The title of 110.31 in both the 2008 and the 2011 National Electrical Code (NEC) indicates this section deals with enclosures for electrical installations operating at more than 600 volts (V) nominal. Within 110.31 are subsections covering the fire resistance of electrical vaults as well as indoor installations, outdoor installations and enclosed equipment accessible to unqualified personnel. The text within Section 110.31 is not written in a clear and concise manner and should be redone. However, Panel 1 has done an excellent job of rewriting and clarifying subsection 110.31(A) in the 2011 NEC, covering the requirements for electrical installations in vaults. Panel 1 could use the rewritten subsection as a template for the rewrite of the entire section. Close examination of the text in the 2008 NEC may help in understanding the inconsistencies and may provide a model for a comprehensive change in this text for the 2014 NEC.

The opening paragraph in 110.31 describes the access of high-voltage enclosures as follows: “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 persons only. The type of enclosure used in a given case shall be designed and constructed according to the nature and degree of the hazard(s) associated with the installation.” The first sentence deals more with the accessibility of the equipment by qualified people only, and the following sentence addresses the type of enclosure based on the hazard. These two sentences need to be split off and incorporated into two separate subsections.

The next paragraph covers walls, screens or fences for enclosing an outdoor electrical installation to deter access by those who are not qualified to be close to high-voltage equipment. In this paragraph, the text states that a fence shall not be less than 2.1 meters (7 feet) in height or a combination of 1.8 meters (6 feet) or more of fence fabric and 300 millimeters (1 foot) or more extension using three or more strands of barbed wire or the equivalent. The distance from the fence to live parts shall be not less than given in Table 110.31. This entire paragraph should be incorporated into existing 110.31(C) to create a clear description of the requirements for outdoor installations.

Section 110.31(A) has been totally rewritten for the 2011 NEC to address electrical vaults for higher voltage installations (similar to how transformer vaults are treated in Part III of Article 450) as follows with underlines as new text and cross outs as deleted text: “(A) Fire Resistance of Electrical Vaults. Where an electrical vault is required or specified for conductors and equipment operating at over 600V nominal, the following shall apply:

1 “Walls and Roof. The walls and roof shall be constructed of materials that have adequate structural strength for the conditions, with a minimum fire rating of 3 hours. For the purpose of this section, studs and wallboard construction shall not be permitted.”

2 “Floors.” [Here the text has not been changed and still requires floors to be concrete where in direct contact with the earth.

3 “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 3 hours. The authority having jurisdiction shall be permitted to require such a door for an exterior wall opening where conditions warrant.”

“Exception to (1), (2) and (3): Where the vault is protected with automatic sprinkler, water spray, carbon dioxide, or halon, construction with a 1-hour rating shall be permitted.”

4 “Locks. Doors shall be equipped with locks, and doors shall be kept locked, with access allowed only to qualified persons. 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.”

5 “Transformers. Where a transformer is installed in a vault as required by Article 450, the vault shall be constructed in accordance with the requirements of Part III of Article 450.”

“Informational Note No. 1: For additional information, see ANSI/ASTM E119-1995, Method of Fire Tests of Building Construction and Materials, NFPA 251-2006, Standard Methods of Tests of Fire Resistance of Building Construction and Materials, and NFPA 80 2007, Standard for Fire Doors and Other Opening Protectives.”

“Informational Note No. 2: A typical 3-hour construction is 150 millimeters (6 inches) thick reinforced concrete.”

The remainder of 110.31 is the same in the 2011 NEC as it was in the 2008 edition, but as can be seen by the underlined text above, the revised 110.31(A) dealing with vaults for above 600V in the 2011 NEC is very clear, concise and easy to understand.


ODE is a staff engineering associate at Underwriters Laboratories Inc., based in Peoria, Ariz. He can be reached at 919.949.2576 and mark.c.ode@us.ul.com.