That it does not exist. Article 310 jumps from 310.15 to 315.60. In between
there, you find the Table 310.15x tables.
Actually, you can use that column. But only if every component (including
connectors) is rated for a 90 degree application [110.14(C)]. Connectors
typically don't come rated for 90 degree usage. So to get a connector of
adequate ampacity, you must use the larger conductors in the 60 DegrC column. If
you use a connector that fits the conductor in the 90 DegrC column, the
connector will be too small to handle the expected current.
But what if a portion of the cable runs through a space thatís at 85 degrees
because steam pipes run in that space? You have no splices or connectors in that
portion, so there's no reason you can't use the 90 column for the conductor. To
understand this better, read Annex D, Example D3(a).
[Table 310.15(B)(16)]. No. The limit for this table is three
conductors. See Annex B for information on how to solve this problem.
Each Table has its particular scope defined, and you can read that in the header
space above each table. Regarding these two tables,
Table 310.15(B)(16) is for three conductors and Table 310.15(B)(17) is for a
single conductor. Be sure to always read the header carefully before selecting a
particular table. It's also a good idea to mark the page (e.g., with a stick on
note or clip-on bookmark) so you don't inadvertently use the wrong table.
I assume you're looking at
Table 310.15(B)(16). Your first mistake is you are using the 90
degree column, just because THHN is listed there. You have to use the 60 degree
column, unless you aren't going to have any connectors in the run where these
conductors are going.
This means the table goes up to only 555A. You could do the calculations
described in Annex B to determine just how ungainly and huge a conductor you
would need. But, it's probably not available and you probably don't want to hear
the electricians complain about how bad the cable pull was. Not that you'd have
the connector crimpers and dies readily available, either....
So what you do is run parallel conductors. You can run two 750 MCMs in parallel
for each phase and solve this problem nicely. This size conductor is big, but
run often enough that you won't need exotic equipment to install it. Nor will
you need special panel terminations and other "gotchas" that an "off table"
installation would entail.