Wiring Commercial Garages and Repair and Storage Facilities

A commercial garage may not seem like much of an electrical challenge at first glance, but upon closer inspection you may discover more than just the usual lights, fans, air compressors, and lifts. If the garage has work pits, fuel storage systems, or fuel dispensing equipment, your installation needs to comply with the requirements of Art. 511. Otherwise, it could jeopardize the safety of those who work in or visit the facility.


Art. 511 applies to areas used for service and repair operations of self-propelled vehicles — including passenger automobiles, buses, trucks, and tractors — that use volatile flammable liquids or gases for fuel or power. You can apply Art. 511 requirements to residential or repair garages for watercraft, 4-wheelers, or motorcycles, but it's not a Code requirement.

Installations that likely fall within the scope of Art. 511 include automobile service/repair centers; service and repair garages for commercial vehicles, such as trucks and tractors; and service and repair garages for fleet vehicles, such as cars, buses, and trucks.

Location, location, location. To determine if Art. 511 applies, you must decide whether the characteristics of the garage deem it a hazardous location. First, carefully review Art. 500 against the planned uses of the garage, and then refer to NFPA 88A-1998, “Standard for Parking Structures” and NFPA 88B-1997, “Standard for Repair Garages.” You must conform to Art. 514 if you're wiring in an area used for motor fuel dispensers or in a location where people transfer flammable fuel to vehicle fuel tanks.

Don't fall into the common trap of classifying the entire garage the same way. A garage may have a vehicle service and repair area that must comply with Art. 511. It could also have another area for fuel dispensing, which must comply with Art. 514. But it may also have other areas, such as show rooms, offices, or a parts department, that don't fall under Art. 511 or Art. 514. While all of these areas must comply with NEC Chapters 1 through 4, each area must comply with the Chapter 5 requirements particular to its specific characteristics.

Elevations. Thinking of an area in terms of the plan view in a drawing is two-dimensional. Classifications include the third dimension of height. Thus, the elevation of a given volume of space is a critical factor in its classification.

The service and repair area up to 18 in. above the floor is classified as Class I, Division 2 (Fig. 1). Where compressed natural gas (CNG) vehicles are repaired or stored, the area within 18 in. of the ceiling is also classified as Class I, Division 2. However, if mechanical ventilation provides a minimum of four air changes per hour, the authority having jurisdiction (AHJ) can declare the area nonclassified.

Any pit or depression below the service/repair floor is considered a Class I, Division 1 location (Fig. 2). This pit requirement has two exceptions:

  • If mechanical ventilation provides a minimum of six air changes per hour, the AHJ can classify the area as a Class I, Division 2 location.

  • If a lubrication or service room doesn't have dispensing equipment, the AHJ will classify it per Table 514.3(B)(1).

An example of this second exception would be an oil change and lubrication service pit. Table 514.3(B)(1) declares this area as unclassified if it meets certain ventilation requirements.

Fig. 3 illustrates the stipulation in 511.3(B)(4) that areas adjacent to classified locations, such as stock rooms, switchboard rooms, and other similar locations, aren't classified if they're mechanically ventilated at a rate of four or more air changes per hour or when the adjacent area is effectively cut off by walls or partitions. The AHJ may declare areas adjacent to classified locations unclassified for reasons of ventilation, air pressure differentials, or physical spacing.

Wiring and equipment requirements.

You must install wiring and equipment within a Class I location per the requirements of Art. 501, one of which calls for threaded rigid metal or intermediate metal conduit. However, raceways embedded in a masonry wall or buried beneath a floor aren't considered to be within the Class I location unless the raceway actually enters the classified area (Fig. 4). A raceway that enters a Class I, Division 1 or 2 location must be threaded rigid or intermediate metal conduit. You can use rigid nonmetallic conduit (RNC) with an equipment-grounding conductor underground if it has at least 2 ft of cover (Fig. 5). If the raceway enters a classified location, the underfloor area takes on the classification of the above area, even if the conduit passes unbroken through the classified area and terminates in an unclassified area. The concern is that volatile gas could seep into the underfloor area through the conduit openings in the floor.

The requirements of Art. 514 govern fuel-dispensing units — other than liquid petroleum gas, which is prohibited — that are located within buildings. Portable lighting equipment, such as lamps and accompanying cords, must be supported or arranged in such a manner that they aren't installed in the locations classified in 511.3, or are an approved type for use in Class I, Division 1 locations.

Wiring and equipment above Class I locations

Per 511.7, all wiring above a Class I location must be within a raceway or in certain cable types. You can use flexible hard usage cords for pendants if you install them according to the requirements of 400.4. Electrical equipment installed in a fixed position must be identified for a Class I location or positioned above it. Equipment with make-and-break contacts installed less than 12 ft above the floor level, excluding receptacles, lamps, and lampholders, must be completely enclosed or constructed to prevent sparks or hot metal particles from escaping.

Lampholders and lamps installed for fixed lighting over travel lanes or other places where physical damage is likely must be located at least 12 ft above floor level, unless the luminaires are totally enclosed or constructed to prevent sparks or hot metal particles from escaping.

You must also install seals for raceway, cable, and boundaries per the requirements of 501.5. Where the Class 1, Division 1 boundary is beneath the ground, you can install the sealing fitting at a point past where the conduit leaves the ground. However, you can't use a union, coupling, box, or fitting between the conduit seal and the point where the conduit leaves the ground [501.5(A)(4) Exception 2].

Special requirements.

You can't locate battery chargers — or batteries being charged — within a Class I, Division 1 or 2 area. That means you need a separate room for battery charging if, for example, you store fuel in the garage. Equipment for charging an electric vehicle must be installed per Art. 695.

The Code requires GFCI protection for all 125V, single-phase 15A and 20A receptacles for service and repair operations like electrical automotive diagnostic equipment, electrical hand-tools, and portable lighting devices (Fig. 6). While OSHA requirements for GFCIs aren't as strict as those of the NEC, you shouldn't automatically adopt the lesser requirements. The AHJ will likely enforce the Code requirements, and all indications point to OSHA moving toward NEC compliance.

On the surface, this seems like quite a few extra requirements for an installation that goes beyond pure parking. However, if you read those requirements carefully, you'll notice a trend: the purpose of Art. 511 is to ensure a separation between volatile fumes and ignition sources. If you keep that purpose in mind when designing, building, or retrofitting a garage, you should have no problem complying with Art. 511. And you'll sleep better at night knowing you've installed a safe electrical system.

Are you still confused by the Code? For additional information on Code-related topics please visit www.mikeholt.com or send an e-mail directly to the author at mike@mikeholt.com.