Defining NFPA 37

Wesley Stiles, PE, Director of Electrical Engineering for the York office, is featured in the December Issue of Consulting-Specifying Magazine for the article below.

NFPA 37: Standard for the Installation and Use of Stationary Combustion Engines and Gas Turbines, 2015 edition, is an important yet often overlooked player in NFPA’s extensive lineup of codes and standards. The stated purpose of the document is to provide “minimum fire-safety requirements for the installation and operation” of permanently installed engines.

The origins of NFPA 37 trace back to 1905, where it was published by the National Board of Fire Underwriters, and subsequent versions were published originally by the NFPA technical Committee on Explosives and Combustibles and currently by the NFPA Technical Committee on Internal Combustion Engines.

That history becomes important when looking at what NFPA 37 is—and what it isn’t. NFPA 37 was a project originated by Fire Underwriters with the main goal of creating a standard set of installation, operation, and maintenance criteria to minimize the likelihood of fire caused by fuel-fired engines inside of or adjacent to structures. While NFPA 37’s requirements are most often applied to generators, the scope of the standard applies to all stationary, engine-driven equipment. The scope of NFPA 37 does overlap with other sections, such as:

NFPA 110 and the NEC (specifically generator-specific Articles 700, 701, 702, and 708) serve to create a baseline for the electrical integrity, reliability, and operation of emergency systems. NFPA 37 primarily is concerned with making sure the engine itself and its associated components (fuel supply, fuel tanks, environment, etc.) are not installed in a manner that creates a potential fire hazard. NFPA 37 is not concerned with the electrical distribution system fed from a generator or the maintenance protocol to ensure that the emergency system is available when needed; instead, the standard ensures that the engine itself does not pose a threat or hazard to the structures around it.

The goal of NFPA 37 is to provide a roadmap for safe installation which, if followed, will function to not only minimize the likelihood of a fire but also then contain the spread of a fire if required.

  • Location, construction, and installation requirements serve to prevent a fire in the engine from spreading to adjacent spaces or structures.
  • Ventilation requirements keep the equipment space from overheating, compromising wiring or the engine itself, as well as preventing buildup of combustible fumes, which could otherwise ignite or cause explosion.
  • Requirements for fuel piping provide installation, material, and equipment guidelines that ensure clean, reliable distribution of fuel to the engine.
  • Automatic-shutoff requirements are designed to automatically cut off flow of fuel to an engine when it is not running or running under abnormal operating conditions. In addition, requirements for manual, remote shutoffs allow maintenance staff or first responders to disable the unit without having to physically approach it or enter a space in which there may be a dangerous event.
  • On-site fuel-storage requirements serve to minimize potential for spills, overfilling, and leaks from fuel tanks and the engine itself. Tank size limits in conjunction with location and construction regulations help minimize the ability of an active fire to spread.
  • Other requirements found within the standard include wiring methods, mandatory controls, and exhaust requirements.

While the purpose of this article is to highlight significant requirements for commercial emergency and standby generator installations, it’s important to remember that NFPA 37 applies to all stationary engines and turbines, both residential and commercial installations. Refer to the full standard for additional requirements for those applications.

Locations and Installation

NFPA 37 identifies four separate categories for generator locations:

  • Dedicated rooms within a building
  • Dedicated detached structures adjacent to a building
  • Exposed on the roof of a structure
  • Outdoors.

While this particular code does not dictate whether an engine located within a building needs to be installed in a separate room, Section 4.1.2 provides requirements of the room for when it is. (Note that in the case of an emergency generator, a separate room is required by NFPA 110, Section 7.2.) NFPA 37-2015 Section 4.1.2 notes that an engine room within a structure must have walls, floors, and ceilings that are 1-hr rated, the only exception being the ceiling of a room on the top floor of a building can be noncombustible or protected with an automatic fire protection system.


The standard also provides requirements for installations on building roofs and outdoor installations. In both cases, the minimum separation distance from the engine housing is 5 ft to any opening in an adjacent building and 5 ft to any structure with combustible walls, building openings, overhangs, and vegetation. There are two listed exceptions to the minimum distance (see Figure 1):

  • Exception 1: When the adjacent wall or structure has a fire rating of at least 1 hr.
  • Exception 2: When the weatherproof generator enclosure is “constructed of noncombustible materials and it has been demonstrated that a fire within the enclosure will not ignite combustible materials outside the enclosure.”

Exception 2 is easily the most frequently discussed and debated requirement in the entire standard. There have been formal interpretations, proposed amendments, and public comments on the topic. Many manufacturers have enlisted third-party testing agencies to show that their units meet the requirements of Exception 2, and, therefore, can be installed closer than 5 ft from adjacent structures. As currently written, there are no specific parameters given for the actual enclosure fire test.

This often leaves room for interpretation. For example, what is the exterior cladding of the adjacent structure? Different materials have different combustion profiles. Is the generator running or not running during the test? Research has shown that in an air-cooled unit, when running, the fan can serve to extinguish a fire within the enclosure sooner than if it were not operational at the time.

