under floor wiring

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Old 07-05-02, 06:27 AM
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under floor wiring

I put some emt under the floor before I poured and used thhn,,, I probably should have used thwn correct? Whats the potential for a problem with it?
 
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Old 07-05-02, 06:36 AM
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Where is "under the floor"? Is it below the outside grade level.

Most THHN is also listed with THWN.

You only have a problem if the EMT gets flooded or condensation builds. Knowing your own installation and it's location, do you think it will get wet?
 
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Old 07-05-02, 06:49 AM
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It is inside above grade. I have a couple of runs that go across in front of big overhead doors so the floor gets cold and damp so condensate could be a problem I supose. The building humidity is low though. I dont see it getting flooded really.
 
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Old 07-05-02, 11:05 AM
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EMT is not allowed to be set in concrete or in contact with the earth unless it is coated with a protection against corrosion. Emt set in concrete without corrosion protection will rot away rather quickly.

COPIED SECTION OF NEC 2002

ARTICLE 358 Electrical Metallic Tubing: Type EMT

358.2 Definition.
Electrical Metallic Tubing (EMT). An unthreaded thinwall raceway of circular cross section designed for the physical protection and routing of conductors and cables and for use as an equipment grounding conductor when installed utilizing appropriate fittings. EMT is generally made of steel (ferrous) with protective coatings or aluminum (nonferrous).


358.12 Uses Not Permitted.

(B) Corrosion Protection. Ferrous or nonferrous EMT, 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 corrosion protection and judged suitable for the condition.
(C) Wet Locations. All supports, bolts, straps, screws, and so forth shall be of corrosion-resistant materials or protected against corrosion by corrosion-resistant materials.
FPN:See 300.6 for protection against corrosion.
According to the 2001 UL General Information for Electrical Equipment Directory (White Book), category FJMX, galvanized steel electrical metallic tubing (EMT) installed in concrete, on grade or above, generally requires no supplementary corrosion protection. Galvanized steel electrical metallic tubing in concrete slab below grade level may require supplementary corrosion protection. 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.

358.12 Uses Not Permitted.

(2) Where protected from corrosion solely by enamel
(3) In cinder concrete or cinder fill where subject to permanent moisture unless protected on all sides by a layer of noncinder concrete at least 50 mm (2 in.) thick or unless the tubing is at least 450 mm (18 in.) under the fill

300.6 Protection Against Corrosion.

Metal raceways, cable trays, cablebus, auxiliary gutters, cable armor, boxes, cable sheathing, cabinets, elbows, couplings, fittings, supports, and support hardware shall be of materials suitable for the environment in which they are to be installed.
Section 300.6 applies generally. For specific applications, see the particular article covering the appropriate cables, raceways, or enclosures.
(A) General. Ferrous raceways, cable trays, cablebus, auxiliary gutters, cable armor, boxes, cable sheathing, cabinets, metal elbows, couplings, fittings, supports, and support hardware shall be suitably protected against corrosion inside and outside (except threads at joints) by a coating of approved corrosion-resistant material such as zinc, cadmium, or enamel. Where protected from corrosion solely by enamel, they shall not be used outdoors or in wet locations as described in 300.6(C). Where boxes or cabinets have an approved system of organic coatings and are marked “Raintight,” “Rainproof,” or “Outdoor Type,” they shall be permitted outdoors. Where corrosion protection is necessary and the conduit is threaded in the field, the threads shall be coated with an approved electrically conductive, corrosion-resistant compound.
Zinc chromate paste is one type of electrically conductive compound that could be approved.
(B) In Concrete or in Direct Contact with the Earth. Ferrous or nonferrous metal raceways, cable armor, boxes, cable sheathing, cabinets, elbows, couplings, fittings, supports, and support hardware shall be permitted to be installed in concrete or in direct contact with the earth, or in areas subject to severe corrosive influences where made of material judged suitable for the condition, or where provided with corrosion protection approved for the condition.
Where ferrous or nonferrous metal conduit has corrosion protection and is judged suitable for the condition, it may be installed in concrete, in contact with the earth, or in areas exposed to severe corrosive influence. Special precautions are normally necessary for installing aluminum conduits in concrete, and specific approval by the authority having jurisdiction may be necessary.
Metal raceways installed in the earth can be coated with an asphalt compound, plastic sheath, or other equivalent protection to help prevent deterioration. Also, metallic raceways are available with a bonded PVC coating.
Galvanized rigid steel conduit and steel intermediate metal conduit do not generally require supplementary corrosion protection.

Hope this helps

Wg
 
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Old 07-05-02, 09:55 PM
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Thanks,,, I guess I was pressed for time when I did it and seem to remember thinking about it. I need to do some corrections and can remove a few sheets and run it behind the walls. I hould have asked or looked and run plastic or ridgid. It can be fixed,,, good winter project. Thanks again.
 
