Differences Between Intermediate Metal Conduit and Rigid

Steel conduit is used in electrical systems. Its strength and wall thickness protects the wires running inside of it. It also acts as a grounding conductor in a correctly installed electrical conduit system. Rigid Metal Conduit (RMC) and Intermediate Metal Conduit (IMC) are very similar in application. However, there are some distinct differences between the two.

Rigid and intermediate metal conduit are similar in appearance to water pipe.


RMC is the thickest and heaviest type of steel conduit. IMC has a thinner wall and weighs about one third less than RMC. However, the lighter weight does not reduce its strength--IMC is considered as strong as RMC.

Color Coding

Metal conduit is sized according to designated trade sizing. The numbers given to each size reflect the size of the diameter of the pipe. Trade size Number 2, for example, has an inside diameter of just over two inches. Thread protectors are added to the uncoupled ends of both types of conduits to preserve the exposed threads and to identify the pipe sizes. RMC is manufactured in sizes ½ to 6, while IMC Is typically available in sizes ½ to 4. The thread protectors are colored blue for whole sizes of RMC (1 to 6) and the thread protectors for all half sizes (1/2, 1 ½, 2 ½, etc.) are black. Red thread protectors are used for ¾ and 1-¼ trade sizes of RMC. Whole sizes of IMC (1 to 4) have orange thread protectors instead of blue, all half sizes are color-coded yellow, and ¾ and 1 ¼ sized IMC are given green thread protectors on uncoupled ends.


Galvanized RMC has a zinc coating inside and out. Electro galvanized RMC is coated on the outside only with a protectant that guards against corrosion on the inside. In addition, RMC sometimes uses a coating that is not metallic based (like PVC). The PVC coating is often added when the conduit will be buried underground in concrete that contains chlorides. Intermediate metal conduit is coated on the outside with a zinc-based coating. It is protected by an organic corrosion-resistant coating on the inside.


Rigid and intermediate metal conduit are interchangeable in most applications. They share the same thread and use the same couplings and fittings. They are used in the same locations and have the same types of support systems. RMC and IMC are both used indoors and outdoors, as well as underground. In addition, RMC may be installed either covered or exposed if it has been galvanized. RMC with coatings that are not zinc may have some temperature limitations, which are typically noted in the manufacturer's label. Both types of conduit allow you to install wiring without running boards and provide additional protection to the electrical system. RMC costs more in comparison to IMC, which is often one of the deciding factors in which type to use.