OSHA requirements are set by statute, standards and regulations. Our interpretation letters explain these requirements and how they apply to particular circumstances, but they cannot create additional employer obligations. This letter constitutes OSHA's interpretation of the requirements discussed. Note that our enforcement guidance may be affected by changes to OSHA rules. Also, from time to time we update our guidance in response to new information. To keep apprised of such developments, you can consult OSHA's website at https://www.osha.gov.


April 8, 1997

Richard L. Freeman
Colonel, United States Army
Assistant Executive Director
Department of Defense Defense
Logistics Agency Headquarters
Suite 2533
8725 John J. Kingman Road
Fort Belvoir, Virginia 22060-6221

Dear Colonel Freeman:

This letter is in response to your request for interpretation of 29 CFR 1910.399, Subpart S, classified electrical installations.

As you have explained your facilities and the activities conducted there, OSHA classifies them as "industrial plants" as defined in 29 CFR 1910.106(e). These facilities deal with container and portable tank storage as defined in 29 CFR 1910.106(d). Since you have not described the specific makeup of each facility, we are providing the following guidance for your review. In following the electrical installation requirements specified, both situations that you have described concerning "sealed" and "closed or sealed" containers will be covered.

You have stated that "new" disposal storage warehouses are designed with a mechanical ventilation system providing six air changes per hour, and automatic fire suppression systems. You have not described how "existing" facilities are ventilated and provided with protective systems. You have specified, no mixing or transfer operations are conducted in these facilities and the liquids are not used in these facilities. Since we do not know what existing facilities are like, the following guidance is provided to cover either type (new or existing) of facility, your safety, health and fire protection officials are going to have to make the final determination as to which section applies as they are "on-scene" and have the necessary knowledge to make that determination. The electrical equipment in these facilities should be installed according to 29 CFR 1910.106(e)(7), Electrical. Paragraph (e)(7)(i), Equipment, addresses the installation of equipment and recognizes three basic storage/use areas and has requirements for each area. Overall, paragraph (e)(7)(i)(a) requires all wiring to meet Subpart S. More specifically, paragraph (e)(7)(i)(b) addresses locations where flammable vapor-air mixtures are present under "normal" operations. Paragraph (e)(7)(i)(c) addresses locations where flammable vapor-air mixtures are present under "abnormal" operations. And, paragraph (e)(7)(i)(d) addresses locations where "ordinary" wiring may be used.

OSHA's primary concern is "employee" protection. In the conditions that you have described where flammable and combustible liquids (Hazardous Material and Hazardous Waste), and where no liquid transfer takes place, there is no associated generation of vapor-air mixtures, and the fire hazard from an employee exposure perspective is limited. Only in limited, "abnormal" conditions such as an accidental spill or release of a flammable/combustible liquid will the potential for a fire/explosion develop. Other OSHA requirements in 29 CFR 1910.106 for proper drainage and spill control, proper ventilation, and adequate fixed fire protection systems along with an appropriate emergency action plan for employee evacuation as specified in 29 CFR 1910.38 can provide sufficient protection to employees in the event of an accidental release. Keep in mind that the employees must be trained on the hazards to which they are exposed and in the emergency plans and actions required to protect them.

Should you have further questions, please contact [the Office of General Industry Enforcement at (202) 693-1850]. Thank you for your continuing interest in occupational safety and health.


John E. Plummer, Director
Office of Federal Agency Programs

[Corrected 2/4/2004]


January 30, 1997

Mr. Ron Cain
Safety Specialist
Federal Agency Programs
Occupational Safety and Health Administration
Washington, DC

Dear Mr. Cain:

Request your office provide an explicit interpretation pertaining to the requirements for classified electrical wiring (application of Standard 1910.106) to our particular situations discussed below. Previous informal contacts with Mr. Thomas Seymour, Acting Director, Safety Standards Office, resulted in referral to your office.

The Defense Logistics Agency, as part of its global logistics mission, has hazardous material (HM), hazardous waste (HW), and flammable and combustible liquid storage facilities worldwide. The HM and HW are composed of all types of chemicals, including flammable liquids, gases and solids. At supply activities, commodities are received in lots from manufacturers in sealed containers (usually drums), which are sorted, stored and then configured into individual shipments (again usually drums). This process is normally without any liquid transfer taking place. Where liquid transfer is required, this operation is conducted in a separate location with proper electrical and mechanical equipment installed to prevent ignition of any vapors generated during the transfer operation.

At disposal activities, HM and HW (sometimes stored together) are turned-in and either reutilized by the Department of Defense (DoD), transferred to other government agencies, donated to approved recipients or sold to the public. DoD only requires hazardous property to be turned-in for disposal in closed containers (sealed containers are not required). All HW that is ultimately disposed of is done via government disposal contracts. The contractors must open the HW containers to obtain samples to verify the waste profile. New disposal storage warehouses are designed with a mechanical ventilation system providing six air changes per hour, and automatic fire suppression systems.

The issue is the correlation of definitions from 29 CFR Subpart "S" Safety Standards for the Electrical System, Section 1910.399, n relation to applicable sections of Subpart "H", Hazardous Material, NFPA 30 and NFPA 70. Your interpretation should specifically address the following questions:

1. What classification of electrical wiring is required in our supply function warehouses that only store sealed containers of hazardous materials?

2. What classification of electrical wiring is required at our disposal function warehouses that may accept either closed or sealed containers?

In either situation, any flammable vapor-air mixtures would be present only under abnormal operations. With the significant cost differential between Class I, Division 2 and Ordinary wiring, and our requirement for construction of facilities to accommodate mission requirements at various locations world-wide, we need to have your interpretation of the standard as it relates to our operations. Our goal is to construct facilities to accommodate mission requirements, to meet all applicable standards to protect our personnel, and the public, in the most cost effective manner.

Thank you in advance for your expeditious response to this request.

Colonel, United States Army
Assistant Executive Director