Intermediate metal conduit, or IMC, is a rigid steel electrical conduit designed for outdoor exposure and strong connections. It was designed specifically to protect insulated electrical conductors and cables. It does the work of a similar metal conduit, rigid metal conduit (RMC), but weighs about a third less. By utilizing IMC in any areas allowed, you can all but eliminate the need for a heavier-wall conduit.
IMC Conduit Basics
IMC conduit is coated with a hot galvanized coating on the exterior and a special corrosive-resistant coating on the inside to extend the conduit’s lifespan. Common conduit sizes range from ½” to 4”. It is sold in 10-foot lengths, with or without threaded ends. IMC was originally introduced by Allied Tube & Conduit Corporation. It is manufactured in accordance with UL (formerly Underwriters Laboratories) safety standard 1242 and ANSI C80.6. The National Electrical Code book has a specific article about IMC, article 342.
IMC conduit is joined with threaded couplings for safe and secure connections. These often require large pliers or pipe wrenches for installation. IMC can be cut with pipe cutters or band saws, but you must file the interior side of the pipe to be sure the burr doesn't damage the outer coating of the wire as it is being pulled into the conduit. Once cut, the conduit can be threaded on the cut end, allowing the user to make whatever lengths are needed.
IMC vs. RMC
In addition to its lighter weight, IMC offers some other advantages over RMC. It has a larger interior diameter than RMC, and the smoother interior of the pipe allows for easier wire pulling through the conduit. Allied Tube & Conduit Corporation has done extensive studies on conduit strength and reliability of IMC.
On this product data sheet, they outline the testing and guidelines of their product in making it a viable alternative to RMC. If you have any question of whether this conduit can stand up to RMC or exceed it, this article paints a pretty good picture of IMC’s strength and reliability.
IMC vs. EMT
IMC is compared to RMC, but regarding weight, it lies between RMC and EMT, or electrical metallic conduit. EMT is commonly used for branch circuit wiring in homes (where the wiring runs are exposed) and in commercial buildings. EMT is never threaded and is not designed for strong connections like IMC and RMC. In residential wiring, IMC is usually limited to service entrance assemblies and exposed runs to and from exterior service panels, but otherwise, EMT is the standard for household installation.
IMC is allowed anywhere RMC is used, particularly for residential wiring. This may include service masts, panel connections, and other outdoor applications. Always check with the power company and the local building authority for specific requirements in your area.
Don't Forget the Fittings
IMC conduit comes in a variety of sizes to fill your conduit needs. Remember to include the accessories that you may need to complete the project. Threaded couplings, locknuts, whether regular or self-sealing and bushings, are all on this list.