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NFPA 110-2019

NFPA 110-2019
Concepts and Changes


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Codes such as NFPA 101: Life Safety Code, NFPA 99: Health Care Facilities Code and the International Building Code dictate when it is legally required to install an emergency power supply system to support the emergency and essential electrical systems within a building. However, simply having an emergency power supply doesn’t guarantee that it will be reliable or will perform as intended for the duration of a utility outage. Those codes often lack clear and comprehensive performance, testing, and maintenance requirements for an EPSS. NFPA 110: Standard for Emergency and Standby Power Systems is intended to fill that void for applications where the legally required emergency power supply is a generator.


There are many common misconceptions about what NFPA 110 applies to. It is not the role of NFPA 110 to determine whether a generator is legally required or not. NFPA 110 is focused solely on the EPSS’s performance and the associated installation, testing, and maintenance as they relate to maintaining that performance. NFPA 110 applies to the complete EPSS and, as such, is not limited to just the generator but also addresses supporting auxiliary systems and related power transfer equipment. NFPA 110 does not:

  • Apply to other portions of the EPSS such as the normal utility source, electrical distribution equipment upstream or downstream of an automatic transfer switch, feeder conductor sizing, overcurrent protection size, grounding, and other design considerations normally governed by NFPA 70: National Electrical Code.

  • Allow uninterruptable power supplies/battery inverter systems, fuel cells, or any other form of on-site energy storage or generation system for use as an EPS. The use of stored energy systems for emergency power is governed by NFPA 111: Standard on Stored Electrical Energy Emergency and Standby Power Systems.

  • Determine which loads are legally required to be connected to the EPSS, and which type, class, or level of performance should be provided to support those loads.

  • Apply to portable generators that are not permanently installed.

  • Determine if natural gas or diesel fuel is a more reliable or appropriate fuel source in any given part of the country.

  • Have installation/construction requirements that retroactively apply to existing generator systems unless specifically directed by the authority having jurisdiction. However, the routine testing and maintenance requirements of NFPA 110 still apply to existing generators. Adherence to NFPA 110 testing procedures can improve the reliability of all generator systems since those procedures continuously evolve with each code revision cycle based on industry research, investigation of generator common cause failures after disasters, etc.


Energy sources that may be used as an EPS as defined in NFPA 110, Chapter 5 are limited to “rotating equipment” consisting of a generator driven by one of three prime mover types:

  • Otto cycle engine (spark-ignited four-stroke engine, typically natural gas type for generator applications).

  • Diesel cycle engine (compression ignited engine).

  • Gas turbines (continuous flow of air and fuel through a rotating compressor, ignitor, and turbine).

Both NFPA 99 (section and NFPA 70 (sections 700.12E and 701.12F) permit the use of fuel cells for emergency power. However, they are not approved as an EPS in NFPA 110. Motions have been made repeatedly during the public comment period for this and previous versions of NFPA 110 to include fuel cells as an approved EPS. Those motions have been consistently voted down by the NFPA 110 Technical Committee.

The primary reason for this rejection is the lack of supporting technical data and operational experience that would demonstrate that they have equivalent reliability, performance, and fault tolerance compared to a standard generator when used as an EPS. Note: The standards development site at is an excellent resource for further insights on the rulemaking process for all NFPA standards. As a consensus-based standard, draft reports with substantiation, comments, and ballot results for all proposed changes are publicly posted online.


NFPA 110 quantifies the performance of emergency power supply systems in three different categories noted as type, class and level. The formal NFPA 110 definition for each is as follows:

  • Type: “The maximum amount of time, in seconds, that the EPSS will permit the load terminals of the transfer switch to be without acceptable power.” The amount of time ranges from uninterruptable to 120 seconds with 10 seconds being the most common for life-safety loads. See below:

  1. NFPA 110 emergency power supply system types

  2. TypeMaximum allowed time until power restored

  3. UNo interruption allowed: uninterruptible

  4. 10 seconds

  5. 60 seconds

  6. 120 seconds

  7. No time limitation: manual operation

  • Class: “The minimum time, in hours for which the EPSS is designed to operate at its rated load without being refueled or recharged.” This amount of time ranges from five minutes to 48 hours (see below). There is also a “Class X” classification for other run times that do not fall into one of the preset time intervals such as a 96–hour requirement for certain health care facilities located in seismic design categories C through F as defined by ASCE 7: Minimum Design Loads for Buildings and Other Structures.

