You can only use the lowest common temp. rating, including terminations, per 110-14(C). For instance, feeders would include OCPD temp rating at main switchboard/panel, conductor temp. rating, and temp. rating at lugs at subpanel. SWBD and panels are typically 75°C per the listing info on the panel covers, and THWN is 75°, so 75° can be used. For branch circuits, the OCPD ratings are 75° or 60°, per what's printed on the device, the conductor rating (usually 75° for THWN), and the utilization device (usually 60°); so you usually have to use 60° for branch circuits. I rarely ever see a combination of conductors and terminations that allow the use of 90°.
[This message has been edited by energy7 (edited 11-30-2004).]
The temperature rating of insulation is about heat generated by current flow as well as heat imposed upon the conductor by the surrounding ambient just as you said. Some more examples: Most luminaires require a specific insul temp. Flourescents 90deg Some incandescants with the bulb base up and wiring compartment on top - 90deg and higher Romex is required to be 90deg conductor type because of the ambient.
There are no UL Listed overcurrent devices, of any type or manufacturer, that are rated for use with wire/cable sized using the 90C ampacity column.
Needs to be qualified a little bit by saying for 600v or less. Once you get over 600v. there are terminals rated for 90 degree C and the 90 degree column in wiring tables is acceptable for termination.
[This message has been edited by George Little (edited 12-01-2004).]