When are expansion joints used, In./in./°f, For conduit installed outdoors – Carlon Expansion Joints for PVC Rigid Nonmetallic Conduit User Manual
Page 2: Where installed inside buildings, Continued)
When are expansion joints used?
(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 joints, 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 10
of Chapter 9 of the National Electric Code
®
.
(See Table below)
Expansion Characteristics of PVC Rigid Nonmetallic Conduit
Coefficient of Thermal Expansion = 3.38 x 10
-5
in./in./°F
Length Length Length Length
Change in
Change in
Change in
Change in
Temperature
Inches per
Temperature
Inches per
Temperature
Inches per
Temperature
Inches per
Change
100 ft. of
Change in
100 ft. of
Change of
100 ft. of
Change of
100 ft. of
Degrees F
PVC Conduit
Degrees F
PVC Conduit
Degrees F
PVC Conduit
Degrees F
PVC Conduit
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
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
2.0
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 joints 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 joints are not
required inside unless conduit is located in an area of widely varying temperatures. Attics of buildings
are areas where temperature swings exceeding 100°F are common and expansion joints are required.
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