Considerations for emergency generator systems

Learn about factors to consider when designing a generator for an emergency power system

By Bianca Jimenez and Rick Baca January 1, 2023
Courtesy: Michael Robinson, SmithGroup


Learning Objectives

  • Understand the physical components of an emergency power system.
  • Become familiar with interdisciplinary design factors and physical considerations of emergency power systems.
  • Learn key talking points for a better-integrated design for emergency power systems.


Generator insights

  • Commercial buildings often use generators to provide power for backup, emergency or life safety.
  • Specifying a generator requires the engineer to ask several questions and to consider the siting of the genset.

When designing a new facility one of the first space planning questions by the design team is, “Do we need a generator?” For the engineer, the answer can be obvious — or the project application may be complex and require a code study to determine the need and system configuration.

There are standard building systems that often require defined operational time during a normal power outage, such as egress lighting, but often a designer will plan for a generator if longer runtimes are required or if there are larger operational loads. If a battery cannot support the code requirement, the engineer will look at generators as the next option.

Examples of emergency generator systems applications are fire pumps, high-rise buildings, atriums, chemical exhaust systems and hospitals. Less common examples can include critical operations power systems facilities or high-density storage facilities.

While egress lighting and other life safety systems could be served with a central or distributed battery system, a generator becomes a practical application when other legally required and optional standby loads are introduced into the generator system, as these tend to be considerable electrical loads. In some cases, even when a central battery system or distributed batteries would suffice, the client will find a generator more practical to maintain.

Engineering design has a priority obligation to public safety and the liability risk is there to reinforce professional ethics. The National Society of Professional Engineers state as its first canon that “Engineers, in the fulfillment of their professional duties, shall: 1. Hold paramount the safety, health and welfare of the public.” The engineering review of emergency systems is essential for every project and should be taken seriously.

Figure 1: Perspective rendering of utility yard including generator, nedium-voltage transformer and medium-voltage switch, located behind an architectural screen wall, emphasizing the visual cohesion between the utility space and the building exterior, at the Applied Research Building in Tucson, Arizona. Courtesy: Ryan Haines, SmithGroup

When codes and standards require generators

Some projects do not require a generator distribution system per code, but are required by the client for operational purposes. Other projects may want to combine standby power systems with the emergency power system, which is allowed if there is a proper load shedding schemes provided. There may also be facilities where medium-voltage generators greater than 600 volts become practical. While often overlapping in architectural considerations, this article will not discuss those unique details.

All devices and control devices in the emergency system must be tested and listed by an agency approved by the authority having jurisdiction for their intended use. There are many different types of listings for testing. UL Solutions’ 2200 is required for stationary engine generators. UL 1008 is required for transfer switch equipment. UL 924 is required for emergency lighting automatic load control relays.

It is also essential to understand that only some devices are intended for emergency operations. Light fixtures, for example, can be used for egress, but only certain portions of the facility lighting systems are designed for use as an egress system. The egress lighting system can be provided with 90-minute battery backup or supplied from an emergency generator system.

For example, power packs are listed for normal and UL 924 operation. Misapplying and/or missing the listing and testing requirements can be a considerable complication resulting in cascading implications.

Figure 2: This shows the interior generator installation for a hospital facility located in Phoenix, Arizona. Courtesy SmithGroup

What is in an emergency system?

NFPA 110: Standard for Emergency and Standby Power Systems includes two important definitions for emergency systems, emergency power supply, or EPS, and emergency power supply system, or EPSS. EPS is “the source of electric power of the required capacity and quality for an emergency power supply system,” which is often the generator itself. EPSS is ‘’a complete functioning system … needed for the system to operate.”

A typical system will consist of the generator(s), transfer switches, load banks, temporary generator connections, distribution boards, panelboards, breakers and all the pathways in between. This will apply to all systems requiring emergency support, such as egress lighting, emergency exhaust or critical heating, ventilation and air conditioning.

Where do we put a generator?

The generator and emergency systems can physically reside in the building’s interior, exterior, above grade or even on the roof — and likely a combination of locations if access is limited to qualified persons. Subgrade installation is a possibility but usually avoided due to the potential of water intrusion during a flood scenario. Several factors will determine location, including engineering judgment, code requirements and project parameters, to “minimize the probability of equipment or cable failure” (NFPA 110 7.1.1).

When installed within building interior spaces, EPS units must be in a separate room with a two-hour rating with exclusively EPSS equipment per NFPA 110 7.2.1. Also, consider that if the generator is interior to the building, the motor will likely cause vibrations that some facilities may not tolerate.

When installed outdoors, weather-rated enclosures maintain the functionality of the EPS and are a viable solution, provided the enclosure is rated to resist the ingress of weather to the level required by the local building codes. Appropriate weather-rated housings and a single enclosure often simplify the design for operations and points of risks. The noise attenuation of the outdoor enclosure is also a consideration.

