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The essential rule is that if more than one ampacity can be determined for a given conductor, the lowest value must be used. There are exceptions, however.
For example, suppose a small portion of a conductor is operating at a high temperature because it is exposed to a higher ambient temperature than an adjacent portion or because it has more adjacent current-carrying conductors.
In that instance, some of the heat in the higher temperature portion will move by conduction into the adjacent length that is operating at a lower ambient temperature. Therefore, in some cases it is permissible to disregard limited lengths of conductors that might operate at higher temperature.
This is the case with Section 310.15(A)(2), Exception , which says: "Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less."
Heat can be transmitted or transferred in multiple ways. Typically, heat is being dissipated from a conductor by convection and radiation. Another way to get rid of heat or move heat is through conduction.
© Hughes Associates,Inc. 2012