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#43460 - 10/14/04 01:23 PM Expansion Joints  
drillman  Offline
Member
Joined: Mar 2003
Posts: 97
Somewhere in Texas
I just went thru the code book to find out where expansion joints are required on steel conduit. My conclusion is that they are not.

Is that correct?

Now if it is correct, I am still going to need to buy some. On my facility there is a 1,000 ft plus conduit run of 2 inch IMC exposed outdoors. Theres J boxes about every 400 feet. At some j boxes the conduit has shrunk and pulled out by 3-4 inches.

Doing the math in the code book for expansion of steel conduit I get 3/4 of an inch for 100 feet for 100 degree F change. So a 400 foot run will be 3 inches. Enough to cause problems.

Right now I am thinking of ordering enough of those things to install at each jbox.

Good or bad idea?

Thanks.


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#43461 - 10/14/04 01:34 PM Re: Expansion Joints  
Geno  Offline
Member
Joined: Aug 2001
Posts: 16
Syracuse, NY USA
300.7 (B)


#43462 - 10/14/04 02:26 PM Re: Expansion Joints  
earlydean  Offline
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Joined: Dec 2003
Posts: 751
Griswold, CT, USA
You would only need expansion joints where the conduit is likely to buckle or separate due to expansion or contraction from temperature changes, where frost heave is a problem or where the building is "jointed" for expansion. You can avoid some expansion joints by installing the runs of conduit with a 90 degree bend or two at appropriate points. The expansion or contraction due to temperature change will be absorbed by the flexion at the 90 degree bend.
Pipe fitters use this solution on long runs of pipe with 4-90 degree bends every 200 feet or so of pipe. Electricians cannot do this without pull boxes or "C" condulets.
It only takes one 90 degree bend to allow for the expansion. The pipe fitters use four because they need to get the pipe back to the origional direction.
In your situation, I would suggest you 90 down from your run to each "J" box, then 90 up back onto the run. You need to install the "J" boxes below the run of conduit because it is easier to pull the wire that way. As the pipe expands or contracts with the temperature, the 90s will flex and absorb the expansion and contraction. A 12 inch nipple between the boxes and the 90s would make the flexion less noticable, too.
Otherwise, you are correct, you need expansion fittings at each box.


Earl

#43463 - 10/14/04 07:21 PM Re: Expansion Joints  
drillman  Offline
Member
Joined: Mar 2003
Posts: 97
Somewhere in Texas
Geno: Yes I saw that. One of the parts of the code that leaves up to the installer what to do. It appears that the installer was not thinking to hard here.


earlydean: that is a good idea I did not think of. I sort of forgot one detail in my post, this conduit is run on a chain link fence which probably expands and contracts also. Right now we are looking to feed the equipment from a closer building and scrapping this mess.


#43464 - 10/15/04 03:30 AM Re: Expansion Joints  
e57  Offline
Member
Joined: May 2003
Posts: 2,876
S.F.,CA USA
At each box and some mid-point at couplings too. 400 feet between boxes with 3 inches per hundred, just at the boxes would not be enough. Anyway, you're gonna have to re-pull, and modify the whole run, why not start from scratch? As not only is the run on a chain link fence an eye sore, but if it doesn't have a ground wire in the run your fence is now the ground conductor, even then, still not hip. A closer building and underground sounds like a good perminant installation.


Mark Heller
"Well - I oughta....." -Jackie Gleason


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