Steel Tube Institute Addresses Use of Standard Rigid Couplings in Hazardous Locations

Atkore has received periodic questions on the use of our standard couplings on Rigid and IMC conduit in Class 1 Division 1 hazardous locations during the last year. Often, these questions arise in the form of requests for the explosion proof listing associated with the coupling, which does not exist. As the frequency of these questions increased, Atkore reached out to the Steel Tube Institute to see if other members were receiving the same questions. It was determined that all US steel conduit manufacturers were facing the same inquiries, and this was an industry wide inquiry. The Steel Tube Institute Technical Committee, which includes members from all US conduit manufacturers including Atkore, discussed the issue with appropriate parties. Following those discussions, the Steel Tube Institute developed the letter below regarding this particular question:

Dear Valued Customer,

This letter has been provided to clarify the use of rigid conduit couplings in hazardous locations. National Electrical Code® (NEC ®) Article 501.10(A)(1)(a) permits all threaded rigid metal conduit, threaded intermediate metal conduit (types RMC and IMC) and their couplings for use in Class I, Division 1 hazardous locations. The conduit threads and conduit coupling are designed so that five full threads are engaged when the connection is wrench tight. This thread engagement was designed to contain internal explosions. The threaded connections allow the burning gas to cool as it escapes so that any explosion would be confined to the inside of the conduit. This connection is not designed to prevent an explosion, full prevention is not possible but the possibility is mitigated.

Should you wish to further mitigate the possibility of an explosion, a “sealoff” or a “sealing fitting” can be used. The seal-off minimizes the passage of the gasses from traveling freely through the conduit. These devices are designed to stop or localize explosive gases from traveling through the conduit.

The National Electrical Code, Article 501.15 discusses Sealing and Drainage and the requirements for “seals.” Article 501.15(A)(1)(1) only permits explosion proof fittings, including explosion proof couplings, between the sealing fitting and the explosion proof enclosure.

For further information, please see the Steel Tube Institute website at

Please note that the above letter was prepared by the Steel Tube Institute for information purposes only, and that the final determination of product acceptability and compliance with any applicable codes lies with the jurisdiction having responsibility for the installation. Installers should consult both the NEC and the AJH for any further questions.

In addition, please note the informational letter provided above is in reference to couplings only, and not fittings. Fittings must be specifically listed for hazardous locations. Standard rigid couplings are not considered fittings and therefore, are not listed or subject to UL 514B, the Safety Standard for Conduit, Tubing, & Cable Fittings. Rigid steel conduit and couplings are listed to UL Safety Standard #6 and IMC is listed to UL Safety Standard #1242.

If you still have questions regarding the use of steel conduit or couplings in certain applications please contact Raymond Horner, Director Codes & Standards at or (708) 915-1547.