Know specific, unique applications of generators in mission critical facilities

Electrical engineers should consider many factors when selecting generators and generator systems for mission critical facilities

By Brian Rener, PE, LEED AP, SmithGroup, Chicago  May 23, 2024
Example of paralleled generators. Courtesy SmithGroup

Generator insights:

  • Topics include code requirements for the load type to be powered, backup, standby and emergency generators.
  • Special facilities such as data centers, hospitals or campuses may lead to considerations for medium-voltage or paralleling systems.
  • Additional systems considerations include a growing desire for sustainable and renewable alternatives to traditional diesel and natural gas fuels.
  • This article assumes that the engineer is familiar with engine generator basics and therefore focuses on some specific or unique considerations for using a generator and their applications.

This article has been peer-reviewed.This article has been peer-reviewed.An electrical engineer often faces the question of when af generator is required or desired on a project. It might be a code requirement, it might be the owner’s request or the engineer might recommend one based on the specific facility type or concerns with local utility power reliability.

Once the decision is made to design and specify a generator system, there are numerous other important considerations that include the type of generator, its fuel sources, sustainability goals, power distribution options, procurement challenges and how the generator will be tested before operation.

NFPA 70: National Electrical Code (NEC) has several articles that apply to emergency power backup sources. It is important to review the code requirements when deciding on the classification of an alternate power source.

The most pertinent codes, standards and guidelines the design engineer must become familiar with are:

The NEC generally classifies three types of “generator backup” power systems:

  • Legally required standby.

  • Optional standby.

Health care is the one major facility type that has completely different classifications, which we will review later in this article.

NEC Article 700 classifies emergency systems as “those systems legally required and classified as emergency by municipal, state, federal or other coded or by any governmental agency having jurisdiction.” The NEC further states that “these systems … automatically supply illumination, power or both … essential for safety to human life.”

In practical terms this normally includes providing power to egress lighting, fire detection and protection, certain types of elevators, public safety communications or any system where loss of power would cause serious endangerment to life or health.

Further, section 700.12 states that “Current supply shall be such that, in the event of a failure of the normal power supply, emergency lighting, emergency power or both shall be available within the time required for the application, but not to exceed 10 seconds.” NFPA 110 contains additional requirements on classification of startup times.

The NEC also contains requirements for legally required standby systems in Article 701. Code-required standby systems may include communications, selected ventilation or smoke removal systems, lighting or certain types of industrial processes that may create hazards or hamper firefighting operations if power were not available. The code-required standby systems must be available within 60 seconds.

Another classification of loads is defined as optional standby systems and outlined in Article 702. Optional standby loads consist of electrical loads that do not affect life safety but would result in unacceptable financial or operational losses to a facility. Typical loads could include data processing, communication systems, refrigeration, selected heating, ventilation and air conditioning (HVAC) loads and manufacturing or critical industrial processes. We will discuss a few of these common applications below.

Generators for mission critical facilities

NEC Article 708: Critical Operations Power Systems was first introduced in the 2008 version of the NEC. These are systems, operations or facilities designated by local, state or federal government as “mission critical.” Examples include police or fire stations or other facilities for reasons of public safety, national security or business continuity. Additionally, this section has some notable requirements for engineering practices like commissioning, which have long been practiced in data centers, health care and other previously unclassified “mission critical” facilities.

Article 517 covers the requirements for health care facilities. While the topic of health care generator systems requires an in-depth discussion, we will summarize the key differences.

Hospitals require generators but they don’t use the term “emergency” or “standby” as discussed in Article 700. The hospital generator electrical system is called the “essential electrical system.” The essential system has three branches: equipment, critical and life safety. A one-line power diagram of health care systems is shown in Figure 1.

Figure 1: Example of a hospital generator distribution diagram. Courtesy SmithGroup

Figure 1: Example of a hospital generator distribution diagram. Courtesy SmithGroup

The hospital equipment branch is primarily mechanical systems used to support patient areas; there is some latitude to include additional systems on this branch related to hospital operations beyond direct patient care areas.

The critical branch generally services patient care areas, life support systems and other loads essential for patient care.

The life safety branch must meet the requirements of NEC 700, with a few specific exceptions as outlined in NEC 517.26. Typical loads include egress lighting, exit signs and fire alarm equipment.

Each hospital branch and system is required to be separated from other sources with a few exceptions and must be fed from a separate automatic transfer switch.

Generators for data centers and science buildings

Regarding NEC Article 702 “optional standby systems” when would an engineer or client want a generator that was optional and not required by code?

Data centers are one example. They are the largest growing use for generators in the United States. The demand and shortages of generators are due, in part, to the significant purchase of generators for data centers. In every aspect of our lives from social media, to video streaming, big data and artificial intelligence, Americans rely on the continuous operation of data centers.

