Understanding General Requirements of the NEC, Part 17
Article 110 provides general requirements for all installations. While Part II provides the requirements for systems operating at or below 1,000V ac or 1500V dc (nominal), Part III provides the requirements for systems operating over 1,000V ac or 1500V dc (nominal).
Some parts of Part III have changed with the 2026 revision to the NEC. One of those changes is the title now includes “1500 Volts dc”. This stating of both the ac and dc voltages is a general change throughout the NEC with the 2026 revision.
Where Part II systems tend to have enclosures you walk up to, Part III systems tend to have enclosures and vaults you walk (or crawl) into. Thus, the second section (110.31) of Part III is about electrical installations that are “in a vault, room, or closet or in an area surrounded by a wall, screen, or fence.” The gist of it is that such a space “is considered to be accessible to qualified persons only.”
The differences between Part II and Part III are further illustrated in the requirements for indoor installations and outdoor installations. For Part II installations, there’s no mention of unqualified persons. In Part III, there is.
For Part III indoor installations that are accessible to:
- Unqualified persons, the equipment must be metal-enclosed [Sec. 110.31(B)(1)]. There are also requirements for marking and for diverting foreign objects away from energized parts if said objects are inserted through ventilation (or similar) openings.
- Qualified persons only, they must comply with Sec. 110.34 (workspace and guarding), Section 110.36 (circuit conductors), and Sec. 495.24 (minimum space separation) [Sec. 110.31(B)(2)].
For Part III outdoor installations that are accessible to:
- Unqualified persons, they must comply with Art. 267 Part I [Sec. 110.31(C)(1)]. Article 267 provides the requirements for branch circuits and feeders over 1000V ac, 1500V dc, nominal.
- Qualified persons only, they must meet the same requirements put forth for indoor installations [Sec. 110.31(C)(2). This subsection further stipulates these must comply with the first paragraph of Sec. 110.31 which means any enclosures used must be designed and constructed per the “nature and degree of the hazards associated with the installation.”
Table 110.31 provides the minimum distance from the fence to live parts, broken down by three nominal voltage ranges:
- 1001V ac to 13,700V ac or 1501V ac to 13700V dc, 10 ft.
- 13,800V ac or dc to 230,000V ac or dc, 15 ft.
- Over 230,000V ac or dc, 18 ft.
Working space requirements for Part III equipment installations necessarily exceed those for Part II equipment installations. You will find the enhanced distances described in Sec. 110.32 Work Space About Equipment and Sec. 110.34(A) Working Space. Table 110.34(A) provides the minimum depth of working space at electrical equipment, and this is broken down into 5 voltage ranges.
About the Author
Mark Lamendola
Mark is an expert in maintenance management, having racked up an impressive track record during his time working in the field. He also has extensive knowledge of, and practical expertise with, the National Electrical Code (NEC). Through his consulting business, he provides articles and training materials on electrical topics, specializing in making difficult subjects easy to understand and focusing on the practical aspects of electrical work.
Prior to starting his own business, Mark served as the Technical Editor on EC&M for six years, worked three years in nuclear maintenance, six years as a contract project engineer/project manager, three years as a systems engineer, and three years in plant maintenance management.
Mark earned an AAS degree from Rock Valley College, a BSEET from Columbia Pacific University, and an MBA from Lake Erie College. He’s also completed several related certifications over the years and even was formerly licensed as a Master Electrician. He is a Senior Member of the IEEE and past Chairman of the Kansas City Chapters of both the IEEE and the IEEE Computer Society. Mark also served as the program director for, a board member of, and webmaster of, the Midwest Chapter of the 7x24 Exchange. He has also held memberships with the following organizations: NETA, NFPA, International Association of Webmasters, and Institute of Certified Professional Managers.