Authorities having jurisdiction (AHJs) have argued that currently there are no provisions for an inspector or reviewer to reject “proof” from a manufacturer that a particular enclosure meets the second exception. In order to assure safety, it is recommended that a system installer maintain the 5-ft separation wherever possible and only rely on the exceptions when absolutely necessary for a particular installation.


In the case of a generator that is installed in a dedicated detached structure, such as an equipment out-building, those installations have similar requirements to those listed above with a minimum separation of 5 ft from the structure (not the engine) to an adjacent building. For dedicated structures, the minimum requirement is superseded when either the outbuilding or the adjacent wall are 1-hr rated, or when the outbuilding has an automatic fire-suppression system (see Figure 2). Note that if a project is designed to meet the NFPA 110 requirement for all generators within a building to be located in a rated room, the installation inherently qualifies for Exception 2.

Other notable requirements concern ventilation and storage in the engine space. Engine rooms and structures are required to have dedicated ventilation systems sized adequately to meet combustion requirements of the engine and prevent buildup of fumes and products of combustion in the space. Storage of combustible materials other than those required for day-to-day operation (manuals, filters, etc.) is strictly prohibited.

Fuel Supply

The fuel-supply requirements of NFPA 37 are broken up into two categories, gaseous and liquid. For the purposes of this standard, “gaseous” refers to engines where fuel is delivered in vapor form (including natural gas, liquid propane vapor, and biogas) and “liquid” refers to engines where fuel is delivered in liquid form (including diesel and fuel oil). Liquid propane, which is stored as a liquid but delivered to the engine as a vapor, is considered a gaseous fuel.

Chapter 5, Fuel Supply—Gaseous, provides explicit requirements regarding the distribution of vaporous fuel to an engine and required safety features within that distribution system. The components required to make up the system gas train, which is the “portion of the fuel supply piping starting with and including the equipment-isolation valve and extending to the point at which the fuel enters the prime mover,” shall include the aforementioned equipment-isolation valve, a pressure regulator (if required), two automatic safety-shutoff valves, a manual leak-test valve, low- and high-pressure limit control (for engines with more than 732-kW input), vent valve or valve-proving system (for inlet pressures more than 2 psi), a gas filter or strainer, and “any other components or equipment that the manufacturer requires for safe operation.” The gas train is located at the engine and, in many cases, all required components downstream of the isolation valve are furnished with the unit by the generator manufacturer. If required, the gas regulator, vents, or any relief valves must be installed so that they are located a minimum of 5 ft from any building opening.

In addition to the gas train, Section 5.4 requires the installation of a manual-shutoff valve to isolate the fuel supply. Note that this is different from the equipment-isolation valve. In the case of multiple engines, each individual engine would have an equipment-isolation valve after the last tap in the branch piping serving that unit, but there would only be one manual-shutoff valve used to shut off the flow of fuel to all engines simultaneously.

Chapter 6, Fuel Supply—Liquid, is most frequently applied to diesel engines. This part of the standard includes requirements for fuel-storage tanks—location guidelines and size restrictions as well as piping, valve, and fitting rules. Per Section 6.3.2, fuel tanks located inside buildings shall not be more than 660 gal when located in a shared space. Fuel tanks may be larger if they are located in a dedicated room, but the installation and construction of that room must meet additional requirements of Section 6.3.5 (for tanks up to 1,320 gal) or 6.3.6 (for tanks larger than 1,320 gal).

All fuel tank size restrictions are provided with the exception that tanks of any size may be installed within engine or mechanical rooms provided the room is “designed using recognized engineering practices with suitable fire-detection, fire suppression, and containment means to prevent the spread of fire beyond the room of origin.” The exception is subjective, so any attempt to take credit for the exception should be discussed with the AHJ early in the project.

The standard also requires all fuel tanks located within a building or on a building roof to be equipped with a suitable spill-containment system (wall, curb, or dike capable of containing a quantity of fuel equal to the capacity of the largest single tank present). An exception to this would be the installation of a drainage or overflow system designed to remove any spilled or leaked fuel to an approved “safe area” outside of the structure. NFPA 30 contains additional design requirements governing storage of fuel.

In addition to fuel tank size requirements, the standard also provides guidelines for ventilation of fuel tank rooms, approved methods of filling tanks, mandatory tank alarms and safety shutoffs (such as shutting down tank fill pumps as a result of high-level alarms), and relief and piping requirements.

Exhaust Systems

The exhaust system guidelines in NFPA 37 are relatively straightforward. Chapter 8, Exhaust Systems, requires that the exhaust system is designed, constructed, and installed in such a manner that it doesn’t present a safety or fire hazard during normal operation. This includes ensuring that all penetrations through the building structure are appropriate for the temperatures that they will be exposed to and that all products of combustion are adequately and deliberately removed from the space without interfering with other building systems. Where exposed, exhaust systems must be guarded to prevent potential injury to personnel.

For the purposes of a typical generator installation, the responsibility for meeting the requirements of NFPA 37 falls equally on the generator manufacturer, the design professional, and the installer. Fire is never fully preventable, but by following the guidelines set forth in this standard, the likelihood of a catastrophic event and the potential for loss of property or life are significantly reduced.