Underground RMC Protection

[RSquirrel]

For a proposed underground feed (transformer to CSED - combination of 3” PVC and 3” Galvanized RMC), a portion of the RMC will be imbedded in soil and a portion will pass through a 4” thick concrete pad.

Q1. Since some of the galvanized coating of the RMC is removed in the threading process, should the exposed threads be “painted” with some sort of sealant after assembly? If yes, what product?

Q2. Is it general practice to wrap the RMC to prevent bonding to the concrete? If yes, what material?

 

[don_resqcapt19]

I require a corrosion protection tape such as 3M;s Vinyl Corrosion Protection Tape 50 for any concrete to earth transition as that is a severely corrosive condition. I also require that at concrete to air and earth to air transitions as those are also corrosive areas, but the worst is the concrete to earth transition,

300.6(A) requires an electrically conductive corrosion-resistant compound to be applied to any field cut threads. Crouse Hinds STL is one such compound as is T&B's Kopr-Shield. There are others.

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[tom baker]

What I used for protecting the field cut threads was to spray with cold galvanizing. Then over that I used STL or kopper shield.
Hint: kopper shield is messy, it gets home before you do, but it works great, I have gone back after 25 years and easily taken apart threaded fittings.
Our DOT requires 2 coats of cold galv on RMC couplings as they do not hold up in corrosive or wet locations

 

[marmathsen]

300.6(A) requires an electrically conductive corrosion-resistant compound to be applied to any field cut threads. Crouse Hinds STL is one such compound as is T&B's Kopr-Shield. There are others.

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It appears that there are differing opinions on appropriate couplings for joining RMC underground or in wet conditions. It doesn't sound like many people feel that the common threaded coupling included with the RMC is an approved means.

That said, if using a compression coupling in lieu of the aforementioned threaded coupling, the threads need to be cut off per NEC 344.42.

So since 300.6(A) only requires corrosion protection of the threads, is the implication that the raw end of the conduit after the threads are removed DOES NOT REQUIRE PROTECTION? Or is that simply missing the point and one should protect the end cut as well?

 

[petersonra]

seems like making it all PVC would be easier.

 

[marmathsen]

seems like making it all PVC would be easier.

If only that was allow by the POCO

 

[don_resqcapt19]

It appears that there are differing opinions on appropriate couplings for joining RMC underground or in wet conditions. It doesn't sound like many people feel that the common threaded coupling included with the RMC is an approved means.

That said, if using a compression coupling in lieu of the aforementioned threaded coupling, the threads need to be cut off per NEC 344.42.

So since 300.6(A) only requires corrosion protection of the threads, is the implication that the raw end of the conduit after the threads are removed DOES NOT REQUIRE PROTECTION? Or is that simply missing the point and one should protect the end cut as well?

I have never seen anything that suggests that you can't use the coupling supplied with the conduit for an underground installation. I also have never seen a different coupling used in place of the one that comes with the conduit. The conduit and coupling are listed products and the product listing includes underground installation.

What is the concern of using the supplied coupling underground? No matter what coupling is used or what type of raceway is used, even a continuous raceway like HDPE, installed underground will eventually fill with water in most areas of the world.

 

[marmathsen]

I also have never seen a different coupling used in place of the one that comes with the conduit.

Really? You've never seen the compression fittings used? At some point you need to cut the pipe to length. I'm guessing that means you only ever see field threaded installations. I don't own a pipe threader so...there's that.

What is the concern of using the supplied coupling underground? No matter what coupling is used or what type of raceway is used, even a continuous raceway like HDPE, installed underground will eventually fill with water in most areas of the world.

Oh I know it. But regardless, NEC 314.15 states:
"In damp or wet locations, boxes, conduit bodies, outlet box hoods, and fittings shall be placed or equipped so as to prevent moisture from entering or accumulating within the box, conduit body, or fitting."

I do notice it doesn't say "prevent from entering the conduit".

 

[don_resqcapt19]

Really? You've never seen the compression fittings used? At some point you need to cut the pipe to length. I'm guessing that means you only ever see field threaded installations. I don't own a pipe threader so...there's that.


Oh I know it. But regardless, NEC 314.15 states:
"In damp or wet locations, boxes, conduit bodies, outlet box hoods, and fittings shall be placed or equipped so as to prevent moisture from entering or accumulating within the box, conduit body, or fitting."

I do notice it doesn't say "prevent from entering the conduit".

I have never used a compression fitting for rigid conduit and have never worked for contractor that did not have threading equipment. I have seen compression fittings used with rigid, but have have never seen one used in an underground run.