Generally conductors below 100amp are rated according to the 60degree column of 310.16.
You must rate the wire to the lowest value of any component in the circuit. The degree rating of the breaker, and any devices, lugs, or equipment, have to be taken into concideration.
8 copper = 40amps for 3 or less conductors.
You have to derate if you have more than three conductors or have ambient temperatures greater than 86 degrees F.
To derate the wire use the 90 degree column if using thhn/thwn.
[This message has been edited by russ m (edited 12-19-2004).]