RMC vs. IMC

[brandon2177k]

What are the main differences between RMC and IMC? Is RMC any stronger than IMC? Will it last longer when buried outside? Thoughts Please...

 

[celtic]

Intermediate metal conduit (IMC) is a thinner-walled rigid metal conduit that is satisfactory for uses in all locations where rigid metal conduit is permitted to be used. Also, threaded fittings, couplings, connectors, and so on, are interchangeable for either IMC or rigid metal conduit (RMC). Threadless fittings for IMC are suitable only for the type of conduit indicated by the marking on the carton.

Above from 2002 NECH commentary.

 

[don_resqcapt19]

I have not used IMC in many years, but when I did, I found that it was harder to bend and thread than rigid. Not sure why, maybe a different type of steel? I would expect that it would not last as long as rigid in the ground based on its appearance. It looks like IMC is plated and does not have the same amount of galvanizing for corrosion protection, however the listing information is the same for both rigid and IMC.
Also on the 1/2 and 3/4 sizes you had to be careful with putting stress on the conduit where it was threaded into fittings. The depth of the threads is the same as on rigid so a result of the reduced wall thickness, there was less pipe left to provide strength.
Don

 

[paul]

I had the opposite experience as Don when I worked with IMC. It was easier to thread and bend. I had to order it in, but I would much rather work with it than I would with RMC.

 

[steve66]

The NEC describes IMC as being "Steel", but RMC "is generally made of steel (ferrous) with protective coatings or alumnium (nonferrous)."

So IMC is steel, and RMC can be either steel or alumnium.

 

[Dave58er]

IMC is made of stronger steel allowing it to have thinner walls with equivalent or often greater strength. Check out Chapter 9 table 4 and annex C table C.4/C.8 for comparisons.The thinner walls give you a larger inside diameter that can help with fill. As far as underground I'm not sure. RMC is galvanized inside and out. IMC is galvanized outside and has a "UL-listed, corrosion-resistant, organic or inorganic coating on the interior".

I've found it easy to thread if you loosen your dies and run a factory thread into them, then lock them down. It usually needs full shoe benders bacause of the thinner walls. I could not get hickeys to work, but some people can.

Should also mention, if you haven't used it, IMC is a lot lighter.

 

[brandon2177k]

I have worked with IMC and RMC, but more so with IMC. We really only use RMC for risers, everything else is IMC or PVC, if applicable. I was just curious of other peoples thoughts on each type.

 

[steve66]

Why do you use RMC for risers? I would have guessed RMC would be better for horizontal runs, since it probably wouldn't sag as much.

Steve

 

[ramdiesel3500]

We find IMC to be a bit more prone to buckling or kinking when bending. It also tends to crush into a triangle cross-section during threading if threading dies are a bit dull. Otherwise its great! For some reason, the IMC we get here has a much better outer surface finish than the RMC.