When using conductor ampacity Table 310.15(B)(16), it is important to understand why some conductor types are shown in both the 75°C column and the 90°C column. Types THHW and XHHW are listed in both temperature columns. A statement on the table, just above the temperature refers the reader to Table 310.104(A).

In Table 310.104(A), type THHW conductors are shown as 75°C rated conductors when used in wet locations and 90°C rated conductors when used in dry locations.

Similarly, type XHHW conductors are 75°C rated conductors when used in wet locations and 90°C rated conductors when used in dry and damp locations.

Selecting the correct temperature column from Table 310.15(B)(16) for the above conductor types depends on the intended location of the conductor.

For complete code sections, refer to the actual NEC^® text at NFPA.ORG. Once there, click on their link to free access to the 2017 NEC^® edition of NFPA 70.