PVC Expansion Joint Question

[wwhitney]

I'm curious about the language in 352.44 that requires expansion fittings only for "a straight run between securely mounted items such as boxes, cabinets, elbows, or other conduit terminations." That would imply that a run with elbows (that are not securely mounted) could be allowed to thermally expand and contract without an expansion fitting.

However, the support requirements of Table 352.30 mean that the PVC conduit will be constrained every 3-6 feet (for sizes up to 3"). So in the case of "fixed end - long straight run - elbow - short straight run - fixed end", the short run will be supported within at least 3-6 feet of the elbow. If that support doesn't allow transverse movement, the entire longitudinal expansion or contraction of the long run has to be taken up as a transverse deflection in that 3-6 feet of conduit. I would think this could lead to excess bending force on that 3-6 feet of conduit, and/or excessive angular deviation of the elbow.

So are there supports for PVC conduit that would allow both longitudinal movement of the conduit as well as some transverse movement in one direction? Maybe a piece of unistrut with a clamp that could move back and forth in the unistrut, between stops.

Cheers, Wayne

 

[FionaZuppa]

A rubber base does not allow the amount of movement that is required for PVC conduit. The movement required at any given support could be a number of inches.

a spring stand-off can sway in any direction a # of inches. soft rubber mounts can move inches. one joint of PVC wont even move that much. all depends on the mounting stuff,

 

[don_resqcapt19]

...

So are there supports for PVC conduit that would allow both longitudinal movement of the conduit as well as some transverse movement in one direction? Maybe a piece of unistrut with a clamp that could move back and forth in the unistrut, between stops.

Cheers, Wayne

The B-Line B-2417 series of straps are designed for that purpose.

 

[wwhitney]

The B-Line B-2417 series of straps are designed for that purpose.

Hmm, the catalog page for them says "designed as a guide to permit longitudinal movement of pipe." The product appears to be a U-shaped piece of metal that goes inside an (included) oversized pipe clamp, so that the pipe clamp can be tightened down and leave an annular free space around the pipe. The catalog page lists a "minimum radial clearance" of 1/16" to 5/32", depending on pipe size. So that would provide a little lateral movement.

Is it possible to use these B-2417 pipe guides with a pipe clamp that isn't full tightened, allowing the pipe clamp to move relative to the strut?

Thanks, Wayne

 

[wwhitney]

BTW, here's an example of what I have in mind:

Say you have two fixed points on a flat roof that are 100' 6" apart and need to run some 2-1/2" PVC conduit between them. Rather than running the PVC straight from point to point and using an expansion joint, the run starts off angled 5.7 degrees off the straight connecting line. It goes 50', jogs perpendicularly 10', and then continues the final 50'. [5.7 degrees is arctan (0.1). The hypotenuse of a 10' - 100' right triangle is 100.5'.]

If the roof is exposed to a 100F temperature range, then the 100 ft run will expand 4 in over that swing. Make the install at the middle of the temperature range, so the expansion or contraction will be 2 in per 100ft at either extreme. That means the 50 ft runs will range from 49' 11" to 50' 1", and the 10 ft run will range from 9' 11.8" to 10' 0.2".

The 2-1/2" PVC requires strapping every 6', so one strap can be placed in the middle of the 10' run, which would be a fixed point. That requires a strap within 1' of the 90 degree elbows on each 50' run. Those straps would have to be able to move 0.2" transversely to accommodate the length change in the 10' run. Straps on each 50' run would have to accomodate proportionally less transverse movement as the distance to the end fixed point goes down. The PVC elbows would have to accomodate 1 degree of angular deviation, so a range of 89 degrees to 91 degrees. Likewise the central strap on the 10' run would have to allow the 10' run to rotate 1 degree either way.

Is anything like the above a practical way to comply with NEC 352.44?

Cheers, Wayne

 

[don_resqcapt19]

Hmm, the catalog page for them says "designed as a guide to permit longitudinal movement of pipe." The product appears to be a U-shaped piece of metal that goes inside an (included) oversized pipe clamp, so that the pipe clamp can be tightened down and leave an annular free space around the pipe. The catalog page lists a "minimum radial clearance" of 1/16" to 5/32", depending on pipe size. So that would provide a little lateral movement.

Is it possible to use these B-2417 pipe guides with a pipe clamp that isn't full tightened, allowing the pipe clamp to move relative to the strut?

Thanks, Wayne

There is no intent or requirement to allow the clamp to move relative to the strut. The intent is to allow movement perpendicular to the strut to allow for for thermal expansion and contraction of the raceway. I specify them for rigid conduit where I have specified expansion fittings.