A run of trade size 1 rigid PVC conduit is installed exposed and horizontally across a basement ceiling. What is the maximum spacing permitted between supports for this conduit?
- 3 feet
- B5 feet
Five feet is the support spacing for trade sizes 1-1/4 through 2, one row down in the table. It is too far apart for trade size 1.
- C10 feet
Ten feet is the support interval for metal raceways such as EMT. Plastic conduit will visibly sag long before 10 feet.
- D6 feet
Six feet applies to trade sizes 2-1/2 through 3 in the PVC support table, not to trade size 1.
Why A is correct
Thermoplastic conduit has almost no beam strength and creeps under its own weight once a building warms up, so the Code ties support spacing to stiffness, which rises with trade size. Table 352.30(B) starts at 3 feet for everything through trade size 1 and widens from there. A trade size 1 run sits in that first row, so straps go on 3-foot centers no matter how the metal pipe beside it is hung.
What this question is testing
This item tests whether you can use a size-dependent support table rather than applying a single memorized number to every raceway type.
On the job
PVC droops badly in a warm building, and a sagging run looks unprofessional and traps water at every low point. On outdoor and rooftop runs the sag also fights against the expansion fittings you installed. Small PVC in particular surprises people: at trade size 3/4 you need a strap every 3 feet, which is a lot more hardware than the 10-foot spacing you use on EMT of the same size. Price the straps into the bid.
Memory technique
Small plastic pipe is floppy: 3 feet for anything an inch or under.
Exam tip
PVC support spacing is the one that changes with trade size. Small pipe, tight straps.
Where to look it up
Open the PVC conduit article and go to the support table in the .30 section; the left column is trade size and the right column is maximum spacing.