Protecting Field-cut Threads Against Corrosion
Factory-cut threads receive a corrosion-protection coating at the factory, but what about field-cut threads? Section 300.6(A) of the NEC says, “Where corrosion protection is necessary and the conduit is threaded in the field, the threads shall be coated with an approved electrically conductive, corrosion-resistant compound.” The 2014 edition of the NEC added an Informational Note to section 300.6(A) to clarify what field-cut threads are. The Informational Note explains that “Field-cut threads are those threads that are cut in conduit, elbows or nipples anywhere other than at the factory where the product is listed.” To meet the requirement to protect the threads, zinc-rich paint or other coatings acceptable to the AHJ may be used. If the AHJ requires listed coatings for this purpose, compounds are available under UL category “FOIZ,” Electrically Conductive Corrosion-resistant Compounds. Zinc-rich paint is available from a variety of manufacturers and has as many formulations. Not all zinc-rich paint products are electrically conductive. It is always good practice to check with the paint manufacturer if you are in doubt about a specific application.
It is important that the corrosion protective coating applied to conduit threads not inhibit the conduit system from breathing. Moisture through condensation due to temperature changes or ingress of water from other sources must have a path to drain. Section 230.53 Raceways to Drain reads, “Where exposed to the weather, raceways enclosing service-entrance conductors shall be suitable for use in wet locations and arranged to drain. Where embedded in masonry, raceways shall be arranged to drain.”
When conduit systems are installed in some Class 1 location, any flammable gases or vapors entering a conduit system that become ignited and explode need the threaded path way between the conduit and coupling to cool hot gases to a temperature below the ignitable temperature of the surrounding air. Be aware, if using a product under the UL category of “FOIZ,” that the UL FOIZ.Guideinfo says, “These compounds have not been investigated for use in hazardous (classified) locations.”
If you have any questions or concerns about corrosion protection, don’t hesitate to contact the Steel Tube Institute.