Search Interpretations


Mr. John Coursen

Product Line Manager

Structural Composites Industries

325 Enterprise Place

Pomona, CA 91768

Dear Mr. Coursen:

This responds to your email in which you seek confirmation of the proper bonfire test procedure when two or more compressed natural gas (CNG) fuel containers are connected to a common manifold under Federal Motor Vehicle Safety Standard No. 304, Compressed Natural Gas Fuel Container Integrity (Standard No. 304). Specifically, you ask if it would be proper to perform bonfire testing as specified under paragraph S8.3 with the entire group of interconnected containers and their shared pressure relief devices (PRDs) being tested, as opposed to testing containers individually. Based on the information you have provided, we would not conduct the Standard No. 304 test in the manner you describe, because the standard specifies that in the bonfire test, NHTSA tests CNG fuel containers individually. However, this does not prevent you from performing additional testing of your interconnected CNG fuel containers as a group if you wish to do so.

By way of background, the National Highway Traffic Safety Administration (NHTSA) is authorized to issue Federal motor vehicle safety standards (FMVSSs) that set performance requirements for new motor vehicles and items of motor vehicle equipment (see 49 U.S.C. Chapter 301). NHTSA does not provide approvals of motor vehicles or motor vehicle equipment. Instead, manufacturers are required to self-certify that their products conform to all applicable safety standards that are in effect on the date of manufacture. NHTSA selects a sampling of new vehicles and equipment each year to determine their compliance with applicable FMVSSs. If our testing or examination reveals an apparent noncompliance, we may require the manufacturer to remedy the noncompliance, and may initiate an enforcement proceeding if necessary to ensure that the manufacturer takes appropriate action.

In your letter, you describe a system of CNG containers in which two or more fuel containers are interconnected by a manifold, with multiple PRDs attached to the manifold for venting the fuel containers as a group. You state that two CNG containers, with a common manifold with two PRDs, would be the expected configuration.

Standard No. 304 is an equipment standard, which specifies requirements for the integrity of [CNG] motor vehicle fuel containers (Standard No. 304, S1).[1] Standard No. 304 regulates CNG fuel containers individually with their PRDs: note, for example, that every use of the term CNG fuel container in the standard is singular, not plural. Regarding the bonfire test in particular, both S7.3 and S8.3 refer to the CNG fuel container, not to containers or to fuel container systems. This is reflected in our enforcement offices laboratory test procedure for Standard No. 304, which tests each CNG fuel container individually, and states that each shall be equipped with a pressure relief device or integral thermal protection system.[2]

Even though Standard No. 304 does not require multiple interconnected fuel containers to be tested as a group, you may perform such a test yourself. The FMVSSs provide a minimum threshold of safety, as established by performance and testing requirements. However, you are free to perform additional testing of your products in order to ensure that they do not pose an unreasonable risk of safety when used on the road.

If you have any further questions, please do not hesitate to contact Rebecca Schade of my staff at (202) 366-2992.

Sincerely yours,

Anthony M. Cooke

Chief Counsel



[1] Note that multiple linked containers would be evaluated as a system under Standard No. 303, Fuel system integrity of compressed natural gas vehicles. Standard No. 303 is a vehicle standard, as opposed to an equipment standard like Standard No. 304. Standard No. 303 specifies fuel leakage limits for the vehicle when subjected to crash tests. The responsibility to meet Standard No. 303 is borne by the vehicle manufacturer in cases in which your system is installed on a new vehicle by or with the express authorization of that vehicle manufacturer. 

[2] NHTSAs Laboratory Test Procedure for FMVSS 304, Compressed Natural Gas (CNG) Fuel Container Integrity (TP-304-03, Dec. 8, 2003) is available at The quotes above are taken from page 13 of that document