  #6  
Old 07-06-02, 07:12 AM
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wg,
All EMT that is currently available has corrosion protection (galvanized). It is common practice to intall EMT in concrete on or above grade without additional corrosion protection. See the quote below from the UL White Book.
...Galvanized steel electrical metallic tubing installed in concrete on grade or above generally requires no supplementary corrosion protection. Galvanized steel electrical metallic tubing in concrete slab below grade level may require supplementary corrosion protection. In general, galvanized steel electrical metallic tubing in contact with soil requires supplementary corrosion protection. Where galvanized steel electrical metallic tubing 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. ...
You can read the complete guide information at the UL website. The guide code is FJMX.
Don
 
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Old 07-06-02, 09:31 AM
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Thanks guys. It is amazing how many code faults I thought I had found,, thinking all the while I had most of them figured out. I will at least add a green wire to those subsurface circuits. I got thinking about welding benches also. I used hospital orange recepts and just bonded thru the steel to the building so a small ground conductor wouldnt carry welding current by accident. I see this isnt legal either and really the boxes on the bench should be insulated and use a ground wire. The grinders we use are double insulated but other things are 3 wire,,, so this isnt code legal I guess. I have been fixing these things one at a time as I go and should be caught up shortly. I found another subpanel that needs to be bonded today.
 
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Old 07-06-02, 11:16 AM
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resqcapt19, seems like every piece of thin wall I have seen entering or exiting concrete of earth over a few years turned into a bucket of rust. Are you saying this doesn't happen in your area?

Wg
 
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Old 07-06-02, 01:00 PM
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I've read that code section many times, and I've always wondered how I'm supposed to figure out whether a piece of conduit is protected from corrosion or not, and if so by what. I've never seen any markings on the conduit that would tell me.
 
  #10  
Old 07-06-02, 01:16 PM
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resqcapt19
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wg,
If it is directly buried or in burried concrete I have seen the same problems. If it is in a slab on or above grade I haven't seen that problem. The worst case is where a conduit passes from burried concrete to the earth, even rigid won't last for more than 2 or 3 years in that conditon. If you lay it on the earth and pour concrete over it, it won't last even if it is inside the structure.
Don
 
  #11  
Old 07-06-02, 05:00 PM
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Wgoodrich
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AS John said reading the rule concerning this subject in the NEC is almost like listening to a politician talk. John didn't say that I just applied it to what he said.

I remember years ago thinwall EMT was forbidden to be embeded in concrete or direct contact with the earth. Then products have come out like the metal clad lot cable and an EMT that is made like the metal clad lot cable and the NEC has tried to address that new product issue by the wording now in the NEC. Metal clad lot cable is a metal clad cable that has a rubber coating just like sealtite and allowed to be installed just about anywhere. Rubber coated EMT also would meet the same capabilities.

My thinking is that the wording in EMT uses permitted and protection from corrosion was addressed in an attempt to address new products with the same name yet rubber coated.

The above thoughts is why I limited my comments but mainly provided the copy of hte NEC rule that applid.

If it were me as an electrician I would install the rubber coated EMT or Metal Clad Lot Cable without concern but would install PVC and refrain from installing normal EMT or metal clad cable embedded in concrete or in contact with the earth. If I had to install normal EMT then I would demand a corrosion resistant mateeial such as tar on the normal EMT.

As an inspector I can only point out the problems that I have experienced with rotted material becuase the rule requiring corrosion resistant preparation of embedded normal EMT is as clear as mud. Nothing that an inspector can lay his hat on as to a violation of installling normal EMT embedded in concrete whether above ground or inground. Wish the would revisit that rule for a rewrite. I have to pass EMT embedded in concrete becuase of the generically written requirment of corrosion protection all while knowing their conduit will not be there coming out of that concrete in a short time but only rust will be left.

Disheartening.

Wg
 
  #12  
Old 07-06-02, 07:18 PM
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resqcapt19
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wg,
You are seeing this problem with EMT that is installed in interior slabs??? I've never seen that. Yes there is a serious problem with exterior installations. As far as using EMT in interior concrete applications, the UL listing says it is generally ok without additional corrosion protection and that is the basis for the code rule.
Don
 
  #13  
Old 07-06-02, 08:19 PM
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Wow,,, I dont feel so damm dumbafter all,,, it confuses you guys too,,hahaha
 
  #14  
Old 07-06-02, 08:52 PM
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EMT inside above grade buried in concrete will not have a crossion problem. There must be thousands of multifloor buildings in the Chicagoland area done that way with no problem. BUT put steel pipe below grade mix in a little dampness and the pipe will rust apart right were it exits the concrete.
 
  #15  
Old 07-06-02, 09:00 PM
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The areas that I am referring to are often cement decks, and sidwalk type areas in homes. The problem I am referring to is exposed to exterior conditions. I have seen the rotting problem along outside walls inside a structure also such as poured slabs on the first floor area. One more place that has been my experience is EMT that was installed years ago in the poured walls of wet basements.

I can't say I have experienced problems of rotting EMT inside elevated floors.

Wg
 
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