  1. NFPA 110 emergency power supply system classes

  2. ClassMinimum acceptable amount of runtime at full load

  3. 0.0835 minutes (0.083 hours)

  4. 0.2515 minutes (0.25 hours)

  5. 22 hours

  6. 66 hours

  7. 48 hours

  8. X Other runtimes as dictated by code or user

  • Levels: Level 1 “where a failure of the equipment to perform could result in loss of human life or serious injuries,” which typically corresponds to legally required NEC Article 700 and NFPA 99 life safety loads, or Level 2 “where a failure of the EPSS to perform is less critical to human life and safety,” which corresponds to legally required NEC Article 701 standby loads, which does not directly and immediately impact life safety such as communication systems, sewage ejectors/sump pumps, etc.

Note that these categories apply to the performance of the complete EPSS, not just the generator that is used as the EPS. Improper selection of individual auxiliary system components (starting batteries, fuel transfer systems, remote cooling system components, etc.) can severely impact overall system reliability and performance. When determining which type, class, and level of performance should be provided, the entire system needs to be evaluated, not just the generator. Simply stating a requirement for an “NFPA 110–compliant generator” within the specifications provides inadequate guidance for what is really required to ensure a reliable system.


Like most other NFPA standards, NFPA 110 is updated every three years. Most revisions usually amount to little more than administrative housekeeping — changing language and organization so that they are consistent with NFPA’s “Manual of Style” and editing sections so that they don’t conflict with other standards and code.

While the pace of NFPA 110 major revisions, such as the adoption of emerging energy conversion technologies, is glacial, there are always a handful of significant changes that are of particular importance to engineers. The 2019 version of NFPA 110 has the following notable changes:

  • Section 3.3.5 and 5.2.5: definition and role of field evaluation bodies.

  • Section 5.3.5: generator room temperature.

  • Section changes in battery chargers.

  • Section acceptable means of remotely starting a generator.

  • Section emergency stop button quantity and location.

  • Section generator coolant temperature while testing.

  • Section 8.3.7: diesel fuel testing.

We’ll examine each of these changes and their potential impact on the design engineer.


The AHJ inspects installations for compliance with the applicable codes and standards. It is the AHJ’s discretion to determine what is approved and what is rejected. For installations to be approved, you typically have to use equipment that has been listed per the applicable standards by Intertek Testing Services NA Inc., UL, FM Global, or an equivalent nationally recognized testing laboratory (NRTL). Given that most AHJs do not have the technical expertise to evaluate a generator for compliance with all of the applicable technical standards, such a requirement for equipment certification by a third party is not unreasonable.


NFPA 110 also requires prototype testing to validate the capability of a fully assembled generator, not just the individual components, to survive and function after being exposed to abnormal operating conditions such as excessive vibration (earthquakes), short circuit faults, etc. This testing is usually performed on a representative sample of that particular model of generator, not every generator of that model that is manufactured. This is because factory prototype testing can potentially damage a unit and/or reduced its useful service life due to the stresses placed on it during testing. Proper evaluation of the tests also may involve a full teardown of the unit. Given these considerations, it is not feasible to perform this type of complicated testing on every generator.

While the expectation is that most generator manufacturers should be able to provide certification that their product meets all applicable standards, there are significant exceptions. Generators are often highly customized to an extent where they may technically no longer be equivalent to the prototype model. There is also currently no listing process for medium voltage stationary engine-generator assemblies. To address these situations, the 2019 version of NFPA 110 added sections 3.3.5 and 5.2.5, which state the definition and role of field evaluation bodies.

FEBs are intended to provide a technically competent third party that will review the installed equipment for risks related to shock, fire, and mechanical hazards. Most NRTLs have field evaluation services but the quality of those services is still determined by the individuals that the NRTL sends to the field. Although NFPA 110 allows the use of FEBs, NFPA does not specifically approve or evaluate the field evaluation services provided by individual testing laboratories. The AHJ still has to be satisfied that the FEB has the appropriate knowledge/experience and understands the scope and extent of the evaluation.

