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Old 07-14-2013, 09:21 AM   #1
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Buried EMT - Hot tub installation


Hello all -

This month I decided to take on two jobs and installed a sprinkler system and run electrical wire to my back yard for a hot tub. I live in Houston, and the soil in my yard is extremely hard, dry clay.... seemingly impossible to dig.

I'm using 4 conductors of #6 THWN due to the distance from the panel (about 100').

At first, I intended to run grey PVC conduit 18" deep per the NEC but found the soil so difficult to dig, that I returned the PVC for metal conduit which can be buried 6" deep. I thought what I was buying was 'rigid conduit', but what I actually installed in the ground at a depth of 6" is UL-listed galvanized EMT. The tubing sits in the bottom of a shared trench that has a sprinkler pipe or two in it and I've already filled in the trench.

It seems now that the NEC probably intended me to use heavier 'Rigid' conduit with threaded ends (this would been a real pain compared to compression fittings and hacksaw, and I didn't know the difference at the time). Am I really going to have corrosion problems with the buried EMT?

Other than EMT / Rigid confusion, as far as I can tell everything else conforms to code.

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Old 07-14-2013, 09:43 AM   #2
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Buried EMT - Hot tub installation


Ever see these at your local rental store? Makes digging a trench WAY easier and faster than by hand.


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Old 07-14-2013, 09:57 AM   #3
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Buried EMT - Hot tub installation


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Originally Posted by hades281 View Post
Hello all -

This month I decided to take on two jobs and installed a sprinkler system and run electrical wire to my back yard for a hot tub. I live in Houston, and the soil in my yard is extremely hard, dry clay.... seemingly impossible to dig.

I'm using 4 conductors of #6 THWN due to the distance from the panel (about 100').

At first, I intended to run grey PVC conduit 18" deep per the NEC but found the soil so difficult to dig, that I returned the PVC for metal conduit which can be buried 6" deep. I thought what I was buying was 'rigid conduit', but what I actually installed in the ground at a depth of 6" is UL-listed galvanized EMT. The tubing sits in the bottom of a shared trench that has a sprinkler pipe or two in it and I've already filled in the trench.

It seems now that the NEC probably intended me to use heavier 'Rigid' conduit with threaded ends (this would been a real pain compared to compression fittings and hacksaw, and I didn't know the difference at the time). Am I really going to have corrosion problems with the buried EMT?

Other than EMT / Rigid confusion, as far as I can tell everything else conforms to code.
EMT is not RMC or IMC.... Im guessing you didnt get a permit or inspection for the job either.

Last edited by stickboy1375; 07-14-2013 at 10:06 AM.
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Old 07-14-2013, 10:06 AM   #4
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Buried EMT - Hot tub installation


Thanks for responses so far -

fj5gtx - We had a trencher but its maximum depth was 10 inches and we had real problems getting it any deeper than 8". It had a large disc with four teeth on it rather than a chainsaw-like bar and chain.

stickyboy1375 - 6" is the minimum acceptable depth for rigid. This link is for California but mirrors what I read elsewhere on the subject: http://www.ci.oceanside.ca.us/civica/filebank/blobdload.asp?BlobID=22230. The confusion is that I thought I was using rigid (and thus complying) the entire time.
Rigid metal conduit shall be buried a total of six (6) inches

The only real difference between galvanized rigid and EMT appears to be carbon steel wall thickness. Basically if the galvanized coating fails to protect the metal and rust is allowed to form, it's a matter of how long it will take for perforation to occur rather than if.
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Old 07-14-2013, 10:07 AM   #5
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Buried EMT - Hot tub installation


Quote:
Originally Posted by hades281 View Post
Thanks for responses so far -

fj5gtx - We had a trencher but its maximum depth was 10 inches and we had real problems getting it any deeper than 8". It had a large disc with four teeth on it rather than a chainsaw-like bar and chain.

stickyboy1375 - 6" is the minimum acceptable depth for rigid. This link is for California but mirrors what I read elsewhere on the subject: http://www.ci.oceanside.ca.us/civica/filebank/blobdload.asp?BlobID=22230. The confusion is that I thought I was using rigid (and thus complying) the entire time.
Rigid metal conduit shall be buried a total of six (6) inches

The only real difference between galvanized rigid and EMT appears to be carbon steel wall thickness. Basically if the galvanized coating fails to protect the metal and rust is allowed to form, it's a matter of how long it will take for perforation to occur rather than if.
I edited my post after verifying, either way, EMT is not RMC or IMC.... You could have used 2" thick concrete on top of the conduits to conform to code.
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Old 07-14-2013, 10:13 AM   #6
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Buried EMT - Hot tub installation


To confuse matters further, NEC allows EMT to contact or be buried in the ground but there are caveats - in some cases, "supplemental protection against corrosion is required."

