One of My Favorite Topics…Aviation, That Is!
In the world of National Electrical Code® “Special Occupancies,” one of the articles, Article 513, addresses electrical installations in aircraft hangars. Let’s talk about hangars!
Class 1, Divisions 1 and 2
A key to proper application of Article 513 is understanding Class 1, Division 1 and Class 1, Division 2 areas. The differentiation between Division 1 and Division 2 hinges on concentration levels of flammable gases, liquid-produced vapors, or combustible liquid-produced vapors. This all involves the vapor-to-air ratio.
Class 1, Division 1 is defined in NEC® Article 500.5(B)(1) and (B)(2). A simple explanation is that IF a concentration of flammable gases, liquid-produced vapors, or combustible liquid-produced vapors exist in the area during “normal operating conditions…frequently because of repair or maintenance operations or because of leakage, “or “in which a breakdown or faulty operation of equipment or processes might release” these concentrations, we are in a Class 1, Division 1 area. [NEC, Article 500, 500.5(B)(1)].
A Class 1, Division 2 area, on the other hand, is defined in 500.5(B)(2), and, again in a simplification, is an area in which flammable gases, liquid-produced vapors, or combustible liquid-produced vapors are NOT present during normal operations but in the case of “accidental rupture or breakdown of … containers or systems or in the case of abnormal operation of equipment.”
In both cases, refer to NEC 500.13(B)(1) and (B)(2) for all requirements to properly classify the work area.
Areas of Concern
These differences matter greatly according to what area of the aircraft hangar is being addressed. Let’s break these down one at a time.
In any areas below the floor, including any sump, fuel or oil pit or any other area below the hangar floor, we have a Class 1, Division 1 area. The article gives no exceptions to this area classification.
All other areas above the floor up to 18 inches above the floor level that is “not suitably separated from the hangar” are designated as Class 1, Division 2 areas.
A third area of classification involves the aircraft storage area itself. Article 513 designates any area(s) five feet horizontally from the aircraft power plant or any aircraft fuel tanks are Class 1 Division 2. These areas go upwards from the floor to five feet above the upper surface of wings and engine enclosures. [see NEC Article 513.3(A), (B), and (C)]. Other conditions are introduced in aircraft painting areas, which are not discussed in this article.
Unclassified Areas
So, what if a hangar includes a separate office area, an area to store parts, a restroom, or similar areas? Article 513.3(D) covers these areas and states that they are, in fact, unclassified IF proper ventilation is provided at a rate of at least four or more air changes per hour, or have a positive pressure design, or the area(s) is(are) effectively separated from the hangar area by “walls or partitions.”
If the hangar contains sumps, pits, under-floor storage areas, or even electrical conduits in the floor area, all of these are to be considered Class 1, Division 1 areas, which introduces full application of NEC Article 501 “Class 1 Locations.” This can complicate hangar wiring and may seriously increase the cost associated with these areas.
What Wiring Methods are Allowed?
All wiring that is not in these locations must be installed in either MI, TC, or MC cable, or in metal raceways. These wiring types and their requirements are explained in NEC Chapter 3, known as “Chapter 3 compliant wiring methods.”
If you delve into Article 336, you can glean all information needed for “Power and Control Tray Cable.” Tray cable has limitations of use, however, that appear in 336.10. The NEC specifically prohibits TC in areas “exposed to physical damage.”
Article 332 covers MI cable, known as mineral-insulated metal sheathed cable. If you have no experience with MI cable installation, it looks like thin copper tubing on the exterior and is very time-consuming to install, involving specialized tools and attention to detail to properly install. The advantage of MI cable is the wide range of applications, as its only “Uses Not Permitted” include underground runs that are unprotected from physical damage, and areas that might destroy the protective sheath, which is usually a copper alloy.
The last allowed wiring method, besides metallic raceways, is MC cable. This is metal-clad cable that includes conductors in an overall metal cladding, usually spiral-wound. This sheath can be aluminum, which is typical, or steel, which is also common. MC cable must have listed MC connectors for every connection point. These connectors can get costly, as every section of wiring run will use two of these connectors.
When running MC cable, the installer needs to make sure that the installation hugs building surfaces tightly to avoid potential damage to the sheath during building usage. This requirement is specified in NEC Article 330.15.
When included in the floor slab, all raceways that penetrate the floor are still considered Class 1, Division 1 areas until they are properly sealed using a listed seal-off fitting as described in NEC Article 501.
Equipment Requirements
Unique to aircraft hangars is a code requirement for items like vacuum cleaners, air compressors, fans, etc. If such equipment is not listed as Class 1, Division 1 or Class 1, Division 2 compliant, such equipment must only be used if mounted at least 18 inches above the floor.
Proper signage must be installed on mobile equipment permanently that states “WARNING – KEEP 5 FT CLEAR OF AIRCRAFT ENGINES AND FUEL TANK AREAS” or similar wording [see NEC Article 513.10(D)]. As such, all flexible cords for this equipment have to be identified as extra-hard usage and identified for the location being operated in and must include an equipment grounding conductor.
You can find the definition of extra-hard use cord types in NEC Chapter 4 Article 400 “Flexible Cords and Flexible Cables.”
Other Requirements
Article 513 includes other requirements not discussed in this article. Ultimate care must be exercised to ensure proper wiring methods and daily operations in aircraft hangars. . The key to understanding vapor danger levels is to understand the critical concentrations of vapors between the lower explosive limit and the upper explosive limit. Within these ranges, such fumes are extremely volatile. Hence, areas with ratings of Class 1, Division 1 and Class 1, Division 2 must be highly respected to avoid potential catastrophic results.
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