Rigid Metallic Conduit Direct Burial

[KWH]

is rigid conduit rated for direct burial as is. I had to rewire some sight lighting and install pvc/ remove existing rigid. The existing conduit was in terrible shape and you could'nt pull 10ft of wire out of a straight piece of pipe. This is a YMCA built in the 80's and if they ever have to replace wiring to parking lot lighting/ misc. there will be major problems.

 

[jumper]

Yes and no. clear as mud.

344.10 Uses Permitted.
(A) Atmospheric Conditions and Occupancies.
(1) Galvanized Steel and Stainless Steel RMC. Galvanized
steel and stainless steel RMC shall be permitted under
all atmospheric conditions and occupancies.

344.10
(B) Corrosive Environments.
(1) Galvanized Steel, Stainless Steel, and Red Brass
RMC, Elbows, Couplings, and Fittings. Galvanized steel,
stainless steel, and red brass RMC elbows, couplings, and
fittings shall be permitted to be installed in concrete, in
direct contact with the earth, or in areas subject to severe
corrosive influences where protected by condition.

 

[petersonra]

I have never quite understood why anyone would bury RMC when PVC will last almost forever and RMC is often subject to serious corrosion underground. There does not appear to be an upside to RMC underground. Classified areas I suppose.

 

[jumper]

I have never quite understood why anyone would bury RMC when PVC will last almost forever and RMC is often subject to serious corrosion underground. There does not appear to be an upside to RMC underground. Classified areas I suppose.

:thumbsup: Usually EEs spec Rob Roy/OCAL in classified areas in my limited experience. Metal+dirt=corrosion IMO.

 

[don_resqcapt19]

From the UL Guide Information for Rigid Ferrous Metal Conduit (DYIX)

[h=4]Corrosion Protection and Coatings[/h] Galvanized rigid steel conduit installed in concrete does not require supplementary corrosion protection.
Galvanized rigid steel conduit installed in contact with soil does not generally require supplementary corrosion protection.
In the absence of specific local experience, soils producing severe corrosive effects are generally characterized by low resistivity (less than 2000 ohm-centimeters).
Wherever ferrous metal conduit runs directly from concrete encasement to soil burial, severe corrosive effects are likely to occur on the metal in contact with the soil.
Conduit that is provided with a metallic or nonmetallic coating, or a combination of both, has been investigated for resistance to atmospheric corrosion. Nonmetallic outer coatings that are part of the required resistance to corrosion have been additionally investigated for resistance to the effects of sunlight.
Nonmetallic outer coatings of greater than 0.010-in. thickness are investigated with respect to flame propagation detrimental effects to any underlying corrosion protection, the fit of fittings and electrical continuity of the connection of conduit to fittings.
Conduit with nonmetallic coatings has not been investigated for use in ducts, plenums, or other environmental air spaces in accordance with the NEC.
Rigid metal conduit with or without a nonmetallic coating has not been investigated for severely corrosive conditions.

 

[John120/240]

Use PVC for underground conduits. However Rigid elbows are a smart alternative to prevent

the jet line from cutting in to your conduit. Bury the elbows at least 18" below grade & no

additional grounding is required.

 

[GerryB]

I have never quite understood why anyone would bury RMC when PVC will last almost forever and RMC is often subject to serious corrosion underground. There does not appear to be an upside to RMC underground. Classified areas I suppose.

I've used it for residential runs when I had to do the digging. Six inches is a lot easier than 18.

 

[Ponchik]

Use PVC for underground conduits. However Rigid elbows are a smart alternative to prevent

the jet line from cutting in to your conduit. Bury the elbows at least 18" below grade & no

additional grounding is required.

I agree and i also wrap the 90s with PVC tape.

 

[Glennsparky]

Will aluminum RMC have a corrosion problem when going from concrete to soil?

 

[hardworkingstiff]

The only time I've seen aluminum conduit in use was when I went behind someone that fed a submerged pump for a gas tank with it (code violation if I remember correctly) and it was completely ate up. I had to break concrete and install new conduit.

 

[petersonra]

I've used it for residential runs when I had to do the digging. Six inches is a lot easier than 18.

good point.

 

[renosteinke]

Remember your Article 110 requirements regarding suitability for the environment.

Not all soils or 'direct bury' situations are the same. For example, aluminum conduit might last forever in Chicago's dirt, but wouldn't last a month on the alkalai flats of central Nevada.

RobRoy? Please .... It's amazing just how effective 'pipe wrap' tape is at stopping RMC from corroding. The 'critical' area seems to be just a few inches below the ground, where the dirtis loose enough to allow air in. Once you get a foot or so down, corrosion is not usually an issue.

As much as I like plastic for direct burial, there is a role for steel at bends, and where the pipe exits the ground.

 

[GerryB]

Remember your Article 110 requirements regarding suitability for the environment.

Not all soils or 'direct bury' situations are the same. For example, aluminum conduit might last forever in Chicago's dirt, but wouldn't last a month on the alkalai flats of central Nevada.

RobRoy? Please .... It's amazing just how effective 'pipe wrap' tape is at stopping RMC from corroding. The 'critical' area seems to be just a few inches below the ground, where the dirtis loose enough to allow air in. Once you get a foot or so down, corrosion is not usually an issue.

As much as I like plastic for direct burial, there is a role for steel at bends, and where the pipe exits the ground.

What are the alkalai flats? Does the word alkaline come from that? Curious and to lazy to look it up. I looked it up. A lot of salt I guess.

 

[renosteinke]

Yes, the words 'alkali' and 'alkaline' are related. These are area where there were once large bodies of water, but now are only dry scrub land. Watch the Shanai Twain video "That don't impress me much" and you'll see a good example of such a place.

Also called 'salt flats,' these are cenerally flat valley bottoms and the ground is rich in various minerals, typically mineral salts, including common table salts. Add water, and the solution typically has a pH of 12 .... which is pretty good for neutralizing battery acid.

Want to have some fun? Stick a piece of aluminum into a pail of lye-based drain opener. That's how aluminum reacts to alkaline solutions. That's why you would not want to use aluminum conduit in an alkaline soil.