Looking at the spec for the EMT that I used, it states: Flo-Coat coating combines zinc, a conversion coat and a clear organic top-coat for protection against corrosion and abrasion.

That's at least one additional coat beyond plain galvanizing (zinc). Can the additional treatment applied at the factory count as secondary corrosion protection the same way that a layer of anti-corrosive paint can? (A layer of this paint appears to be a cited secondary method and would satisy EMT installation below 6", the way I read it. And I can't imagine any layer of paint that I can apply could have better adhesion than something factory applied...?)

Last edited by hades281; 07-14-2013 at 10:17 AM.
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Old 07-14-2013, 10:15 AM   #7
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Buried EMT - Hot tub installation


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Originally Posted by hades281 View Post
To confuse matters further, NEC allows EMT to contact or be buried in the ground but there are caveats - in some cases, "supplemental protection against corrosion is required."

Looking at the spec for the EMT that I used, it states: Flo-Coat coating combines zinc, a conversion coat and a clear organic top-coat for protection against corrosion and abrasion.

That's at least one additional coat beyond plain galvanizing (zinc). Can the additional treatment applied at the factory count as secondary corrosion protection the same way that a layer of anti-corrosive paint can? (A layer of this paint appears to be a cited secondary method.)
You cant have EMT buried at 6", so why worry about the other issues?
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Old 07-14-2013, 10:23 AM   #8
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Buried EMT - Hot tub installation


stickyboy - I cannot find where the NEC disallows EMT for burial, or calls out a depth different than 6", with exception that "supplemental corrosion protection" may be required.

Depths greater than 6" seem to be written in paragraphs mentioning PVC, direct burial wire, etc.
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Old 07-14-2013, 10:24 AM   #9
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Buried EMT - Hot tub installation


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Originally Posted by hades281 View Post
stickyboy - I cannot find where the NEC disallows EMT for burial, or calls out a depth different than 6", with exception that "supplemental corrosion protection" may be required.

Depths greater than 6" seem to be written in paragraphs mentioning PVC, direct burial wire, etc.
Yes, you can bury EMT, but EMT is NOT RMC or IMC....
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Old 07-14-2013, 10:27 AM   #10
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Buried EMT - Hot tub installation


stickyboy - How deep, then? I understand (after installation of course) the difference between EMT and RMC. My home improvement store didn't even carry IMC.
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Old 07-14-2013, 10:31 AM   #11
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Buried EMT - Hot tub installation


Do you have a link to the EMT you did install? If it's the same EMT that they have at the big box stores I will argue that that type is not listed for direct burial.
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Old 07-14-2013, 10:35 AM   #12
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Buried EMT - Hot tub installation


Unless the EMT you buried has some sort of PVC coating or other protective method, I give your EMT 30-40 years before it is dissolved back into the earth.

"The UL WhiteBook, Category FJMX, states that in general, galvanized steel EMT in contact with soil requires supplementary corrosion protection. Where galvanized steel EMT without supplementary corrosion protection extends directly from concrete encasement to soil burial, severe corrosive effects are likely to occur on the metal in contact with the soil."

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Old 07-14-2013, 10:50 AM   #13
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Buried EMT - Hot tub installation


Link to conduit: http://www.homedepot.com/p/Allied-Tu...9#.UeKoaY21Eqw

It's definitely not PVC coated. Hopefully I'm understood here - I'm not necessarily advocating this as the best way to interpret the NEC. This is the installation I've currently got, and the questions now are:

1) How long will this last
2) Can I (or anyone) prove where this is expressly prohibited by the NEC rather than just being "unusual practice"
3) Does the manufacturer 3-coat process constitute galvanizing plus supplemental protection in the eyes of the NEC
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Old 07-14-2013, 10:51 AM   #14
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Buried EMT - Hot tub installation


stickyboy - Yes I read that and it's not encouraging. That being said, my EMT is not in any concrete encasement, and it *possibly* (?) has supplementary corrosion protection by way of the "conversion coat and clear organic top-coat."
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Old 07-14-2013, 10:53 AM   #15
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Buried EMT - Hot tub installation


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stickyboy - Yes I read that and it's not encouraging. That being said, my EMT is not in any concrete encasement, and it *possibly* (?) has supplementary corrosion protection by way of the "conversion coat and clear organic top-coat."
dude, without some type of pvc coating, i give it 30 years with direct contact of the earth.


Every piece i've ever pulled out of the earth was dissolved.

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