pvc expansion fittings

[kjless]

i have 4 inch pvc incased in concrete from the pole to the meter socket and also coming up thru the concrete sidewalk into my meter. i didnt think i needed exspansion fittings but the inspector told me to do the calculation to see if i need them can someone tell me what that might be. thank you

 

[don_resqcapt19]

Look at 352.44 in the 2008 code for the calculations, but also look at 300.5(J). This second section may require an "expansion" type fitting even where the calculations don't.

 

[kjless]

I saw that i guess my real ? is how is the calculation done to find if i stay under a qtr inch expansion

 

[btharmy]

I have NEVER applied the thermal expansion tables for pvc to buried conduits, probably never will. However, if someone wanted to push the issue I suppose I might have to humor them.

table 352.44 gives a (length change) per 100' of conduit based on expected temp changes given in 5deg incriments. If you expect a 50deg temp change and your conduit is 50' between "securely mounded items" (this is where my argument is established, the entire run is "securely mounded" it is not hanging loose between supports or boxes) your expansion is calculated to be =

2.03" of expansion per 100', divided by 2 because conduit run is 50', giving you 1.015" of expansion. You would need an expansion joint.

Keep in mind, buried in the earth is low 50deg in summer, maybe low 30deg in winter if above the frost line. That is only 20deg. (Check your local climate and frost line.)

If your conduit is installed below the frost line you don't need to figure for expansion because it is never going to change temp. The risers aren't long enough to figure unless you are putting the meter base on the third floor.