IMC vs. GRC for Service Masts

Question: I have an issue with our power company regarding the use of IMC (intermediate metal conduit) for service masts. Several contractors have installed IMC in this application only to be rejected by the power company. The power company says that GRC means galvanized steel RMC. I asked the power company to refer me to the NEC for clarification on how they prove that GRC and galvanized steel RMC are the same thing. My problem, of course, is that GRC is not found in the NEC. Ironically, nor is GRC defined in the power company install guide, but they are emphatic that GRC is clearly mentioned in the NEC under RMC. My question is:

  1. Is it an understood fact that GRC and galvanized steel RMC are one and the same?
  2. Is it possible to prove that IMC has the same strength as GSRMC when it comes to the weight stress of an overhead service on the mast?

Please also let me know whether or not you wish to be quoted. Thanks for your time.

Answer: Thank you for your questions. GRC is a common industry acronym used for galvanized rigid conduit. GRC is the same product as UL-6 electrical rigid metal conduit — steel (ERMC-S) and rigid metal conduit (RMC) covered in NEC Article 344, which also covers aluminum RMC, stainless RMC and brass RMC. IMC is covered in Article 342 and is a steel galvanized conduit just like GRC, but it is made of a different type of steel with a thinner wall thickness. The wording in Article 342 and Article 344 is identical; therefore, both products are permitted in exactly the same installation applications. IMC is slightly stronger due to the different types of steel. However, for some reason, there are utilities that only permit the Article 344 rigid metal conduit for service masts. Generally, they will change their installation specification when they are made aware of these facts. You may quote me, and I will be happy to answer any additional questions you have.

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