The field evaluation process typically involves a document review, visual and mechanical inspections, and applicable tests to ensure safe and reliable operation. The applicable standard for this evaluation is >NFPA 791: Recommended Practice and Procedures for Unlabeled Electrical Equipment Evaluation. There are some limitations in field evaluations compared to factory prototype testing. For example, field tests have to specifically be nondestructive. As such, some tests that have an unacceptably high rise of equipment damage may possibly be omitted. If the FEB finds the generator to be compliant with the applicable standards, they will provide a field label and issue a detailed report to the AHJ. If non-compliant, the report will provide an itemized list of issues that can be used as a guide for corrections.


Cold engines are typically slower to start and accelerate to full speed. However, in a Level 1 EPSS, load acceptance has to be unusually quick, 10 seconds. Any type of delay is not acceptable, regardless of weather conditions. With this in mind, it should be clear why NFPA 110 has requirements for maintaining the temperature of the EPS itself (water jacket and batteries) in addition to the room or enclosure in which it is located in. Section 5.3.5 was revised with an important clarification for the required temperature of the generator equipment room or outdoor housing.

Before the 2019 edition of the standard, it was simply stated that the room or housing needed to be maintained at 40°F. However, the language was such that it could be interpreted as meaning that the temperature had to be maintained at that temperature at all times. This would be unreasonable in that it would be nearly impossible when the generator was operating during the winter. After all, how would you realistically maintain that temperature in the equipment room when the outside air louvers are open and the generator is drawing in as much air as possible?

The 2019 revision added the important clarification that these environmental conditions only when the equipment is not operating. Again, the intent of the standard is only to prepare the generator to start quickly as possible.


Ask a generator vendor what the most common reason is for a generator failing to start and picking up the load. The most likely answer is starting battery failure. Unfortunately, batteries are often poorly maintained and seldom replaced at the 24 to 30-month intervals recommended in the annex of NFPA 110. As such, any changes in NFPA 110 that affect batteries potentially have an outsized impact on the overall reliability of the EPSS.

Typically, generators are provided with two methods of charging the starting batteries; a charger/alternator that’s mechanically driven by the prime mover and a separate alternating current-powered battery charger. Having both provides some level of redundancy. However, earlier versions of NFPA 110 had an exception that allowed the mechanically driven charger to be omitted if the AC-powered charger had a high-low rate capable of fully charging the batteries during running conditions.

That exception in section has been revised. That exception is now only allowed for Level 2 EPSS installations. The mechanically driven charger is required for all Level 1 installations. This change was made to address perceived reliability issues with AC-powered battery chargers compared to a charger that was mechanically driven by the generator’s engine. AC-powered battery chargers have several potential points of failure including the branch circuit wiring, overcurrent protection devices, the charger, etc.


Section was changed to add a temperature compensation in battery chargers used for a Level 1 EPSS. This was added because the proper charge rate is dependent on temperature. Extreme heat and cold will reduce charge acceptance, necessitating slower charging until the batteries come up to temperature. If the charge rate is not temperature compensated, battery cells may age excessively and prematurely fail. The type of batteries (lead-acid vs NiCad) will also impact the temperature-dependent charge rate.


The 2017 edition of the NEC brought sweeping changes in Articles 700 and 701. One of these changes was a new requirement that the generator start circuit integrity is properly supervised. This type of supervision usually requires a normally closed starter circuit.

However, generators were traditionally started by a contact closure signal (consistent with requirements in previous versions of NFPA 110) from an automatic transfer switch. This older NFPA 110 requirement meant that the starter circuit was normally open and therefore difficult to monitor for proper circuit continuity.

Section in the 2019 edition of NFPA 110 deleted the requirement that remote engine starting be initiated by closing a switch or set of contacts. While the official technical committee statement inferred that this was a relaxation of the requirement so that either normally open or normally closed starting circuits would be allowed, this change effectively harmonized generator starting circuit requirement between NEC and NFPA 110.


ESTOP buttons are furnished to allow the generator to be shut down without having to enter the generator room/enclosure. In scenarios where the ESTOP might be needed, remotely mounting the button reduces the chance that the person pushing the button would be directly exposed to an unsafe condition such as a fuel fire or catastrophic mechanical failure.


While ESTOP buttons were required in previous editions of NFPA 110, it was unclear exactly where it had to be located and if more than one ESTOP button was acceptable. The language was ambiguous enough that it could be interpreted that only one ESTOP button was allowed and that there had to be significant physical separation between a generator in a weatherproof enclosure and the location of the ESTOP button. If there is more than one generator, it is reasonable to expect that there may be a need for more than one ESTOP button.

