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Section 110.14(C) says that the current rating that is assumed for a given conductor size will be based on Table 310.15(B)(16). So for a 60° terminal, the load would be limited to the value found for a given conductor size in the 60° column.
This is really a limitation on the load, not on the ampacity of the wire. The table just says that regardless of the wire's ampacity (even if it is higher than the ampacity given for the conductor in the 60° column), the wire cannot carry any more current than would cause it to operate at 60° C at the terminal.
Determining a minimum conductor size is the goal, but in this case based only on the load to be imposed on the conductor at the terminal. The ampacity from the 75° or 90° C column can be used if there are larger overcurrent devices, larger wires, or specific equipment terminal markings that override the assumptions. However, both ends of the conductor must be considered, and the lowest terminal temperature rating will determine this minimum size.
© Hughes Associates,Inc. 2012