While the electrical engineer may find the generator aesthetically pleasing and worthy of showcasing, there are applications where a generator should be inconspicuously located, either for architectural design or facility/operational considerations. This is important for certain government projects and other sensitive facilities where the generator provides redundancy and resiliency for people or critical systems. An obvious and easily accessible generator can make for a high-value target. As an integrated product, the generator can be incorporated into the visual parameters of the project while providing a level of security when required.

While the electrical rooms can be placed as needed for the architectural program, it is more efficient, constructible, cost-effective and manageable if the interior and exterior equipment are adjacent. Ideally, the generator feeders should be in a direct route to the distribution board. This becomes more critical where there are paralleled generators. A clear and direct route is beneficial because the underground organization can become complicated with adjacent wet and dry systems. A simplified route also has the benefit of reducing the length and, therefore, cost. The actual maintenance, observations and testing become simplified when the generator is adjacent to the space with the transfer switches due to accessibility.

While the generator and distribution system are in areas accessible to only qualified personnel, the associated remote annunciation devices are strategically placed at normally staffed locations to alert them of system status and operational issues.

If the generator is near the building service transformer, required clearances should be reviewed with the AHJ, owner and utility requirements. As well as potential insurance requirements such as FM Global. It is often recommended that an emergency generator be provided with a fire-rated wall in case of fire at either source, or provided the recommended clear distance.

It is also important to consider the practical elements of planning a generator yard. There should be adequate lighting so the generator can be maintained during an outage. That could mean circuiting lighting, access doors and receptacles to a life safety circuit. It is a great benefit to the project or building to have an early run-through of major building components and potential outage procedures with building facilities personnel during design.

Furthermore, with climate change becoming an increasing issue in electrical systems resiliency, the floodplain is not an item that can be ignored. Great care should be used with locating the emergency systems. Consider if the equipment is in a 100-year or even 200-year flood plain. Is the equipment located in an area with hurricanes and tornadoes? These resiliency considerations will guide the configurational basis of the system.

If the EPS (generator) is located outdoors and Level 1 EPSS equipment is located indoors, the system must have a room separate from the normal service per NFPA 110 7.2.3. When the EPS is located indoors, the room per NFPA 110 7.2.1, must have a two-hour fire rating.

It is often recommended that the EPSS room also has a two-hour fire rating because of the 90-minute operating time required for the egress system or protection requirements for specific types of emergency support systems. Regularly a combination of interior and exterior locations are utilized, but in all scenarios planning to mitigate risk in all forms is critical for an optimal installation.

The NFPA 70: National Electrical Code restricts the height of the disconnecting means to 6 feet, 7 inches per NEC 2017 240.24(A) to allow access with the ability to use “portable equipment” when the “overcurrent device is adjacent to the utilization equipment.” Depending on the AHJ, there can be various interpretations.

There are various ways to meet this requirement and the recommended method should be reviewed and approved during design. It could be that a readily accessible ladder or strategically located emergency power off switch meets the intent or a permanent perimeter platform must be installed.

Generator ventilation

It is tempting to provide the minimum clearances around the generator for maintenance, but it is also important to allow enough breathing room for intake and discharge ventilation. While it has always been good practice to provide enough space to walk around the enclosure doors, NFPA 110-2022 codified the clearance requirements in section that generators are to be provided with “36 inches of working space access for inspection, repair, maintenance, cleaning or replacement from the outside edge of the enclosure or sufficient space to fully open all hinged doors, whichever is greater.”

The ventilation needs can vary by manufacturer and accessories. Ventilation is critical in an environment with high heat, such as the desert, so the hot ambient air does not recirculate the already increasing hot air into the generator and overheat the engine. For exterior units, there are accessory options such as low intake or vertical-discharge hoods that help manage the recirculation and could also tighten the space needed, but the dimensions should be confirmed before reducing the expected generator area.

NFPA 110 7.7.1 calls for provisions “to limit the maximum air temperature in the EPS room or the enclosure housing the unit to the maximum ambient air temperature required by the EPS manufacturer.” An interior EPS room will require intake, discharge and ventilation directly from the exterior through a wall ‘’or a two-hour fire-rated air transfer system” per NFPA 110 7.7.2.

Figure 3: A generator appears in a chiller yard with a utility transformer in the distance, located in Texas. Courtesy: Wade Griffith, SmithGroup

Fuel requirements for generators

There is a common misconception that the larger the generator, the longer the runtime, which is inaccurate. Generators are sized to accommodate the required loads, but the fuel tank capacity determines the actual runtime. NFPA 110 5.5.3 indicates that the tank shall “support the duration of the run specified,” and NFPA 110 7.9 further clarifies that the “fuel tanks shall be sized to accommodate the specific EPS class.”

The generator kilowatt rating and fuel are required to prioritize adequate capacity to Level 1 and Level 2 loads, then optional loads per NFPA 110 7.1.5. The potential runtime consumption rate of the generator varies by manufacturer, but the operational full load of gallons per hour can be requested.

