes. See Table 10 in Chapter 9 for the number of strands in Class B and Class C conductors.


Next month’s column continues the discussion of Article 110.