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The primary issue affecting ampacity is the heat produced by the flow of current in the conductor.
Thus, the need to determine what size conductor is required for a certain amount of current is well understood, but the other factors mentioned in the informational note are also important.
The Informational Note explains that another principal determinant of operating temperature is ambient temperature, which is the temperature of the environment that surrounds the conductor.
Ambient temperature may vary along the conductor length as well as from time to time. So selection of a conductor requires consideration of the entire length of the conductor to determine where the ambient temperature is most likely to be highest and at what time.
A good source for average ambient temperatures at various locations is the ASHRAE Handbook.
© Hughes Associates,Inc. 2012