When considering “the generator cannot ever go down” project parameter, the answer isn’t always to add more fuel. Fuel must be cleaned and maintained — commonly referred to as polishing — and the system must also have the varnish periodically removed. Factors such as representative service in the area, distance from the nearest fuel supplier or availability of fuel in the area need to be considered.

A conversation regarding the client’s experience in the area can clarify the specific requirements to bring balance to performance and value. NFPA 110 clarifies that “tanks shall be sized so that fuel is consumed within the storage life or provisions shall be made to remediate fuel.”

Fuel tanks have specific requirements in NFPA 30, 37, 54 and 58 that may also call for additional protections contingent on the type of tank. NFPA 37: Standard for the Installation and Use of Stationary Combustion Engines and Gas Turbines defines the requirements and maintains that the engines shall be readily accessible for “maintenance, repair and firefighting” (NFPA 37 4.1.1).

NFPA 37-2021 section calls for engine rooms to have “at least one-hour fire-resistance rating” for walls and floors, with the rating maintained for the ceiling unless it is otherwise protected or noncombustible. Section 4.1.4 calls for outdoor generators in “weatherproof housings that are installed outdoors shall be located at least 5 feet from any openings in the walls of structures” or from structures having combustible walls unless complying to the exception of “the structure having a fire-resistance rating of at least one hour” in nonhazardous locations.

There are also exceptions to reduce the 5 feet clearance if fire testing is conducted. Similar requirements apply to engines located on the roof.

Regardless of location, containment of fuel in the possibility of a spill or leak should be coordinated with the requirements of the space. It is possible to get a double-walled fuel tank, but containment curbs or pumps should be coordinated if that is not provided.

There is a limit to the amount of fuel that can be placed interior to a building or on the roof. NFPA 110 7.9.5 calls for a maximum quantity of 660 gallons of diesel fuel “inside or on roofs of structures.” Often when there is a need to extend the potential runtime of the tank in these locations. A refueling system and day tank, above or below ground, will be required and will need extra planning and discussion with the AHJ. The fuel port will need to be accessible and secure.

Usually, when a fuel port is required, a recirculation system will also have to be provided to get from the ground level up or across a long distance. Refueling systems are also costly because the fuel lines have separate fire protection requirements for safety. Direct routing and immediate adjacency are the most effective and economical.

Additionally, it is important to plan a path for refueling. Hose lengths can vary depending on the supplier, but the general expectation is to plan for a minimum 25-foot hose, with some suppliers having options for extensions. Many fuel vendors also have a small pump to push or pull the diesel gas an extended distance.

If the generator uses natural gas, it doesn’t mean fuel planning can be ignored. Natural gas piping will need access to the generator pad and can be costly if the natural gas piping is not near the electrical utility yard. Natural gas may not be considered a reliable source in all jurisdictions. It is important to note that natural gas generators have a practical operational limitation or rating, for EPSS.

Figure 4: Generator with support platform is located at Johnson County’s Medical Examiner Facility. Courtesy: Michael Robinson, SmithGroup

Testing and maintenance

Generators produce considerable sound. It is important to consider the generator’s location for internal occupants and exterior adjacencies. AHJs have sound ordinances and if located adjacent to any residential neighborhoods; these ordinances can be very strict. Sound is measured in decibels and is a condition of distance. When coordinating the decibel level, a clear understanding of the surrounding environment is essential.

The code requires periodic testing for emergency systems. General requirements of NFPA 110 8.4.1 call for documented ESS testing at least monthly for at least 30 minutes. Periodic testing mitigates the likelihood of a failure during a normal power outage. It is important as a team to communicate that emergency systems will operate based on how well they are maintained.

Additionally, the generator breaker is a part of the coordination, short circuit and arc flash studies. These studies are code required for emergency systems and often require physical confirmation of the configuration. It is important to plan the time and coordination between suppliers for this to occur during the submittal review and before project closeout.

The 2017 edition of the NEC 700.3 (F) requires equipment provisions for a straightforward connection of a temporary power source in the event the permanent generator is out of service due to failure or maintenance. It is important to consider where the temporary generator could be located. The cables connecting to the equipment provisions, often a plug-style or triple switch-like configuration, aren’t intended to be in driveways or will require additional protection.

The optimal planned location for a temporary power source is adjacent to the switching means. Planning the parking area for the temporary generator connection can save the operator time during the outage, often saving costs and increasing safety.

Ultimately, the emergency systems cannot be designed in isolation and successful system design will be the product of integrated communication between the AHJ, stakeholders, disciplines and clients.


Author Bio: Bianca Jimenez, PE, is a senior electrical engineer at SmithGroup. She has more than 10 years of experience currently focusing on science and technology projects. Rick Baca is a senior electrical designer at SmithGroup. He has more than 19 years of experience in electrical engineering with a focus in health care.