Good question... for which I do not know the answer. I found the initiating ROP, which I quote in part below...
Originally Posted by MWayne
Substantiation: Acceptance of this new section will harmonize the ampacity
correction factors for various ambient temperatures between the NEC and the
CEC, consolidate the ampacity correction and adjustment factors into a single
section [310.15(B)], and consolidate seven ambient temperature correction
tables into two tables.
The equation proposed to be added is the same equation that is permitted in
310.60(C)(4) to calculate the ampacity correction factors for various ambient
temperatures based on the conductor temperature ratings in the tables. The term
ÄTD was omitted from the equation since it is not necessary to include it for
cables rated below 46kV as the temperature rise due to dielectric heating is
insignificant compared to the conductor losses. This equation also appears in
3.4.1 of IEEE STD 835, IEEE Standard Power Cable Ampacity Tables.
Since the NEC is used internationally, the lower and higher ambient
temperatures were added to provide the appropriate ampacity correction factors
for colder and warmer climates.
Some of the ambient temperature ranges were revised to 5ºC increments to
harmonize with the proposal submitted for the CEC, to correlate with existing
NEC Tables 310.20 and B310.3, and to provide consistency with the 30ºC
The ampacity correction factors for 85ºC (185ºF) that are included in Table
B.310.3 were not included in the revised table since, due to environmental
restrictions, paper and lead cable is no longer commonly used in applications
covered by the NEC. The ampacities remain in Table B.310.3 for reference
in existing installations and the ambient temperature correction factors can be
calculated using the equation in 310.15(B)(2).
This Proposal was generated by the NFPA/CSA NEC/CEC Ampacity
Harmonization Task Group...
I'll never get there. No matter where I go, I'm always here.