PVC Dead ending out of the ground

6

69gp

Senior Member
Location
MA
We have a project in the northeast where PVC conduits are rising out of the ground, secured to unistrut and then open conductors into a cable tray via a weatherhead. Contractor used regular strut clamps to secure the PVC. I said that they either needed to put an expansion fitting in before the first strap or use a clamp that will allow movement. This would be an equal to a B-Line B2417-4ZN https://www.eaton.com/us/en-us/skuPage.B2417-4ZN.html The engineer came back and said that an expansion fitting is not required because the conduit does not go to a fixed point. I disagread with that and said that the unistrut clamps were a fixed point. The engineer approved a methood that I still dont agre with. That is adding a nut on the inside of the strut clamp and tightening up the clamp so that it does not lock tight to the conduit. The probleum now is the strut clamp can slide side to side and will over time fail and just seems like a poor fix. I also don't know if this is an approved methood from the manufacturer. I was only able to find installation instructions for Caddy strut clamps and they state that you tighten the bolt and screw. There was no mention of using a double nut. just for reffrence Caddy clamps were not used. Looking for opinions.
 

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I agree that no expansion fitting is required because with for one thing that short length of exposed conduit the thermal expansion will not exceed 1/4". I like the double nut idea and they could add some hardware to keep the strap from moving laterally. You might be able to argue about earth movement but if the strap allows the conduit to move that isn't an issue.


352.44 Expansion Fittings.
Expansion fittings for PVC conduit shall be provided to compensate for thermal expansion and contraction where the length change, in accordance with Table 352.44, is expected to be 6 mm (1∕ 4 in.) or greater in a straight run between securely mounted items such as boxes, cabinets, elbows, or other conduit terminations.

300.5(J) Earth Movement.
Where direct-buried conductors, raceways, or cables are subject to movement by settlement or frost, direct-buried conductors, raceways, or cables shall be arranged so as to prevent damage to the enclosed conductors or to equipment connected to the raceways.
Informational Note: This section recognizes “S” loops in underground direct burial cables and conductors to raceway transitions, expansion fittings in raceway risers to fixed equipment, and, generally, the provision of flexible connections to equipment subject to settlement or frost heaves.
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infinity said:
I agree that no expansion fitting is required because with for one thing that short length of exposed conduit the thermal expansion will not exceed 1/4". I like the double nut idea and they could add some hardware to keep the strap from moving laterally. You might be able to argue about earth movement but if the strap allows the conduit to move that isn't an issue.
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Its not the expansion in the conduit from thermal I am concerend about. It is the frost heave and thaw cycle. AS for the the strap they make one that is designed to allow the conduit to move. What is there now is loose to meet it is not properly installed. I would have no issuse if they used the correct clamp similar to what I included in my orginal post.
 
69gp said:
Its not the expansion in the conduit from thermal I am concerend about. It is the frost heave and thaw cycle. AS for the the strap they make one that is designed to allow the conduit to move. What is there now is loose to meet it is not properly installed. I would have no issuse if they used the correct clamp similar to what I included in my orginal post.
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So all they need to do is to change the strut strap?
 
That is and always will be a problem with any type of plastic pipe. I agree that an expansion joint is probably not required for a short run but installing one would solve the issue.

Instead of an expansion joint can't you use one of the slip over meter rise things? Forgot what they are called.
 
I saw this innovative technique:

IMG_8152.jpg

But in all seriousness, the strap with the extra nut is a bit hack, especially because there's a product that solves the problem. That said, the engineer blessed it and it's his stamp on the line if something happens. It'll probably outlive us all.



SceneryDriver
 
69gp said:
Yes that is all they need to do in my opinion.
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The proper straps that allow movement are required by the NEC. There is no reason why they cannot use the correct hardware. If a product didn't exist then a more creative solution may be needed like the extra nut.
 
Is that strut tied to anything besides the conduits and the cable tray? If not, if the ground heaves, won't it all just move up and down together as a giant assembly if the strut clamps are tight? Being a cable tray and cables dropping into a tray, I don't see much to damage. If it was all a rigid structure or conduits to fixed boxes, I would be more afraid of earth movement. But I live in an area where it doesn't freeze much and don't see much in the way of long term effects from frost heave.
 
suemarkp said:
Is that strut tied to anything besides the conduits and the cable tray? If not, if the ground heaves, won't it all just move up and down together as a giant assembly if the strut clamps are tight? Being a cable tray and cables dropping into a tray, I don't see much to damage. If it was all a rigid structure or conduits to fixed boxes, I would be more afraid of earth movement. But I live in an area where it doesn't freeze much and don't see much in the way of long term effects from frost heave.
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There are 2-4" rigid conduits in the ground for support of the unistrut that supports the PVC
 
Another idea, for next time, is to put a sleeve, the next size up, over the conduit, and strap to the sleeve. The inside piece can float freely but be thoroughly supported. Also, you can make your own slip riser for substantially less money with the next size up PVC.

I am a firm believer in the importance of expansion couplings (or some method that allows play) here in the Northeast. Just yesterday I noticed an expansion coupling at my house that has 1/2" of unpainted PVC showing.
 
electrofelon said:
Another idea, for next time, is to put a sleeve, the next size up, over the conduit, and strap to the sleeve. The inside piece can float freely but be thoroughly supported. Also, you can make your own slip riser for substantially less money with the next size up PVC.

I am a firm believer in the importance of expansion couplings (or some method that allows play) here in the Northeast. Just yesterday I noticed an expansion coupling at my house that has 1/2" of unpainted PVC showing.
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We have done that on some of our solar sites. Another one that I like for solar feilds is going one size up on the PVC and make a 1' sleeve of rigid conduit to sit on the ground to protect the PVC from mower and week whackers
 
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