Carlon Expansion Joints User Manual

Carl on®Expansion Fittings for

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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 10-5in./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 fittings are used to
accommodate changes in length. This prevents
potentially harmful thermal stresses in the conduit
itself or in any supporting structures.

Male Terminal
Adapter End

Coupling End

The National Electrical Code Requirements

Section 352.44 of the National Electric Code® requires that
expansion fittings 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 352.44 of the NEC, in a straight run
between securely mounted items. Securely mounted items are
boxes, cabinets, elbows, or other conduit terminations.

When are expansion fittings used?

The importance of knowing when to use an expansion fitting is as
important as knowing how to use an expansion fitting. Expansion
fittings are used in aboveground installations where temperature
changes are anticipated to exceed 25°F (14°C).

Expansion fittings 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 10-5in./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.

BarrelPiston Mid-Point Indicator

When are expansion fittings used?

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(continued)

Changes in conduit temperature outdoors can exceed 140°F
considering the heat of summer and cold of winter. This means a
100 foot run of conduit could change approximately 6 inches in
length between summer and winter. If not compensated for with
expansion fittings, this change in length can cause unsightly

conduit bowing, distorted boxes, or broken couplings or fittings.
The anticipated expansion and contraction for various lengths of
runs and temperature changes is given in Table 352.44 of the
National Electric Code®. (See Table below)

Expansion Characteristics of PVC Rigid Nonmetallic Conduit

-5

Coefficient of Thermal Expansion = 3.38 x 10

Length

Change in

Temperature

Change in
Degrees F

Inches per

100 ft. of

PVC Conduit

Temperature

Change in
Degrees F

5 0.2 55 2.2 105 4.2 155 6.3

10 0.4 60 2.4 110 4.5 160 6.5

15 0.6 65 2.6 115 4.7 165 6.7

20 0.8 70 2.8 120 4.9 170 6.9

Length
Change in
Inches per

100 ft. of

PVC Conduit

Temperature

Change in
Degrees F

Length
Change in
Inches per

100 ft. of

PVC Conduit

in./in./°F

Temperature

Change in
Degrees F

Length
Change in
Inches per

100 ft. of

PVC Conduit

25 1.0 75 3.0 125 5.1 175 7.1

30 1.2 80 3.2 130 5.3 180 7.3

35 1.4 85 3.4 135 5.5 185 7.5

40 1.6 90 3.6 140 5.7 190 7.7

45 1.8 95 3.8 145 5.9 195 7.9

50 1.8 100 4.1 150 6.1 200 8.1

For conduit installed outdoors,

the maximum and minimum air temperature expected can be used to
calculate the range of expansion and contraction. Conduit installed out­doors in direct sunlight will be considerably hotter than air temperature
during the day because of the absorption of the sun’s heat.

Expansion fittings should be installed to allow for the
anticipated temperature change. For direct sunlight
exposure, add 30°F to ambient air temperature change.

Where installed inside buildings,

the conduit temperature is the ambient air temperature plus the heat
contributed by the conductors inside the conduit. Generally, expansion
fittings are not required inside unless conduit is located in an area of

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widely varying temperatures.Attics of buildings are areas
where temperature swings exceeding 100°F are common
and expansion fittings are required.