National Electrical Code (NEC) Article 110 covers general requirements for the examination and approval, installation and use, access to and spaces about electrical conductors and equipment; enclosures intended for personnel entry; and tunnel installations. While the scope mentions a number of topics covered, it does not come close to mentioning the huge number of different topics in Article 110.


There are 53 main sections in this article. The first two parts alone contain 26 main sections and numerous subsections. Article 110 also covers voltages, conductors, wiring integrity, interrupting rating, selective coordination, mechanical execution of work, electrical equipment mounting and cooling, arc-flash hazard warning, markings, disconnecting means identification, available fault current, spaces about electrical equipment, enclosure types, and working space and guarding when the equipment is over 1,000 volts (V), nominal.


There are five parts in Article 110. Part I contains general provisions. Part II contains provisions for electrical systems rated 1,000V, nominal, or less. Prior to the 2017 NEC, the voltage threshold for this part was 600V, nominal. While Part II contains three sections, it has a number of subsections. Part III contains requirements for 1,000V, nominal; before the 2017 edition, this part was for conductors and equipment rated over 600V, nominal. Requirements for tunnel installations over 1,000V, nominal, are in Part IV, and requirements for manholes and other electrical enclosures intended for personnel entry are in Part V.


The first section in Part III contains general requirements for conductors and equipment used on circuits over 1,000V, nominal. The voltage threshold of 1,000V is a Code change in the 2017 edition. One of the global changes in the 2014 edition was to raise the voltage threshold from 600V to 1,000V. This was done in a number of places throughout the 2014 edition, but it was not changed everywhere. In the 2017 edition, 600V was changed to 1,000V in even more locations, including Article 110. Sections in Part III include 110.30 through 110.41.


Conductors and equipment used on circuits over 1,000V, nominal, shall be installed in accordance with the requirements in Part III and shall be installed in accordance with the general requirements in Part I of Article 110. The requirements in Part III supplement or modify the requirements in Part I. The last sentence in 110.30 states requirements in 110.30 through 110.41 do not apply to equipment on the service point supply side.


As defined in Article 100, the service point is the point of connection between the facilities of the serving utility and the premises wiring. This last statement is in line with 90.2(B)(5), which states the Code does not cover installations under the exclusive control of an electric utility where such installations consist of service drops or service laterals, and associated metering.


Enclosures for electrical installations are covered in 110.31. This section is divided into four subsections; (A) Electrical Vaults, (B) Indoor Installations, (C) Outdoor Installations, and (D) Enclosed Equipment Accessible to Unqualified Persons. The first sentence in 110.31 states that electrical installations in a vault, room or closet, or in an area surrounded by a wall, screen or fence—access to which is controlled by a lock(s) or other approved means—shall be considered to be accessible to qualified people only. The provision in 110.31 pertains to conductors and equipment rated over 1,000V, nominal.


A similar provision in 110.26(F) pertains to conductors and equipment rated 1,000V, nominal, or less. Both sections state that electrical installations in electrical equipment rooms or enclosures that are controlled by a lock shall be considered accessible to qualified people. The informational note under the definition of “readily accessible” in Article 100 states the use of keys is a common practice under controlled or supervised conditions and a common alternative to the ready access requirements under such supervised conditions as provided elsewhere in the NEC.


An informational note under the definition of “readily accessible” states the use of keys is a common practice under controlled or supervised conditions and a common alternative to the ready access requirements under such supervised conditions that the NEC provides elsewhere. The first paragraph continues by stating the type of enclosure used has to be designed and constructed according to the nature and degree of the hazard(s) associated with the installation.


For example, a 4,160V switchgear is located in an industrial facility’s electrical room. The doors to this electrical room are always locked, but qualified people have keys to access it. The electrical room is designed and constructed according to the nature and degree of the hazard(s) associated with this installation. This installation is permitted because access to this electrical equipment room is controlled by a lock and only qualified people have keys (see Figure 1).


The second paragraph in 110.31 pertains to outdoor equipment for installations other than equipment as described in 110.31(D). When equipment rated over 1,000V, nominal, is installed outdoors, a wall, screen or fence shall be used to deter access by people who are not qualified. The second paragraph of 110.31 continues by stating the fence shall have a height of at least 7 feet. A 6-foot-tall fence can be used, but there shall be at least a 1 foot extension utilizing three or more strands of barbed wire or equivalent. The distance from the fence to live parts shall be not less than given in Table 110.31. The minimum required distance for live parts rated over 1,000V, nominal, but less than 13,800V, nominal, is 10 feet. The minimum required distance for live parts rated 13,800V, nominal, to 230,000V, nominal, is 15 feet. For equipment installed outdoors, the minimum required distance between a fence and live parts is 18 feet when the equipment has a voltage rating over 230,000V, nominal (see Figure 2).


Where an electrical vault is required or specified for conductors and equipment rated over 1,000V, nominal, the installation shall comply with the specifications in 110.31(A)(1) through (A)(5). The first specifications are for the electrical vault walls and roof, which shall be constructed of materials that have adequate structural strength for the conditions, and they shall have a minimum fire rating of three hours. Studs and wallboard construction shall not be permitted. 


The second subsection states vault floors in contact with the earth shall be constructed of concrete that is not less than 4 inches thick. If the vault is constructed with a vacant space or other stories below it, the floor shall have adequate structural strength for the load imposed on it and a minimum fire resistance of three hours. 


The third subsection for electrical vaults containing equipment rated over 1,000V, nominal, covers doors. Each doorway leading into a vault from the building interior shall be provided with a tight-fitting door that has a minimum fire rating of three hours. Where conditions warrant, the authority having jurisdiction can even require such a door for an exterior wall opening. There is an exception to 110.31(A)(1) through (A)(3). In accordance with the exception, one-hour fire rating construction is permitted for vaults protected with automatic sprinkler, water spray, carbon dioxide or halon. 


The fourth subsection for electrical vaults containing equipment rated over 1,000V, nominal, covers locks. Doors shall be equipped with locks, and doors shall be kept locked, with access allowed only to qualified people. The second sentence in 110.31(A)(4) is similar to the requirement in 110.26(C)(3). The provision pertaining to personnel doors for equipment rated 1,000V, nominal, or less is required only when equipment containing overcurrent devices, switching devices or control devices is rated 800 amperes or more. The provision in 110.31(A)(4) is required regardless of the equipment’s ampacity rating. Personnel doors shall swing out and be equipped with panic bars, pressure plates, or other devices that are normally latched but that open under simple pressure (see Figure 3).


The last subsection in 110.31(A) pertains to transformers. Where a transformer is installed in a vault as Article 450 requires, the vault shall be constructed in accordance with Article 450, Part III’s requirements. The sections in Article 450 that apply to transformer vaults are 450.41 through 450.48. The requirements in Article 450, Part III include all of the requirements in 110.31(A) plus some additional ones that pertain to door sills or curbs, ventilation, drainage, pipe or duct systems that are foreign to the electrical installation. 


Next month’s column continues the discussion of electrical installation requirements.