I think we need to keep in mind that the problem is that the PVC WILL expand- and a lot more than anything it's likely to attach to! You simply can't fet around this fact.
Whenever you see a drooping, or wavy, PVC run, you can be sure the pipe was not purchased, or installed, that way. What you see are small bends caused by expansion, aided by the weight of the wires within.
Plumbers can use paired elbows, simply because their elbows are threaded. Pipe movement actually results in the threads tightening / loosening. With glued-up pipe, the angle of the bend will change.
Support the pipe more often, and you will get a series of small bends- rather than a few big ones. Within limits! Too much expansion, and you just might start popping anchors loose.
As for what temperature to use in your calculations: what matters is the temperature of the PVC. A run exposed to the sun will heat up well above the air temerature. You really ought to consider this in your ampacity calculations, as well. With PVC this is especially important, because the material becomes much more pliable with even a small change in temperature.
