A long exposed run of rigid PVC conduit is installed outdoors where the temperature swings widely between summer and winter. At what amount of calculated length change must an expansion fitting be provided in a straight run?
- A1/8 inch or more
One eighth of an inch is below the threshold stated in the Code and would require expansion fittings on runs that do not need them.
- 1/4 inch or more
- C1/2 inch or more
Waiting for a half inch of movement allows visible bowing and puts real stress on couplings and box entries before any relief is provided.
- D1 inch or more
A full inch of unrelieved movement will pull terminations loose or crack fittings long before the fitting would be installed.
Why B is correct
PVC moves several times as much as steel for the same temperature change, and a straight run fixed at both ends has nowhere to put that movement except into couplings and box entries. NEC 352.44 sets the relief threshold at a quarter inch of calculated change because that is roughly where the strain stops being absorbed harmlessly by the run and starts loading terminations. The calculation uses the full seasonal swing, not the daily one.
What this question is testing
This item tests whether you know the trigger threshold for providing thermal expansion relief in a nonmetallic raceway, a requirement driven by the material's high coefficient of expansion.
On the job
PVC moves roughly 4 inches per 100 feet for a 100 degree F swing, so exposed exterior runs of any length trip the threshold quickly. Set the piston of the expansion fitting according to the temperature at the time of installation, not centered by default, or you will run out of travel in one direction. Vertical risers on the outside of a building are the classic case, and a bowed run there is an immediate visual flag to an inspector.
Memory technique
A quarter inch of creep is where PVC needs a slip joint.
Exam tip
The trigger is 1/4 inch of calculated movement; use the expansion characteristics table to get the number for your run length and temperature swing.
Where to look it up
Go to the expansion fittings section of the PVC conduit article and use the expansion characteristics table printed with it.