Codes and Standards

The role of NFPA 110 and its interaction with other codes

The coordination of requirements for emergency and standby power systems between different code publications has improved over recent years, but variations still exist that can cause confusion. It is important for engineers to review all the applicable codes and standards to develop a full picture of the requirements for standby systems. While NFPA 110-2019: Standard for Emergency and Standby Power Systems defines system levels, types, and classifications, it does not determine what situations will require any given level, type, or class. Design engineers must coordinate the requirements of NFPA 110 with other sections of the NFPA, International Building Code, Facility Guidelines Institute, and other authority having jurisdiction requirements as applicable.
By Jared Mason, PE, PMP; WSP USA, Phoenix November 16, 2018

Learning Objectives

  • Understand the role and interaction of NFPA 110 and other applicable codes in the design of standby power systems.
  • Define the level, classification, and type designations as defined by NFPA 110.
  • Explore specific requirements of NFPA 110 that can be challenging or confusing for designers.

Designers have to tackle myriad codes and standards when designing emergency and standby power systems for mission critical facilities, such as hospitals, data centers, and critical operations facilities. In a hospital, for example, designers must review NFPA 99-2018: Health Care Facilities Code; NFPA 101-2018: Life Safety Code; NFPA 110-2019: Standard for Emergency and Standby Power Systems; NFPA 70-2017: National Electrical Code (NEC), Articles 445, 517, 700, 701, 702, and 708; and the Facility Guidelines Institute (FGI) (or equivalent enforced standard). This list is not exhaustive, as there are backup system requirements elsewhere in the code and some authorities having jurisdiction (AHJ) enforce different codes or have their own amendments impacting backup system design.

Codes do not have standardized designations for emergency and standby power systems. System designations used in different locations of the code include emergency, essential, nonessential, legally required, backup power, optional standby, standby, Level 1, and Level 2. The following information will focus primarily on the requirements for emergency and standby power systems as defined in NFPA 110.

System designations

NFPA 110 covers emergency power supplies (EPS) and emergency power supply systems (EPSS). The EPS consists of the source of electric power. The EPSS consists of the EPS and all necessary components up to and including the transfer switches. Components include conductors, disconnecting and overcurrent protective devices, transfer switches, controls, and supporting devices.

System levels

NFPA 110 is applicable to systems designated Level 1 or Level 2 as defined by the code. A Level 1 system failure to perform could result in the loss of human life or serious injury. Examples of Level 1 loads include egress illumination, fire alarm and detection systems, fire pumps, and emergency communication systems. Level 1 systems correspond well to the requirements of NEC, Article 700: Emergency Systems.

A Level 2 system failure to perform could cause hazards or hamper rescue or firefighting operations. Examples of Level 2 loads include HVAC systems, ventilation, smoke-removal systems, sewage disposal, and industrial processes. Level 2 systems correspond with the requirements of NEC, Article 701: Legally Required Standby Systems. The NEC goes on to define a third level of standby power systems with Article 702: Optional Standby Systems. The systems described under NEC, Article 702, do not fall under the purview of NFPA 110 and are not considered Level 1 or Level 2 systems.

System classifications

NFPA 110 establishes definitions for the classification of an EPSS. Classification identifies the amount of time for which an EPSS will operate at its rated load without being refueled or recharged. It is important to note that this requirement of the NFPA is based on rated load, not calculated or actual load, which means that a system designed with 50% spare capacity still requires enough fuel to handle the full-rated load of the system for the time determined by its classification.

The classes established by NFPA 110 range from Class 0.083 (5 minutes) to Class X, which is a designation used to require an amount of time, usually between 48 and 96 hours, as defined elsewhere in the code or as determined by the designer based on system requirements. Health care facilities, for example, fall under Class X per NFPA 99. The actual time requirement chosen by the designer or AHJ should include an evaluation of past outages, fuel-delivery challenges, and environmental conditions. It also should include an evaluation of the facility’s seismic design category, although this recommendation was moved to the Appendix of NFPA 110 during the 2013 version rather than in the code body where it was previously.

Medium-voltage emergency substations on large campuses increase the challenge of meeting mandated power within specific time frames.

System types

NFPA 110 then defines types of EPSS. The type identifies the maximum amount of time that an EPSS will allow the load terminals of the transfer switch to be without acceptable power, which is defined as the power quality, capacity, and stability as required by the load to ensure proper system functions. EPSS types range from Type U (basically uninterruptible) to Type M (manual). Examples of system types are the NFPA 99 requirement for Type 10 (10 seconds) systems for health care essential systems, which corresponds to the NEC, Article 517.32(B), requirement for hospital life safety and critical branches, and the NFPA 101 requirement for Type 60 (60 seconds) systems for ventilation of smoke-proof enclosures, which corresponds to the NEC 701.12 requirement for legally required standby systems. Manual systems (Type M) are further discussed in NEC, Article 702 Optional Standby Systems, and NEC, Article 517.35(B).

