22 Jul 2015

Several questions about the use of IMC have been asked at trade shows and other events over the last year and we realize that there are a many misconceptions concerning IMC.  Here is some clarification:

One of the most common misconceptions of IMC (intermediate metal conduit) is that it is not as strong as GRC (Galvanized Rigid Conduit) and does not offer the same physical protection as GRC.  The truth is that IMC is actually stronger than GRC.  Although crush and impact tests are not required in the product standards, historically, we have done testing and have found the following exemplary mechanical properties:

Tensile Strength55,00062,000
Yield Strength35,00055,000

Another misconception concerns the permitted uses of IMC in the National Electrical Code® (NEC®). There seems to be a misconception that IMC has limitations under the NEC that GRC does not. The fact is that IMC is allowed to be used under “All atmospheric conditions and occupancies” according to Article 342 of the NEC. This same statement is made under Article 344 of the NEC in reference to GRC.  A simple rule to remember for using IMC would be:  If you can use rigid steel conduit, you can use IMC! (Always check local code requirements as well).

Is IMC allowed to be used in wet locations? NEC section 342.10 (D) states IMC can be used in wet locations but all supports, bolts, straps, and screws should be of a corrosion resistant material. The same requirement applies to rigid metal conduit.

Can IMC be used for protection of service entry cables?  Yes, IMC can also be used for physical protection of service entrance cables according to NEC section 230.50(1).

Can you install IMC in a hazardous location? Yes, NEC section 501.10(A)(1) allows threaded steel intermediate metal conduit(IMC) to be installed in Class I, Division 1 and Class I, Division 2 applications. Just like rigid steel conduit.

Have an installation in concrete or another corrosive environment? IMC can  be installed in concrete, in direct contact with the earth, or in areas subject to severe corrosive influences where protected by corrosion protection and judged suitable for the condition. Just like rigid steel conduit.

Those are the answers to the most common questions we receive regarding IMC. We would also like to share some other facts about IMC that may be of interest during the selection, installation, or inspection of IMC.

  • IMC is an excellent equipment grounding conductor and is recognized for use in article 250 of the NEC®.
  • The co-efficient of expansion for IMC and GRC are the same.
  • IMC is considered non-combustible by building codes. Therefore, when used to penetrate a fire wall, the International Building Code – in an exception to its main rule – allows the annular space to be filled with common building materials. (cement, mortar, or grout)
  • IMC is listed to UL ®safety standard 1242. UL 1242 IMC standard includes more performance-based requirements than the rigid steel conduit standard, including a beam strength test and a threaded conduit pullout test.

If you still have questions or concerns about using IMC please contact Raymond Horner at (708) 915-1547 or rhorner@atkore.com

The questions and answers in this article are based on the current edition of the NEC.  However, local codes can differ, and your Authority Having Jurisdiction (AHJ) may have opinions that also differ.  It is always important to verify your planned installation with the local AHJ.