[RE-wrenches] Thermal issues with PVC (was RE: raintite boxmounting angle)

Hans Frederickson hans at fredelectric.com
Mon Aug 24 15:43:54 PDT 2009


Slipped pinky finger sent my post a bit early. Sorry...

Here's the Carlon document for PVC expansion fittings:
http://www.carlon.com/Installation_Training/IT-ISEXPJT.pdf

My feeling is that in general, how thermal expansion/contraction issues are
handled is one of the most important factors in the longevity of solar PV
installations. Not just for conduit, but for racking and module construction
as well. If I recall correctly, an expansion coupling (or lack thereof) was
the cause of the Staples fire.

-Hans 

-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Hans
Frederickson
Sent: Monday, August 24, 2009 3:33 PM
To: 'RE-wrenches'
Subject: [RE-wrenches] Thermal issues with PVC (was RE: raintite boxmounting
angle)

Picking up on Max's comment about PVC conduit coming apart in the heat...
PVC is acceptable for use above ground, but is often not a good choice in
exposed outdoor areas due to thermal issues:

First, PVC conduit is not permitted to be used where ambient temperatures
exceed 50 degrees C (122 degrees F) per NEC 352.12(D). This limitation rules
out using PVC on most roofs, although you will occasionally see it used on
"cool" roofs in our neck of the woods (pacific NW). It follows that PVC
junction boxes would not typically be a good idea on a hot roof unless they
are specifically listed for higher temperatures.

Second, PVC of any significant length installed outdoors generally will
require an expansion fitting. See NEC 352.44 for details. The rule of thumb
we use around here is any piece of PVC conduit longer than 10' installed on
an exterior wall needs an expansion fitting. If you find yourself needing to
install an expansion fitting, this document from Carlon is very helpful:






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