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PVC underground installation #186334 05/05/09 10:13 PM
Joined: Oct 2004
Posts: 1
sparky13090 Offline OP
New Member
Is an expansion fitting needed when a 4 inch s80 PVC is stubbed out of the ground to the bottom of a 400 amp disconnect switch (3R)?

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Re: PVC underground installation [Re: sparky13090] #186338 05/06/09 04:23 AM
Joined: Jul 2007
Posts: 1,335
sparkyinak Offline
nope. Expansion fittings are only required in exposed areas like on roofs or exterior overhead racks.

Last edited by sparkyinak; 05/06/09 04:24 AM.

"Live Awesome!" - Kevin Carosa
Re: PVC underground installation [Re: sparkyinak] #186341 05/06/09 09:33 AM
Joined: Sep 2004
Posts: 68
homer Offline
Sparkyinak is technically correct according to NEC 352.44. There is not enough thermal expansion in the 7' of PVC riser to need an expansion fitting.

However, frost heave can cause damage to PVC. As the ground freezes in areas of cold winters, the ground actually slowly rises. This can put quite a bit of pressure on a PVC riser if it is entering a CT cabinet or other fixed cabinet. PVC also gets very brittle when cold and also as it ages. So over time, pvc conduits in cold areas can be damaged by the frost.

I have been asked to install expansion fittings by inspectors in cold areas, and some inspectors interpret 352.44 to include frost heave movement, even though it is not specifically mentioned. This subject certainly can be debated either way, depending on your point of view.

Re: PVC underground installation [Re: homer] #186342 05/06/09 10:06 AM
Joined: Aug 2007
Posts: 853
leland Offline
352.40 "......shall be fastened so that movement from thermal expansion or contraction shall be permitted."

352.44 calls for expansion fitting if movement of 1/4" or more is expected."In a straight run between securely mounted items...." (2008 NEC)

My direct answer (opinion)Would be: if you anticipate a 1/4" movement or more, Yes, you need an expansion fitting.

Re: PVC underground installation [Re: leland] #186351 05/06/09 11:42 PM
Joined: Jul 2007
Posts: 1,335
sparkyinak Offline
Although the NEC does not spell it out, the NEC handbooks states that expansion fittings are not typically used in underground work. typically pipes are buried below the frost line to prevent upheaval (spelling). Below the frot line the temp is pretty much constant. I also do believe if you read the expansion fittings lititerture, it also talks about not needed in underground work

"Live Awesome!" - Kevin Carosa
Re: PVC underground installation [Re: sparkyinak] #186352 05/07/09 12:24 AM
Joined: Aug 2007
Posts: 853
leland Offline
Agreed Sparky. (never intended to imply expansions under ground)
By the way,I have never seen the conduit move and the wires not.
So when does the 'expansion Conductors' come out? smile

the system worked this long,Why the change? (rhetorical of course)

Re: PVC underground installation [Re: leland] #186359 05/07/09 01:00 PM
Joined: Jul 2004
Posts: 9,569
gfretwell Offline
Aren't concentric knockouts "expansion fittings"? wink
That is usually what I see separate if the PVC moves.

The wire usually has enough slack that it can take up any movement in the raceway.

Greg Fretwell


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