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#97527 - 02/26/06 01:15 PM pvc
watersparkfalls Offline
Member
Registered: 03/08/05
Posts: 210
Loc: Washington...Not DC
if you run pvc on the outside of a building there is a calculation for expansion(never calculated it myself always used emt or ridgid) but how about under a building specifically a crawl, here it isnt exposed to uv light and certainly not as hot in the summer or cold in the winter. but should one consider the expansion of a run under a crawl? what about j boxes do they affect the calcs?

thanks,
h20
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#97528 - 02/26/06 02:56 PM Re: pvc
renosteinke Offline
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Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
You need to look at the problem you're trying to solve, to find your answers.

Materials change size as their temperature changes. Where this change is reflected depends, in part, on geometry.

I would venture a guess that buried pipe isn't going to change temperature very much- so there won't be much in the way of size change either.

Junction boxes in a straight run of pipe won't help to give the pipe anyplace to go- sure, there can be some flexing of the box walls, but you'll still see the pipe flex about in response to temperature swings.
Where there is a radical change in the direction of the run, a lot of the movement will be absorbed. A "saddle" will 'bulge' in response to expansion; an elbow might (depending upon direction) have a lot of torque applied to it, resulting in a broken joint.

When you install the expansion fittings, be aware of the conditions at that time. On a chilly morning, you might want to install the fitting in its' extended position, while on a hot summer day, you might consider the compressed position.
In other words, pipe that's been sitting in the sun is as long as it's going to get, so you have to allow for contraction.
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#97529 - 02/27/06 09:03 AM Re: pvc
Tom Offline
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Registered: 01/01/01
Posts: 1069
Loc: Shinnston, WV USA
I would base the calculation for expansion on the worst case, which would be the highest & lowest outside air temperatures in your area. The only thing I'd skip is the addition of 20 degrees for conduit in direct sunlight. This is a conservative approach, but unless you're planning a very long run, one expansion fitting should do the trick.

In the area I live in, for conduit exposed to the sun, I come up with a calculation of about 7 feet before an expansion fitting is needed.

Expansion & contraction is a real problem, I've seen plenty of installations with busted couplings or the conduit pulled apart exposing the conductors.
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#97530 - 02/27/06 12:47 PM Re: pvc
earlydean Offline
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Registered: 12/22/03
Posts: 749
Loc: Griswold, CT, USA
If you use PVC straps (designed to allow movement and required by 352.30) and run the pipe so that it has at least two 90 degree bends at the ends (go up, go long, go down), then the PVC will expand and contract along the long run, spreading the 90s. No expansion fitting is necessary. Article 352.44 allows this in the last part of the sentence: "...in a straight run between securely mounted items...".
I would leave the risers at least 2 feet long, but remember the first strap must be at no more than 3 feet from the box or LB. I learned this from watching the pipefitters make a long run of stainless welded pipe. They would make a series of turns every 200 feet or so to compensate for the expansion and contraction due to seasons, sun and weather.
Table 352.44(A) shows the expansion of PVC conduit.
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#97531 - 02/27/06 02:56 PM Re: pvc
PCBelarge Offline
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Registered: 06/08/03
Posts: 657
Loc: Dobbs Ferry, NY, USA
352.44
...in a straight run between securely mounted items such as boxes, cabinets, elbows, or other conduit terminations.
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#97532 - 02/28/06 12:42 PM Re: pvc
earlydean Offline
Member
Registered: 12/22/03
Posts: 749
Loc: Griswold, CT, USA
That's why you leave the sweep free, so it is not securely mounted. The first strap is on the other side of the elbow's coupling.
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#97533 - 02/28/06 02:24 PM Re: pvc
HotLine1 Offline


Member
Registered: 04/03/02
Posts: 6833
Loc: Brick, NJ USA
Guys:
Do a Google for 'Carlon', the PVC mfg. They have info on expansion joints that you can print.

John
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