Rigid NonMetallic Conduit, commonly referred to as PVC, will expand and contract over its length as the ambient temperature around the installation fluctuates. RNC has thermal expansion characteristics three times that of aluminum and six times higher than steel.
The requirement for expansion fittings can be found in Section 352.44 of the National Electrical Code. It?s accompanying Table identifies the change in conduit length based on the maximum temperature change expected during the life of the installation. If this change in length exceeds ? in or greater, an expansion fitting is required.
A good rule of thumb is that any straight conduit run exceeding 25 feet and where the temperature change exceeds 25?F, an expansion fitting is probably needed. The temperature change is based on the maximum and minimum temperature expected to occur. Another consideration is exposure to direct sunlight. Most fittings have directions indicating an increase of 30?F to be added to the maximum temperature for proper installation.
For example, assume a 50' run of RNC in direct sunlight with a maximum expected temperature of 95?F and minimum likely temperature of 25?F:
95?F + 30?F ? 25F? = 100?F temperature change. Per Table 352.44(A), this would produce a change in conduit length of 4.06 in / 2 = 2.03 in. (The Table is based on in./100ft.)
The best installation practice is to install the expansion fitting in a horizontal position. If the fitting must be installed in a vertical position, proper arrangement must be assured to prevent the entrance of rain water. Care must also be taken to ensure the initial position of the piston within the barrel takes in account the current temperature conditions to allow the full range of expansion and contraction.
Without the use and proper installation of RNC expansion fittings, the conduit system and its supporting structure could be significantly damaged. This may expose the concealed conductors or energized parts. Be sure to read all installation instructions and follow any UL listing requirements.