With data centers, nearly the entire facility is placed on a combination of standby generators and uninterruptible power supplies (UPS). In most data center projects, all HVAC is on generator power and the data center equipment is also on a UPS fed by the generators.

The code complication for data centers is that they usually include some required Article 700 emergency loads such as egress lighting, fire detection and fire pumps. Rather than combining this emergency load with the optional generator load, it makes sense to provide smaller dedicated generators for core required emergency power and a separate larger generator for the optional backup loads in a data center.

Beyond data centers, certain types of science and technology buildings such as research and development, laboratories, semiconductor manufacturing, biopharmaceutical and chemical processing plants would also have a combination of both emergency and optional generator loads.

In these facilities, a careful review is needed to classify certain systems as either emergency, legally required standby or optional standby. A unique component of some labs and processing facilities is the use of hazardous or toxic chemicals and gases, which would require generator backup to prevent life safety concerns for control, treatment and ventilation.

Additional considerations for generators

There are a few common items for the engineer to consider further when selecting or applying generators in facilities.

Loading and sizing: It is highly recommended that a generator sizing program be used when calculating loads. Inductive loads, such as motors and transformers, present special concerns with inrush, which can result in voltage drops and generator stability. Transformers are commonly overlooked in this regard when being energized by a generator.

Electronic or nonlinear loads (computing, lighting, variable speed drives, manufacturing automation, lab equipment, etc.) may present issues with harmonics and voltage regulation. It is not uncommon for data centers to present leading power factor, which presents problems with voltage regulation.

One item that is can be confusing to engineers who are new to generators it the power factor rating of generators. Engine generators are “kilowatt (kW) limited,” that is, they are rated using a 0.8 lagging power factor. Therefore, engineers need to focus on using the kilowatt rating as well as the kilowatt volt-amp rating of a generator.

An additional consideration for sizing is that most engine-driven generator sets are rated for 77°F. You must derate 0.4% for every 10°F above 77°F. Most generator sets are rated for 3,300 feet above sea level. You must derate the unit by 1.5% for every 1,000 feet above that altitude.

Combining code required emergency loads with optional standby loads: When a single alternate power source, such as a generator, is used to supply both emergency and nonemergency loads, the system must be carefully reviewed. Emergency and nonemergency loads are allowed to be placed on a single generator under certain conditions. Unless the generator has the capacity to handle the full load (including starting currents), load shedding or sequencing must be implemented on the nonessential loads.

Generator power distribution: Per NEC 700.10, emergency circuit wiring must be routed separately from legally required or optional standby circuits. However, legally required standby circuiting may be combined with optional and other loads.

Emergency generator power distribution systems must also have fire protection when installed in buildings with occupancies of 1,000 people or greater or in certain types of buildings that are taller than 75 feet.

This fire protection shall be accomplished by one of two methods:

  • Installing the distribution in spaces protected by sprinklers.

  • Providing a 2-hour-rated enclosure for the circuit wiring.

The fire protection requirements also apply to the physical feeder circuit equipment itself (panels, transfer switches, etc.), which must be in 2-hour-rated rooms or rooms with fire protection (see Figure 2).

Figure 2: Example of indoor diesel generator. Courtesy SmithGroup

Figure 2: Example of indoor diesel generator. Courtesy SmithGroup

Per NEC 700.30, emergency generator power distribution systems also are required under the NEC to be selectively coordinated. This will require a protective device coordination study, looking at fault levels and overcurrent devices to ensure that faults are isolated by opening the protective device nearest the fault, allowing the rest of the system to function. Optional standby systems are not required to be selectively coordinated.

Voltage considerations, paralleling and docking stations

Typically, generators have been selected at 208/120 or 480/277 volts (V). However, with increasing backup generation needs for critical facilities or campus settings, there has been an increasing use of medium-voltage generators, particularly in the 15 kV medium-voltage class. The benefits of medium-voltage generators are to allow larger power ratings, often 2.5 megawatt and above, while being able to use smaller power cables and benefit from longer distribution runs without significant concerns over voltage drop. While there is a modest cost increase to select medium-voltage generators, this needs to be considered with savings in cable sizes as well as other cost changes in the medium-voltage power distribution system.

Medium-voltage generators require additional considerations by the engineer and include evaluation of increased code clearances, requirements for resistance grounding and a discussion with the owner on availability of qualified maintenance staff who are experience in working with medium-voltage systems. Using medium-voltage generators can require additional transformation back down to 480 V, impacting the cost and space requirements of additional transformers.

When power demands are significant or where redundancy (N+1) is needed for backup generation, paralleling is an option for evaluation by the consulting engineer. Historically paralleling was a complex process, but advanced digital controls have improved the design and specification process. One of the more common concerns was the start up time to bring multiple generators online and synchronized. Currently most major manufacturers offer onboard paralleling controls on their generators, which has drastically reduced the startup times (see Figure 3).

