The two tables below provide examples of how the NEC addresses the four principle determinants of operating temperature discussed previously (heat produced by current flow, ambient temperature, ambient medium and adjacent current carrying conductors, not in any particular order).

Table 310.15(B)(16) is used for conductors that are inside raceway or cable or that are directly buried. Table 310.15(B)(17) is used for conductors that are essentially hanging in free air and separated from other conductors, which means that heat is more readily dissipated than under the conditions of Table 310.15(B)(16).

Each table specifies the amount of current that a conductor can carry without being overheated based on a certain temperature rating of conductor insulation, whether it be 60° C, 75° C or 90° C.

The ampacity value that comes from the table is the amount of current that the conductor can carry under the conditions specified in that table without overheating. This accounts for the heat generated internally as the result of load current flow.

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