In outdoor areas, electrical installations must be enclosed by a wall, screen, or fence that is tall enough to prevent unqualified persons from accessing it. The enclosure must be designed and constructed based on the level of hazard associated with the installation. For installations other than equipment described in 110.31(D), the fence must be at least 2.1 m (7 ft) tall, or a combination of 1.8 m (6 ft) or more of fence fabric and 300-mm (1-ft) or more of barbed wire or an equivalent. The distance from the fence to live parts must be at least what is listed in Table 110.31.
Informational Note: See Article 450 for construction requirements for transformer vaults.
When an electrical vault must be used for circuits over 600 volts, the walls and roof must be constructed with materials that have enough structural strength and a minimum fire rating of 3 hours. The floor must be at least 102 mm (4 in.) thick and have a minimum fire resistance of 3 hours. If the vault is constructed with a vacant space or other floors below it, the floor must have enough structural strength to support the load.
Doors leading into the vault from the building’s interior must be tightly fitted and have a minimum fire rating of 3 hours. The authority with jurisdiction may require a door with a minimum fire rating of 3 hours for an exterior wall if necessary. If the vault is protected with a sprinkler system, water spray, carbon dioxide, or halon, then the doors only need to have a 1-hour fire rating.
Doors must be equipped with locks and kept locked, with access only allowed to qualified persons. Personnel doors must swing out and be equipped with panic bars, pressure plates, or other devices that open under simple pressure. If a transformer is installed in a vault as required by Article 450, then the vault must be constructed according to Part III of Article 450.
Additional information on vault construction can be found in ANSI/ASTM E119-1995, NFPA 251-2006, and NFPA 80-2010. A typical 3-hour construction is 150 mm (6 in.) thick reinforced concrete.
Section 110.31 was revised and expanded for the 2011 NEC. It includes more detailed construction requirements for vaults that are required or specified for use with circuits over 600 volts. Doors and locks must be tightly fitted and able to be opened under simple pressure. If a vault is required by Article 450, it must be constructed according to Part III of Article 450. An exception has been added to allow for 1 hour construction when the vault is protected by a fire suppression system. This exception is based on 450.42 and 450.43.