While NFPA 110 defines emergency and standby system class, type, and level and outlines requirements for those designations, NFPA 110 does not determine which specific systems fall under these categories beyond the general Level 1 and 2 designations outlined above. Designers must turn to other codes to determine the required system for any given application.

Energy sources

NFPA 110 allows the use of only liquefied petroleum, liquefied petroleum gas, or natural/synthetic gas for EPS energy sources. NFPA 110 does not require onsite storage in situations where an offsite supply, such as a natural gas line, will reliably provide for the continuous operation of the EPS for the duration defined by the classification. The exception to this, which is only for Level 1 systems, is that locations with a high probability of interruption of offsite fuel must have onsite fuel storage of adequate capacity for the full duration of the class. The offsite fuel still can be the primary source, but provisions are required for automatic transfer between the sources during a failure. Before using an offsite source, it also is crucial that designers review other applicable codes and standards. For example, FGI 2.1- states that where onsite fuel storage is required by other codes, a minimum of 24 hours shall be provided for hospitals.

When providing onsite fuel storage, NFPA 110, Chapter 5.5.3, requires the main fuel tank is to have a minimum capacity of 133% of the full-load running of the EPS for the duration specified by the class. This requirement has three separate aspects that can trip up designers, as follows:

  • The 133% requirement is intended to account for fuel that may be used in tests or over time and for fuel in the top and bottom of the tank that may be below the usable level or above the fillable level.
  • The requirement is for the EPS at full load, not design or actual load.
  • The requirement applies to the main fuel tank and does not allow for calculation of day tanks or fuel-pipe volume. It is important for designers to understand that only the main tank can contribute to the required fuel volumes. While the code does not directly address installations with multiple main tanks, it can be assumed that any primary tank volumes can contribute to the required volume.

While NFPA 110 does allow the use of natural gas for both Level 1 and Level 2 systems, it is key for designers to realize the limitations of sources and how they interact with the required system type as defined above. For example, employing a large natural gas generator for a Level 1, Type 10 system may not result in power availability and transfer within the required 10-second time frame. The start-time delay for natural gas is primarily due to fuel travel time to get to the ignition chamber. This delay is a safety measure to reduce the risk of gas explosions on start-up. Large natural gas generators also experience difficulties meeting the requirements from NFPA 110, Chapter, to handle the full-rated load in a single step.

Other design variables, such as using medium-voltage emergency distribution via substations upstream of the transfer switches, add additional challenges to meeting a 10-second maximum time frame. In these large systems, the start time associated with large generators, the energization time of large substation transformers, and the long run lengths of distribution wiring all combine to increase the difficulty of achieving stable power in under 10 seconds. In addition to the challenges above, NFPA 110, Chapter 6.2.5, requires that a minimum 1-second delay be provided on most systems before initiating the EPS to reduce nuisance starts of the EPS system. (The time delay is 0.5 second for gas turbine units.) This required time delay is counted as part of the overall time allowance provided by the EPSS type and effectively reduces the allowed start time from 10 seconds to 9 seconds.

EPS and EPSS locations

When designing Level 1 systems indoors, NFPA 110 requires EPS installations to be in a 2-hour-rated room. The code allows only EPS, EPSS, and other systems that directly serve the space to be in the room.

It can be particularly challenging for small emergency systems to achieve the required coordination. All images courtesy: WSP USA

The code further requires that EPSS equipment is not be installed in the same room with the normal service equipment where the service equipment is rated 1,000 amp or higher and more than 150 V to ground. While the EPSS equipment is not required by NFPA 110 to be in a 2-hour-rated room, designers should carefully review the requirements of NEC, Article 700.10(D), for high-rise and assembly occupancies as well as the requirements of IBC for smoke control, stair pressurization, and fire/access elevators associated with high-rise construction. The requirements of these other codes may necessitate a rated room depending on the project.

The EPSS room must also be designed and located to minimize damage from flooding, sewer backups, and other events. This requirement doesn’t necessarily preclude EPSS systems from being installed in basements or similar spaces, but it does require that the design professional consider these elements and take corrective actions as necessary to reduce environmental risks to the emergency systems. Corrective actions could include floor drains and raised equipment, for example. This requirement is also not intended to preclude the installation of fire-suppression systems in these rooms. NFPA 110, Chapter 7.11.2, does prohibit the use of carbon dioxide or halon systems unless combustion air is ducted from outside the structure. It also prohibits automatic dry-chemical systems unless they are certified by the EPS manufacturer to not impede EPS


For indoor installations, NFPA 110, Chapter, requires the remote manual stop to prohibit accidental actuation and to be located outside of the room containing the EPS. While NFPA 110 does not give specific requirements for the emergency power off (EPO) to be in a secured location, designers should consider the hazards created by an unauthorized person having access to an EPS EPO. Consideration should also be given to the needs of first responders in an emergency.

