Many of the articles I write for ELECTRICAL CONTRACTOR originate as questions from people who read my columns or attend my workshops. This article is based on a question from an electrical contractor friend who is doing an electrical installation in an extremely large hangar being used for painting aircraft. His question involves the installation of underground raceways below a hazardous Class 1, Division 1 location and whether the Class 1 location extends through the concrete slab and into the sub fill below the slab.


To answer the question, I first went to Article 513 in the 2014 National Electrical Code (NEC). The scope states that Article 513 applies to buildings or structures, any part of which have aircraft that contain Class I (flammable) liquids or Class II (combustible) liquids where temperatures are above the flash point of the liquids. Also covered are buildings or structures that house or store Class I or Class II liquids and in which aircraft might undergo service, repairs or alterations. 


Pertinent to the question is the definition located in 513.2, which states that an aircraft-painting hangar is constructed for the express purpose of spraying, coating or dipping applications and provided with dedicated ventilation supply and exhaust. Article 513 does not apply to locations used exclusively for aircraft that have never contained fuel or unfueled aircraft, such as buildings or structures used as aviation museums, showrooms or aircraft factories.


Based on 513.3, any pit or depression below the level of the hangar floor is classified as a Class I, Division 1 location, and that classification extends up to the level of the floor. The entire hanger area—including any adjacent areas that are not suitably cut off from the hangar—are Class I, Division 2 locations up to a level of 18 inches above the floor. In addition, where aircraft are stored or maintained, 513.3(C)(1) requires the area within 5 feet horizontally of aircraft power plants or aircraft fuel tanks to be classified as a Class I, Division 2 area, and that area will extend upward from the floor to a level of 5 feet above the upper surface of wings and engine enclosures. This is because fuel storage is often located in the wings and fuel is present in the engine. 


Section 513.3(C)(2) covers area classification for aircraft-painting hangars and requires the area within 10 feet horizontally from aircraft surfaces to be Class I, Division 1; the classified area extends from the floor to 10 feet above the aircraft. Since the area within 10 feet of the aircraft is Class I, Division 1, the horizontal measurement of 20 feet from the edge of the Class I, Division 1 area is classified as Class I, Division 2.


All of these dimensions for paint-spray areas for aircraft painting are derived from NFPA 33, the Standard for Spray Application Using Flammable or Combustible Materials. Also unclassified are any adjacent areas that are suitable cut off from the hangar by walls or partitions, or where sufficient ventilation is provided and where flammable liquids or vapors are not likely to be released—such as stock rooms, electrical control rooms and other similar locations.


The previous paragraph outlining the classified areas was necessary to be able to answer the original question, while 513.8 applies specifically to underground wiring. The title states underground wiring and equipment embedded in the slab, under the slab or just installed in an underground application. This section states that all wiring installed in or under the hangar floor must comply with the requirements for Class I, Division 1 locations. Where wiring is located in vaults, pits or ducts, the wiring must have adequate drainage. 


Section 513.8(B) applies to uninterrupted raceways where the raceways are embedded in the hangar floor, buried beneath the hangar floor or buried in the ground. These uninterrupted raceways are considered to be within the Class I location above the floor, regardless of the point at which the raceway descends underneath or rises above the floor.


Underground, underslab or embedded in slab raceway installations in Article 511, covering self-propelled vehicle commercial major and minor repair facilities, are not required to be classified as Class I, Division 1. Article 514 covers motor-fuel-dispensing facilities and provides requirements for underground wiring in 514.8 but treats these applications much differently than those areas in hangars. This section states that any portion of electrical wiring located below the surface of Class I, Division 1 or 2 locations must be sealed within 10 feet of the point of emergence above grade.


Maybe the underslab requirements in aircraft hangars should be reviewed and modified similar to those in articles 511 and 514.