Carlon Expansion Joints for PVC Rigid Nonmetallic Conduit - Manual

Expansion
Joints for PVC
Rigid Nonmetallic
Conduit
All construction materials expand and contract
with fluctuations in temperature. These temperature
changes affect polyvinyl chloride (PVC) more than
traditional materials such as aluminum and steel. For
example, the coefficient of thermal expansion for PVC
(3.38 x 10
-5 in./in./°F) is almost three times as high as
for aluminum and six times as high as for steel.
Thermal expansion and contraction have a significant
effect on long straight runs of conduit. For this
application, expansion joints are used to accommodate changes in length. This prevents potentially
harmful thermal stresses in the conduit itself or in any supporting structures.
Expansion Fittings Required
Section 347-9 of the National Electric Code ® requires that expansion joints be installed for rigid
nonmetallic conduit to compensate for thermal expansion and contraction where the length change
is anticipated to be 0.25 in. (6.36mm) or greater, in accordance with Table 347-9 of the 1999 NEC, in
a straight run between securely mounted items. Securely mounted items are boxes, cabinets, elbows,
or other conduit terminations.
When are expansion joints used?
The importance of knowing when to use an expansion joint is as important as knowing how to use
an expansion joint. Expansion joints are used in aboveground installations where temperature
changes are anticipated to exceed 25°F (14°C).
Expansion joints are usually used where the
straight run length is greater than 25 feet and
expected temperature change is greater than
60°F. Rigid PVC conduit has a coefficient of
expansion of 3.38 x 10
-5 in./in./°F. A 10 foot
length will change in length about 3 / 8 " with a
change in temperature of 100°F. This equates
to more than 4 inches over a 100 foot run.
Barrel
Muzzle
Piston
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