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Electrical Design Considerations for Commercial Garages

Electrical Design Considerations for Commercial Garages

For years, commercial garages have been the source of serious safety hazards, considering their potential for fire and explosion. Service pits, for example, have been the site of workers trapped under burning vehicles, resulting in terrible injuries and/or electrocutions due to concentrations of several flammable liquids and water. Although gasoline is the main culprit, hydrogen is also potentially

For years, commercial garages have been the source of serious safety hazards, considering their potential for fire and explosion. Service pits, for example, have been the site of workers trapped under burning vehicles, resulting in terrible injuries and/or electrocutions due to concentrations of several flammable liquids and water. Although gasoline is the main culprit, hydrogen is also potentially dangerous — as a fuel and as a vapor (given off by lead acid batteries during rapid charging). Compressed natural gas and propane (used as alternative fuels) and volatile hydrocarbons (used as solvents in parts cleaners) also present a threat. And don't forget about grease, kerosene, and diesel fuel. Although they are not as easily ignited, these garage staples can contribute a vast amount of heat to the environment — not to mention the presence of oxy-acetylene welding equipment, which adds to the mix.

Based on the underlying danger that exists in these structures, it's fitting that they're deemed Class I Division 1 zones. To mitigate risks, most commercial garages currently under construction are opting for above-slab lifts, while many older buildings have discontinued their pits. Despite this step forward, amid all of this combustible material, there is still extensive use of electricity in lighting, portable power tools, arc welding, and battery-charging equipment — together with large motors. All of this means the electrical installation must be impeccable. As an electrician or electrical contractor, the best way to start is by complying with applicable Code mandates in the design stage. Let's take a look at what's involved in wiring commercial garages.

Initial assessment

Examining Art. 511, we see the commercial garage has vertical and horizontal boundaries, and the delineated areas are classified according to the degree of hazard encountered. We also see that the authority having jurisdiction (AHJ) can derate some of these classifications if:

  • The building's use will be restricted to the extent that fuel systems will not be opened. (This would preclude major engine work or fuel system repairs, including replacement of fuel filters and fuel pumps.)

  • Positive air ventilation will be in place at all times (even nights and weekends) as long as vehicles are present.

If you're in the design stage of a commercial garage, it's probably better not to take advantage of either of these options. A better approach is to design the whole building so that a given area is considered Class I Division 2, but wiring is kept out of that zone. Moreover, in the life of the building, the installation and maintenance of an air-venting system means a large amount of energy, parts, and upkeep — just to downgrade a classification. How do you avoid these pitfalls? By proper planning on the front end.

Design considerations

Design work for a commercial garage should address all facets of the proposed construction. The first step is to size out the service. Table 220.12 gives the general lighting load for various occupancies. Commercial garages require ½VA per square foot. This may not sound like much, but remember that unlike dwelling units, the receptacle load is not included in this figure. It has to be added to it, along with all other loads. The ½ ampere-per-square-foot is for lighting only. Thus, the designer has to enumerate all receptacles, motors, and other equipment to calculate the total load (see Table).

If any of the motors will be 5 horsepower or higher — or if there will be heavy arc welding work performed in the building — now is the time to install a 3-phase service. In the early stage of design work, a meeting with the electric utility should take place to agree on type of service, rate structure, and where the service drop or lateral will hit the building. Will a masthead be necessary to achieve minimum ground clearance? If so, you can size out and design the service including metering and ground electrode conductor entry into the concrete.

For a large building, over 100 feet in greatest dimension, you may want one or more feeders and subpanels to minimize voltage drop. All structural metal, water pipe, metal ductwork, gas, air lines, and the like should be solidly bonded back to the service neutral at the main bonding jumper. At that point, the grounded neutral and the equipment-grounding conductor separate — never to rejoin.

Branch circuits higher than 18 inches above the slab (and 18 inches below the ceiling if compressed natural gas will be allowed) should be placed in EMT and MC (or flexible conduit as needed). Under this arrangement, continuous ventilation is never required, and a full range of mechanical work is allowed, including fuel tank changes and major engine overhauls.

Although Art. 511 is only a few pages long, it should be carefully scrutinized. The key is to make sure all wiring is located well outside any horizontal or vertical boundaries.

Special concerns

Classified areas within a commercial garage are delineated in 511.3 (B). In the event that flammable fuels will be dispensed into vehicle fuel tanks inside or outside the building, such areas must also conform with Art. 514, Motor Fuel Dispensing Facilities.

