I will give it a try it is pretty straight forward.
First here is the rule, notice it says "securely mounted items" so if the weather head is not attached to the building an expansion joint is unnecessary.
352.44 Expansion Fittings.
Expansion fittings for RNC shall be provided to compensate for thermal expansion and contraction where the length change, in accordance with Table 352.44(A) or (B), is expected to be 6 mm (1/4 in.) or greater in a straight run between securely mounted items such as boxes, cabinets, elbows, or other conduit terminations.
Table 352.44(A) tells us the expansion characteristics of PVC.
I will figure here in New England we could see temperatures of -10F to 100F in between summer and winter.
The table tells us PVC will change 4.46" per 100' of length for a 110F temp change.
NorthernLights said 15' of length, 15' is .15 of 100'
4.46" x .15 = .669" of change
That is close to 3 times the code allowed .25" of change.
For 15' of length the largest temp change allowed without an expansion fitting needed would be only about 40-45F.
[This message has been edited by iwire (edited 11-19-2003).]