Designing emergency and standby generator systems

Consulting engineers who specify emergency and standby power equipment understand that installations for mission critical facilities, such as hospitals and data centers, are required to comply with NFPA 110: Standard for Emergency and Standby Power Systems, in conjunction with NFPA 70: National Electrical Code (NEC). System designers must interpret the requirements of NFPA 110, ensure their designs follow them, and educate their clients about how the standard affects their operations.


This article is peer-reviewed.Learning objectives

  • Know the codes and standards that dictate the specification and design of emergency and standby generators.
  • Consider various design, environmental, and commissioning aspects when designing generator systems.
  • Understand the implications of specifying generators in mission critical facilities

Engineers of emergency power systems must be familiar with the latest requirements of NFPA 70-2017: National Electrical Code (NEC) and NFPA 110-2016: Standard For Emergency and Standby Power Systems. As these standards continue to evolve, as previous design approaches are evaluated over decades of service, and as retrofit projects encounter complications, the lessons learned by the industry must be taken into consideration to ensure modern emergency systems are safe and reliable. This article will focus on the applicable codes, environmental and operational design constraints, and considerations for health care and mission critical facilities while offering commissioning suggestions.

Applicable codes and standards

Figure 1: An engine-generator ceiling air-intake louvers with drip pans. Courtesy: KJWW Engineering Consultants

When applicable, engineers are required to consult code adoption of local municipalities, state authorities, and public health departments. This article will focus on professional design recommendations and the legal requirements associated with the latest available standards at the time of writing. Specifically, aspects of the 2017 version of the NEC and the 2016 version of NFPA 110 are referenced. Sections of the NEC that designers should reference include (but are not limited to):

  • Article 110: Requirements for Electrical Installations
  • Article 240: Overcurrent Protection
  • Article 250: Grounding and Bonding
  • Article 700: Emergency Systems
  • Article 701: Legally Required Standby Systems
  • Article 702: Optional Standby Systems
  • Article 800: Communications Circuits.

Articles 700, 701, and 702 sometimes are confused. Looking at them in terms of increasing levels of importance, Article 702 is optional and usually used purely for backing up utility power, but not for life safety. Examples of optional standby systems include data centers or a commercial business resulting in discomfort, serious interruption of a process, or damage to a product. Article 701 covers issues associated with preventing hindrance of life-saving measures—for example, firefighting, ventilation, and smoke-removal systems. Article 700 covers issues associated with preventing loss of life. Examples are egress lighting, fire alarms, and fire pumps.

Environmental design considerations

Typically, emergency and standby generators are not deployed in ideal environments. Engineers must devise practical, actionable methods to deal with the environment in which generators are installed. The issues that must be dealt with include the general environment, ventilation, exhaust, fuel, acoustical, electrical clearances, locations, installation, and removal considerations.

Environment. When designing rooms that contain an emergency power supply (EPS) and EPSS, engineers should consider the possibility for natural or human-made disasters, including flooding by river, storm water, sewer, sprinkler system, flying debris, physical attack, and lightning as defined in NFPA 110, Chapter 7: Installation and Environmental Considerations. Mission critical equipment should never be relegated to areas that are prone to flooding, particularly below grade in basements. Although it may be a struggle to convince the design team and client that this equipment should be placed out of harm’s way, it will be worth it when storms hit and the critical equipment continues to operate. In addition to keeping equipment safe from flooding, fuel storage and filling locations also should be designed to allow for security and access during natural disasters.


Figure 2: An exterior generator enclosure with intake (left) and discharge (top right) silencers. Courtesy: KJWW Engineering Consultants

Generator sets require combustion and cooling air to enter the generator room or enclosure, and requirements are included in NFPA 110, Chapter 7.7.7. Most of the air is for cooling a unit-mounted radiator. Engineers must ensure the pressure drop of the entire system is within the manufacturer's recommendations, which will determine the size, configuration, and location of the air-intake pathways. When large or multiple generator sets are running, the amount of intake airflow can be significant. The layout of multiple generators should not result in uneven airflow distribution. In some instances, precipitation can be pulled into the controlled space. To prevent this, there are several design decisions that can help (see “Ventilation design considerations”).

In designs where outside air is expected to bring in moisture and dirt, ceiling openings should include both drip pans and intake hoods and these openings should not be located above EPS or EPSS equipment (see Figure 1). Engineers have the option of remote-mounted radiators, which greatly reduce airflow requirements for the room, but these systems will complicate the installation and may reduce engine cooling system reliability.

Exhaust. The exhaust system may require a Department of Natural Resources (DNR) study for the effects of exhaust dispersion using computational fluid dynamics, and engineers should be aware of where exhaust fumes may go (see Wisconsin DNR example). The proximity of the generator exhaust relative to the building air intakes for HVAC systems also requires careful study. Even when separated by several hundred feet, prevailing winds may bring the smell of generator exhaust into a building. When locating generators within a multistory building, the routing of exhaust piping will not only cause heat and noise concerns within the building, but also backpressure concerns for the generator set. U.S. Environmental Protection Agency Tier 4 compliance will require mitigation solutions such as urea injection or selective catalytic reduction, both of which reduce the amount of pollution.

Fuel. NFPA 110, Chapter 7.9: Fuel System covers requirements for fuel sources and references NFPA 30-2015: Flammable and Combustible Liquids Code and NFPA 54-2015: National Fuel Gas Code or NFPA 58-2017: Liquefied Petroleum Gas Code. Fuel sources located in the EPS room include local day fuel tanks or integral fuel tanks whose aggregate size is limited by the amount of fuel that can be stored in one room, defined as 660 gal by the International Fire Code-2015 Section 603 and reinforced by NFPA 110 Chapter 7.9. Requirements for main fuel storage is defined in NFPA 13-2011: Standard for the Installation of Sprinkler Systems and may be underground or aboveground fuel storage. The physical relationship of the fuel pipe routing to and from day tanks and larger storage tanks should be considered for optimum reliability, minimizing the need for solenoid valves on overflow and return lines.

While routed in the generator room, fuel lines should be protected but accessible. A common approach to this is in a grated trench that allows visual inspection, ease of maintenance, and provides some containment in case of a liquid fuel leak. Leak detection with monitoring by the building automation system is recommended in rooms without frequent observation. When routed outside and underground, the depth of frost in winter should be considered and fuel supply lines may require heat traces to prevent stagnant fuel from gelling.

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