Article 110 in the National Electrical Code (NEC), 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. It contains five parts.


Part I, General, includes Articles 110.1 through 110.25. Topics covered include installation and use, listing of equipment, voltages, conductors, wiring methods, interrupting rating, selective coordination, mechanical execution of work, mounting and cooling of electrical equipment, electrical connections, arc flash hazard warning, markings, identification of disconnecting means, available fault current, and lockable disconnecting means.


Part II includes Articles 110.26 through 110.28. While Part II contains only three sections, two of them contain a number of subsections. Part II contains provisions for electrical systems rated 1,000 volts (V), nominal, or less. Prior to the 2017 NEC, the voltage threshold for this part was 600V, nominal, or less. 


Part III contains provisions for electrical systems rated more than 1,000V, nominal, includes 110.30 through 110.41. 


Part IV covers tunnel installations more than 1,000V, nominal. It also contains provisions for the installation and use of high-voltage power distribution and utilization equipment that is portable, mobile or both, such as substations, trailers, cars, mobile shovels, draglines, hoists, drills, dredges, compressors, pumps, conveyors, underground excavators, and the like. Provisions in Part V—the last part in Article 110—apply to installations in manholes and other electrical enclosures intended for personnel entry. 


Enclosures for electrical installations are covered in 110.31, which is divided into four subsections: (A) electrical vaults, (B) indoor installations, (C) outdoor installations, and (D) enclosed equipment accessible to unqualified people. The provision pertains to conductors and equipment rated over 1,000V, nominal. Section 110.31(B) pertains to indoor installations and is divided into two subsections. In accordance with 110.31(B)(1), indoor electrical installations that are accessible to unqualified people shall be made with metal-enclosed equipment. Switchgear, transformers, pull boxes, connection boxes and other similar associated equipment shall be marked with appropriate caution signs.


It may be necessary to refer to Article 490 when referencing requirements in Part III of Article 110. Article 490 covers the general requirements for equipment operating at more than 1,000V, nominal. In accordance with 490.30, Part III covers assemblies of switchgear and industrial control equipment including, but not limited to, switches and interrupting devices and their control, metering, protection and regulating equipment where they are an integral part of the assembly, with associated interconnections and supporting structures (see Figure 1).


The last sentence in 110.31(B)(1) states that openings in ventilated dry-type transformers or similar openings in other equipment shall be designed so that foreign objects inserted through these openings are deflected from energized parts. Section 110.31(B)(2) pertains to indoor electrical installations accessible to qualified people. This section simply states the installation shall be installed in accordance with 110.34, 110.36 and 490.24. Working space and guarding requirements are in 110.34, and circuit-conductor requirements are in 110.36. Section 490.24 covers minimum space separation provisions in field-­fabricated installations (see Figure 2).


Section 110.31(C) pertains to outdoor installations, and it is divided into two subsections. In accordance with 110.31(C)(1), outdoor electrical installations that are open to unqualified people shall comply with Parts I, II and III of Article 225. Article 225 covers requirements for outside branch circuits and feeders run on or between buildings, structures or poles on the premises, and it covers electrical equipment and wiring for the supply of utilization equipment that is located on or attached to the outside of buildings, structures or poles.


Part I covers general requirements for outside branch circuits and feeders. Part II contains requirements for buildings or other structures supplied by feeder(s) or branch circuit(s). Part III covers outside branch circuits and feeders over 1,000V. Section 110.31(C)(2) states outdoor electrical installations that have exposed live parts shall be accessible to qualified people only in accordance with the first paragraph of 110.31. The last part is like 110.31(B)(2) in that is states the installation shall be installed in accordance with 110.34, 110.36 and 490.24.


Section 110.31(D) covers enclosed equipment that is accessible to unqualified people. The first part is similar to the last sentence in 110.31(B)(1) that pertains to dry-type transformers. Ventilating or similar openings in equipment shall be designed so that foreign objects inserted through these openings are deflected from energized parts.


The next sentence states, where exposed to physical damage from vehicular traffic, suitable guards shall be provided. Since this section is not divided into indoor and outdoor locations, this provision applies to both types of locations. Vehicles inside industrial plants are not uncommon. Therefore, electrical equipment must be protected from physical damage whether installed indoors or outdoors.


The next provision is specifically for outdoor equipment rated over 1,000V. Equipment located outdoors that is accessible to unqualified people shall be designed so that exposed nuts or bolts cannot be readily removed, permitting access to live parts. While some provisions in Part III have a similar provision in Part II for equipment rated 1,000V, nominal, or less, there is no requirement in Part II to prohibit nuts and bolts from being readily removed. Where equipment is accessible to unqualified people and the bottom of the enclosure is less than 8 feet above the floor or grade level, the enclosure door or hinged cover shall be kept locked (see Figure 3).


Section 110.31(D) continues by requiring doors and covers of enclosures used solely as pull boxes, splice boxes or junction boxes to be locked, bolted or screwed on. Underground box covers that weigh more than 100 pounds shall be considered as meeting this requirement. The NEC mentions manhole access covers five times. A manhole access cover does not have to be locked, bolted or screwed on as long as it weighs more than 100 pounds (see Figure 4).


Working space requirements for equipment over 1,000V are in 110.32. Sufficient space shall be provided and maintained about electrical equipment to permit ready and safe operation and maintenance of such equipment. Where energized parts are exposed, the minimum clear work space shall not be less than 61/2 feet high (measured vertically from the floor or platform) or not less than 3 feet wide (measured parallel to the equipment).


This section states the clear work space shall be at least 61/2 feet high or not less than 3 feet wide. This is odd since 110.26 (for equipment rated 1,000V, nominal, or less) requires a minimum work space height and a minimum work space width. In my opinion, the clear work space shall be at least 61/2 feet high and not less than 3 feet wide. Section 110.32 continues by requiring the work space depth to comply with 110.34(A). The last sentence in 110.32 for equipment over 1,000V is just like the last sentence in 110.26(A)(3) for equipment rated 1,000V, nominal, or less. In all cases, the work space shall permit at least a 90-degree opening of doors or hinged panels.


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