Yop:
310.20 IMHO is for triplex type (or field assembled) type of conductors, not for 'cable' (conductors within an outer jacket/sheath).
310.16 is where I would take this, and ambient temp may also have to be considered.
Table B 310.3 references multiconductor cables in free air.
As to MV, none of the Annex B tables refer to cable on messenger in free air. The notes refer to 'engineering data' or use of the tables. You could give the MC mfg a call/email for their input, or perhaps they already have the eng. data
