PVC Experience

PVC has one flaw that must always be considered when using it for electrical conduit runs.  PVC has a very noticeable expansion rate when exposed to sunlight and heat.  Of course, when the ambient temperatures cool down, the PVC will shrink back.  However if the conduit is hung on the side of a wall when exposed to the sun, when it expands the conduit will droop down.  When it cools off the droop does not return to its original shape.  As long as PVC is installed in the shade or buried in earth or concrete it will not expand enough to be a problem.

One time, when I first began using PVC conduit, I installed a run on the side of a building, under an eave.  Where it remained in the shade, it would hold fine.  Where it was exposed to the afternoon sun it warped something terrible!  Another concern with the expanding PVC is that the wiring does not expand.  (at least not at the same rate.)  This can cause the longer conduit run to pull the wiring out of its connections.

About that time I discovered expansion joints.  Expansion joints are fittings you fasten at each end of the conduit with a sleeve in the middle.  The sleeve permits the conduit to expand and contract without harming the enclosed wiring.

On one occasion I was laying a long run of PVC conduit to be covered with cement.  The sun came out and bowed the conduit right up into the air above the limits of the forms.  I had to re-do the run and make sure the cement was poured before the sun became too hot.  An expansion joint would have worked very well there.

Recently there have been other advances in “plastic” conduit.  It is more expensive but it is also very useful.

Another aspect of PVC work I have learned the hard way is regarding the gluing of the PVC parts.  When gluing the PVC together, do not let the glue get inside the conduit itself.  That will produce two problems.

It can cause what you put into the conduit to become fastened to the inside wall of the conduit.  Things that can become glued in that manner are wire, fish tapes, etc.  That is annoying to say the least.  The second problem is that a buildup of glue on the inside of the conduit will reduce the available inside diameter.  Not only will that increase the resistance when pulling wire through the conduit, the reduced area can produce a wiring hot spot.  Wire-fill in conduit is predicated on enough air movement to reduce heat buildup.

Some things you have to “Learn by Doing” because they are not generally taught in classes, or the electricians that already have the knowledge forget to pass it on.

We still use steel conduit on large commercial and industrial jobs, but PVC has almost taken over most conduit work.

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