Also, the standard location for the remote ESTOP button for a generator installed outdoors is on the side on the weatherproof enclosure. Revisions to section have clarified that both are acceptable.


The purpose of testing is to demonstrate that a generator can reliably support a connected load. One of the key indicators that a generator is having a hard time supporting a connected load is unstable coolant temperature. The installation acceptance procedures within NFPA 110 detail a requirement for a two-hour full load bank test.

In earlier editions, coolant temperature had to be recorded during this test but there were no specific requirements beyond recording that data. Section now requires that the engine water/coolant temperature stabilize at a constant value relative to the outdoor ambient temperature at least 30 minutes before the completion of the test.


Diesel fuel is not maintenance-free. It will oxidize during long–term storage. This oxidation mechanism is similar to how animal fat becomes rancid. While bio-degradation of diesel fuel may be desirable from an environmental standpoint, it can cause significant generator performance issues.

Oxidation causes the formation of sediment and gum, which usually ends up clogging fuel filters, one of the more common causes of generator failure. As moisture inevitably condenses in partially filled storage tanks, the resulting water can corrode generator fuel system components like injectors and promote microbial growth which will only make the situation worse. When you consider that diesel fuel can sit in storage tanks for months — if not years — in some generator installations, the potential magnitude of the issue becomes a bit clearer.


The formulation/blends of diesel fuel and their associated characteristics have changed significantly in the last few years due to Environmental Protection Agency requirements and the emergence of biofuels. These new fuels have only made the situation worse. The most common diesel fuel in the United States is ultra-low sulfur diesel, which is also referred to as S15. S15 references the 15 parts per million sulfur, which is a significant reduction from the previous limit of 500 ppm. This reduction dramatically improves engine exhaust emissions. However, sulfur is also a natural biocide, and reducing it can promote additional microbial growth.

Bio-diesel generally refers to traditional petroleum-derived fuels blended with a biofuel feedstock. Within the United States, soybean oil is usually the source of the feedstock for biodiesel. In Europe, it is usually canola oil. In the U.S., the relative percentage of bio-fuel is referenced by B-XX, where XX is the volume percent of bio-fuel in the final product.

Bio-diesel blends up to B5 are acceptable to most generator manufacturers. In fact, biodiesel concentrations up to B5 are allowed to be called “diesel fuel” with no separate labeling requirements identifying the presence of bio-fuel. Bio-diesel blends can significantly improve the lubricity of ULSD and reduce emissions (in particular, NOx emissions). However, bio-diesels are also excellent solvents and can dissolve accumulated sediment in fuel storage tanks, which can cause fuel injector deposits, clog filters, etc. There are also concerns over the stability of the bio-fuel components in long-term storage.

A proper fuel testing program can help identify potential issues with a generator’s fuel supply. However, the testing requirements in NFPA 110 were vague and only required on an annual basis. In fact, the standard that was originally referred to within NFPA 110, ASTM D975, was a specification for new virgin diesel fuel, not testing requirements for fuel used in long-term storage.

Although it was not included in the initial release of the 2019 NFPA 110, a proposed tentative interim amendment (TIA 1388) was submitted in late 2018 to address ongoing uncertainty regarding proper diesel fuel testing — namely what to test for, not just when to test. This TIA revises section 8.3.7 and the associated annex material. This revision requires that testing begins on the first day of installation and that trending data be kept for future comparison. This TIA was balloted but not passed by the NFPA Standards Council. However, the nature of the issue merits further consideration.

The testing intervals also have been shorted from 12 to six months. Six-month testing needs to include ASTM tests for microbial contamination, water and sediment, and bio-diesel concentration. Fuel stability tests are also required every 12 months. Finally, the annex material calls for remediation to restore fuel quality should the testing results be outside the acceptable range of the applicable ASTM tests. Many fuel suppliers recommend the use of stabilizers/anti-oxidants, biocides, and fuel polishing (water removal and filtering) to ensure fuel quality when diesel is stored for more than six months.

John Yoon, PE, LEED AP ID+C; McGuire Engineers Inc., Chicago


Author Bio: John Yoon is lead electrical engineer at McGuire Engineers. He is a member of the Consulting-Specifying Engineer editorial advisory board.

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