US Dept of Labor

Occupational Safety & Health AdministrationWe Can Help

Standard Interpretations - Table of Contents
• Standard Number: 1910.309

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

November 5, 1979

Mr. J. D. Spooner
Vice President and General Sales Manager
Weil Pump Company
1530 N. Fremont Avenue
Chicago, Illinois 60622

Dear Mr. Spooner:

This is in response to your recent telegram addressed to Mr. William Simms concerning OSHA's policy on classification of a wet well as a Class 1, Division 1 location.

The General Industry Standard [29 CFR 1910.399(a)], NEC Sections 500 and 501, applies to situations for OSHA-covered employers. Hazardous concentrations of flammable gases, such as methane and hydrogen sulfide, may exist continuously, intermittently, or periodically under normal operating conditions in a waste-water (containing organic matter) collection system. If at least one OSHA-covered employee is exposed to the hazard, then the wet well pump motor must be of a type indicated in the National Electrical Code Section 501-8(a) for a Class 1, Division 1 location. The employee need not be in the wet well to be exposed to the effects of an explosion.

We hope this information will be helpful to you. If we may be of any further assistance, please feel free to call or write.


Grover C. Wrenn, Director
[Directorate of Enforcement Programs]

[Corrected 6/2/2005]

Standard Interpretations - Table of Contents

Thank You for Visiting Our Website

You are exiting the Department of Labor's Web server.

The Department of Labor does not endorse, takes no responsibility for, and exercises no control over the linked organization or its views, or contents, nor does it vouch for the accuracy or accessibility of the information contained on the destination server. The Department of Labor also cannot authorize the use of copyrighted materials contained in linked Web sites. Users must request such authorization from the sponsor of the linked Web site. Thank you for visiting our site. Please click the button below to continue.