What is in this article?:
Understanding which NEC ampacity table to use when sizing circuit protection, and how to determine the minimum ampacity of your conductors
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I question your calculation of the ambient temp correction.
I belive you should use Table 310.15B2a based on 30 deg C coordinated with 30 deg C in Table 310.15B(16).
THHN is rated at 90 deg C, so the 90 deg C column can be used for ambient correction. At 125 deg F, the factor is 0.76 (90 C column table 310.15B2a).
25 A (90 C column table 310.15B(16)) x 0.76 = 19A.
Final result is 14AWG, but with the correct method of ambient temp correction calculation.
The NEC is a bit confusing on the issue of temperature correction. I personally can't decipher the answer just from reading 310.15. But the NEC does provide a way to find the answer.
First, here is why I used the 60 DegrC column. We're using the 60DegrF column of the table, not the 90DegrF column. So it seems logical you would also use the 60DegrF temperature adjustment factors. Even though THHN is rated at 90 DegrC, for purposes of ampacity for this circuit it's rated at 60 DegrC because its terminations are rated at 60 DegrC [110.14(C)].
That said, I see why you might use the 90 DegrC column; we are derating the conductor and not the terminations. Or are we? That approach assumes the terminations are not also in that elevated temperature space. Example D3(a) in Annex D presents such a scenario.
If you look at Example D3(a), you will find clarification on this question. The question being do we use the conductor's maximum temperature rating or do we use the temperature rating of the circuit when selecting the temperature correction factor column?
The answer is we use the temperature rating of the circuit. One place you can see this illustrated in Example D3(a) is under the heading "Ungrounded Feeder Conductors" and please note what it says in the surrounding explanations.
Also, read 310.15(A)(2) and 310.15(A)(3).
I'd like to draw your attention to the fact that the fundament for load calculations is based on Article 220. In the case of branch circuits please refer to 220.10 and in the particular case of general-use lighting circuits, please refer to Table 220.12
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