What are the difference between IMC & RMC. I have an electrical contractor telling me there is no difference. I have always spec'd RMC in an explosion proof area. I know the Code states IMC is legal, But I was always taught RMC was stronger and had better grounding of interferance for control circuits using 4-20ma. I would like to stay with RMC what would be a good argument not to use IMC?

IMC is less expensive and can be used in all areas that RMC can be used. I am also told that it is stronger, but I have never read anything on that. IMC also has a bigger cross sectional area and is easier to bend.

I would not install RMC unless I was forced to.

I have always found IMC much stiffer and more difficult to bend. I always opt for RMC because it is easier to bend.

Really? I haven't bent either of them in about 6 years or so, so I might not be remembering properly.

IMC has a thinner wall, and more tempered/stiffer, not sure why.

RMC is slightly thicker, weighs more, less tempered, but seemingly about the same to bend. Costs a penny more.

I agree that IMC is much stiffer and harder to bend and thead than RMC. I have also seen problems with the IMC breaking at the threads on the 1/2 and 3/4" size. This is caused by the reduced wall thickness of this conduit. There is very little strength left after you cut the threads.
Don

I used a fair amount of IMC and GRC in hazardous locations including 4-20 mA control circuits and VFDs. Never noticed a diffrence in EMI or RFI problems in one over the other.
Although lighter in weight the IMC was indeed harder to bend, and keeping the shoe tight against the pipe for an accurate radius was difficult even at the 3/4" size. I would avoid using it for hand bending.

From an RF standpoint a wrap of properly grounded metal window screen is as effective as RMC. The difference is physical protection.

Thanks for the feed back.

I prefer working with IMC, lighter and easier to bend IMHO. Never had a problem with it breaking underground and Maine frost can break anything. My suppliers don't stock it anymore though so I use RMC all the time now anyway. What are people paying for 3/4 rigid, last time I checked it was 17 bucks a stick up here??

I remember when IMC came out that Aliied company got with Greenlee and went to demonstrate the difference on bending and I believe IMC had more deflection than RMC. They then tolds use to back out the dies when threading the conduit to reduce over cutting. Also explained how it was tested to match the charitistics of RMC and if I remember correctly they matched identically even the explosion test. My only advice is use what you are comfortable with and easily to obtain. As the Electrical Contractor on the job you and your crew will be the only one who knows.