Why do wires have a lower temperature rating?

I can see the insulation breaking (especially if flexed) if it is not rated for low temperatures and possibly the wire itself but is there anything else?

I'm also wondernig how much the temperature rating would matter if the wire will remain stationary.

I'm amused that after obtaining a BSEE I feel that I need a class in wire itself. I've read through primers on Globalspec and a few manufacturer's sites but if anyone has suggestions for an excellent educational resource please let me know.

I've been collecting samples since I find it is helpful to actually see/touch it, especially after learning about the insulation material properties.

Thanks,

David

The last time I checked UL did not have a low temperature standard. Here in Alaska we have learned that XHHW insulation is workable at low temperatures. XHHW is used almost exclusively for outside installations here including the Alaska Pipeline and the North Slope Oil facilities. THHN is the worst. If you want to learn a lot about conductors try the 1957 Neher-McGrath paper and several other references cited in the NEC handbook in Article 310.

There's a factor of 3 difference in the thermal expansion of copper and PVC. The plasticiser will become ineffective below a certain temperature and further cooling will see the PVC shrink 3 times more than the copper it covers, which I imagine would lead to cracking without any movement being required. This is however just speculation on my part as I haven't tried it.