Figure 3: Example of paralleled generators. Courtesy SmithGroup

Figure 3: Example of paralleled generators. Courtesy SmithGroup

In 2017, NEC Article 700.3 introduced a requirement for single use, emergency generators to use a docking station. This was common good practice for critical facilities needing back up power, but is now required for emergency generators.

Generator fuel sources

There are two common fuel choices when considering generators: diesel and natural gas. Natural gas generators can be used in optional standby systems and with generators less than 500 kW (see Figure 4).

Figure 4: Paralleled 500 kW natural gas generators for an artificial intelligence data center. Courtesy SmithGroup

Figure 4: Paralleled 500 kW natural gas generators for an artificial intelligence data center. Courtesy SmithGroup

When considering generators for emergency systems, the possible interruption of a natural gas utility source and the 10-second startup time normally leads engineers to specify diesel. The amount of on-site fuel should be sufficient to provide a minimum of 2 hours of run time per NEC Article 700.12.D.2 Additional time requirements for on-site fuel can be found in NFPA 110.

In many cases where generators are used, the desired run times are much longer. It is not unusual to see 8, 24 or even 72 hours of fuel needed for some mission critical facilities. In these cases, the engineer will need to carefully examine where to store the fuel. “Belly tanks” under the generator are common, as are separate fuel tanks. The size of these storage tanks can be very significant and the use of belly tanks may notably elevate the generator and require catwalks for access. NFPA 110 Article 7.9.5 specifies a maximum of 660-gallon diesel storage inside or on the roof of a structure. This requirement is in part based on concerns over fire and explosion hazards within a building.


Decarbonization goals and desire for renewable energy have led to the development of generators using hydrogen or hydrotreated vegetable oils (HVO).

There has been increased interest in hydrogen as a fuel source. Limitations currently exist in both the size of fuel cells and microturbines using hydrogen. Typically, these range in size from 100 to 200 kW. Conventional natural gas generator manufacturers have begun producing engines that can run on a blend of natural gas and hydrogen.

There remain challenges in the production, delivery and storage of hydrogen on sites. The Department of Energy has been addressing this by building regional hydrogen hubs and various manufacturers have begun to produce hydrogen via on-site generation units (see Figure 5).

Figure 5: Installed hydrogen fuel generator. Courtesy SmithGroup

Figure 5: Installed hydrogen fuel generator. Courtesy SmithGroup

A significant opportunity to move toward decarbonization with standby diesel generators is with HVO, a next-generation fossil-free fuel. This versatile fuel requires no changes to engines, doesn’t degrade over time and can blend with regular fuel. HVO should not be confused with previous generation biodiesel fuels. HVO is produced from organic agricultural waste products, but it uses hydrogen to drastically reduce the carbon content.

Using HVO in existing generators, in place of fossil diesel fuel, can reduce carbon emissions by up to 90%. HVO can be kept for up to 10 years without notable degradation and it is not susceptible to oxidation, water absorption or bacterial growth. As a newer fuel source, production and distribution is quickly ramping up across the country but limited in some locations. HVO carries about a 10% cost premium over standard diesel depending on location.

Supply chain challenges

Along with paralleling switchgear, transfer switches and many other associated electrical equipment, generators are experiencing significant shortages and delivery delays. Generators may require 18 to 24 months to arrive on a project after approval for purchase.

Engineers are strongly encouraged to review all electrical equipment needs with the owner and construction manager to determine if early “long lead” packages need to be released before the completion of construction documents. This will require careful analysis and understanding of risks with the client in releasing these systems for early procurement.

Testing and commissioning generators

For both emergency and legally required standby generators, testing is required. There are two types of tests to be performed: The first is an acceptance test done upon installation but before becoming operational. A good source for acceptance testing is in various codes, such as ANSI/NETA ATS-2021 and NFPA 110 Chapter 8.

The second test is operational testing to be performed periodically, subject to the local authority having jurisdiction, during the life of the system. NPFA 110-2010, for example, has requirements for testing and maintenance procedures. Written records are to be maintained for either type of system. Of note, emergency systems are required to be tested “under maximum anticipated load” whereas legally required systems are required to be tested “under load.”

As mentioned earlier, generators are also used in COPS defined under NEC Article 708. This section goes beyond the testing required for emergencies and legally required standby systems to require commissioning.

Commissioning is a common practice in all mission critical facilities and includes component, system, baseline and functional performance tests.

There are notable differences between emergency and standby generators within NEC. The engineer should be careful in using the terms and understand the requirements, considerations and specifications when designing generator power.

Author Bio: Brian Rener is a principal and electrical discipline leader at SmithGroup. He is a member of the Consulting-Specifying Engineer editorial advisory board.