When designing EPS systems in outdoor enclosures, NFPA 110 requires that enclosures resist the entrance of snow and rain and allows for EPSS equipment to be mounted within the EPS enclosure. For outdoor installations, the remote manual stop is required to be located outside of the enclosure and of a type to prohibit inadvertent actuation. Again, care should be taken by the designer to limit the access of unqualified personnel to the EPO button.

Generator paralleling-gear installations require testing of load shed, load optimization, and other operational features.

Coordination requirements

Chapter 6.5.1 of NFPA 110 requires that the overcurrent protective devices (OCPDs) in the EPSS be coordinated. Coordination of OCPDs ensures that the devices closest to the overcurrent event trip before upstream devices. This ensures, for example, that a 20-amp branch circuit will not cause an upstream feeder circuit breaker to trip, which would result in the loss of emergency loads that were not associated with the overcurrent event. The Appendix of the code further clarifies that the OCPDs should be coordinated to the extent practical and mentions that full coordination may not be feasible or practical.

For health care facilities, both NFPA 99, Chapter, and NEC, Article 517.30(G), clarify the requirements for the essential electrical system OCPDs to be coordinated for the period beyond 0.1 second, but there is no corresponding clarification for other types of facilities. NEC, Articles 700.32, 701.27, and 708.54, all require systems to be selectively coordinated by an engineer or another qualified person, but it leaves the exact level of coordination up to the discretion of the qualified individual. A strict reading of the NEC, Article 100, definition of selective coordination states that coordination needs to be achieved for the full range of available overcurrents and OCPD opening times. This leaves the door open for coordination requirements less than 0.1 second, depending on AHJ interpretation. A requirement for full coordination less than 0.1 second could effectively reshape an entire electrical system and, in many cases, may not be possible to achieve due to technological limitations of the OCPDs operating at such high speeds.

Understanding the above coordination requirements and AHJ interpretation is critical to the design of emergency and standby systems. These requirements have the largest proportional impact on the smallest systems. For small facilities, it is overcurrent coordination that often will determine your overall system size, and not the load experienced by the system. OCPDs have improved dramatically in recent years, with ever thinning and more precise trip curves, but it is still difficult to achieve coordination with small thermal magnetic circuit breakers.

Revised requirements

The 2016 edition of NFPA 110 includes a few notable modifications as mentioned below. A new requirement in Chapter 7.9.13 prohibits the use of automatically actuated valves in the fuel-supply and return lines. This requirement is aimed at reducing the susceptibility of an EPS to single points of failure in the fuel lines. The code does not preclude the use of manual valves.

The 2016 edition also calls for facilities with multiple automatic transfer switches (ATS) to rotate the monthly EPS test-initiating ATS (Chapter This requirement helps ensure that, over time, all ATS are reaffirmed to be able to successfully initiate the EPS. This requirement does not include a maximum time that an ATS can go without initiating the generator, so it could still take years for a facility to rotate through all ATS in very large campus installations.

Specific initial acceptance-testing requirements for parallel systems (Chapter and follow-up maintenance testing (Chapter 8.3.5) also have been added to the code. The initial testing requirement calls for verifying paralleling and load-shed functionality.

Paralleled generators have new NFPA 110 requirements for initial acceptance and maintenance testing.

The maintenance-testing requirements call for the same maintenance as transfer switches but add a requirement for verifying that system controls will operate as intended. The Appendix further clarifies that load optimization, load shed, and other operating features should be tested and adjusted over time as ATS load profiles change. This Appendix note emphasizes important added variables for systems with paralleled generators that facilities need to be aware of. It also is vital for designers of renovations and EPSS modifications to understand how revisions may require this load-shedding and optimization strategy to be adjusted even when they aren’t adding new EPSS equipment.

While the coordination of requirements for emergency and standby power systems between different code publications has improved over recent years, variations still exist that can cause confusion. It is important for designers to review all the applicable codes and standards to develop a comprehensive perception of the requirements for standby systems.

While NFPA 110 defines system levels, types, and classifications, it does not determine what situations will require any given level, type, or class. Designers must coordinate the requirements of NFPA 110 with other sections of the NFPA, IBC, FGI, and other AHJ requirements as applicable.

Jared Mason, PE, PMP; WSP USA, Phoenix
Author Bio: Jared Mason is a senior electrical engineer and project manager at WSP USA.