If there will be a spray room for automotive painting, it must comply with Art. 516, Spray Application, Dipping and Coating Processes. Here again, rather than design to Class I specifications (rigid or intermediate conduit, Type MI cable, etc.), it's possible to locate all lighting behind indestructible translucent or transparent panels. All wiring should be effectively partitioned from the interior of the spray room, as should any motors and associated equipment that drive ventilation fans. Spray fumes should be vented to a dedicated area outdoors high above grade, and separated from any source of ignition and from any air intake.

If effectively partitioned from the garage service areas, offices, parts rooms, and employee lunchrooms are excluded from the classified zone area. However, the entries to all unclassified areas must have doorsills more than 18 inches above the garage floor. This feature should be addressed early in the design phase so that it's reflected in the foundation plan.

Battery-charging equipment and batteries being charged must be in one of these partitioned rooms. Such an area should be fairly large with a high ceiling to allow hydrogen to dissipate. It should be vented, but not back into the service area. A welding room, with noncombustible wall material, should be partitioned off from the main service area and be provided with receptacles sized to match the arc welding equipment that will be used. These will be hardwired back to the panel with no additional disconnect since the welders have unit switches.

So far, we've considered fire and explosion hazards, but there is another serious danger lurking in the commercial garage work environment: electric shock.

Avoiding underlying danger

Electric shock hazards can be offset by Code compliance. As per the Code, all 125V, single-phase, 15A and 20A receptacles in commercial garages where electrical diagnostic equipment, electrical hand tools, or portable lighting equipment are used must have GFCI protection. This can be achieved by installing either circuit breaker or receptacle-type devices. The least expensive but perfectly compliant and effective way to meet this requirement is by using the feed-through capability of a GFCI receptacle to protect downstream receptacles, which should be identified by affixing the stickers provided. Such GFCI protection is needed for outdoor receptacles as well.

These outdoor devices need in-use “bubble covers” in case vehicles will be left unattended with block heaters plugged in. Article 210, which covers GFCI protection for branch circuits, also requires this technology for all receptacles in non-dwelling bathrooms, kitchens, and rooftops, if equipped for servicing heating and air conditioning.

Fire and explosion hazard resulting from electrical wiring juxtaposed with flammable gas and liquid fuels found in commercial garages is better mitigated by locating wiring outside the classified areas, rather than wiring to Class I standards inside those areas or by encumbering the building with a ventilation system that would have to be maintained in perpetuity or by limiting the scope of work permitted. Careful planning and design work can avert human injury and property loss in the future, making for an efficient and profitable operating environment.

Herres is a licensed master electrician in Stewartstown, N.H.

Sidebar: Low-Voltage Accommodations

Increasingly, low-voltage wiring is becoming a part of the electrician's scope of work on commercial garage jobs. For example, telephone and satellite (or cable) Internet, properly grounded, should enter the building at an appropriate place based on outside parameters and be distributed to points of use.

The office, customer service counter, and parts department will need computers connected to the Internet — possibly in a networked configuration. These same areas need telephones every 30 feet within the service area of the garage. Install Cat. 5e or Cat. 6 UTP cable in EMT to all points of use.

Although the Code does not mandate cable fill limitations, four or five cables are the most you can fit in ¾-inch EMT, depending on the geometry of the run. Telephone and Ethernet cables can occupy the same pipe, but not power conductors. You can run 1-inch main lines and ¾-inch drops to the individual points of use. Wall boxes with faceplates incorporating an RJ45 modular Ethernet connector for Internet access, and an RJ25 for telephone will provide excellent service. You can also run coax cable to the office and employee lunchroom if TV is desired. Because the service area can be noisy, there should be public announce capability (audio cabling cannot occupy the same raceway with low-voltage conductors) and an AC-powered telephone ringer.

Sidebar: Lighting the Way

Lighting a commercial garage can also create challenges for the designer. For ceiling heights below 16 feet, low-bay lighting fixtures are the best choice. High-bay fixtures work well where the ceiling height is 20 feet or greater. Ordinary cool white T8 fluorescent fixtures are good over workbenches and other high-use areas.

All lighting should be carefully zoned so that the entire building does not have to be lighted at once. Three- and four-way switching should be provided for walk-through lighting, and a small number of T8 fixtures the length of the garage should be unswitched so that the area is never in total darkness. Battery-powered emergency light units, which also drive exit lights, should be tied into the unswitched lighting circuits. Lighting in a commercial garage is considered a continuous load, rated at 125%, since it is expected to operate over 3 hours.

Receptacles should be provided for all bench areas. They can consist of two duplex straps per 4×4 surface-mount box with raised faceplate. The NEC does not require any particular spacing, so it's up to the designer. They should be abundant to minimize the use of extension cords, and, as previously mentioned, all must be GFCI protected. If these are piped in EMT and mounted along the wall 6 inches above the bench, they will be easy to use and well out of the classified area.

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