The insanity! It is enough to make you run screaming into the night.
This is why electricians get paid so much, and not just to pull wire.
The code is in a logical order, so start by reviewing Section 110.14(C) Temperature Limitations.
You will discover that it is the temp rating of equipment terminations, in subsection#1 and separate wire connectors, in subsection #2, that dictates, one, which type of insulation to use for a specific piece of equipment, and two, what the ampacity of wire will be with that combination of insulation and equipment or termination .
When a job is designed, first to be chosen is the equipment,ie: panelboard, circuit breakers, disconnect, meter sockets, switches, receptacles, etc. Manufacturers will refer to 110.14 to rate their terminations to be UL listed.
Section 110.14 will then dictate to designers and installers how to rate the ampacity of the wire/insulation chosen.
So, before any wire is chosen, you must check the specs of the equipment being installed.
Here is the specs for a popular range receptacle, for example: Product Datasheet -- HBL9450A
I used a 50 amp to get higher than #10 wire, as there are other limitations on #14, #12, and #10 ampacities per Sec 240.4(D).
Note the temp rating is 75 Deg C, so the wire connected can not be rated at the ampacity not exceeding the 75D C column of Table 311.16. A popular wire is THHN/THWN with a max rating of 90 Deg C, so #8 THHN at 90 Deg C is rated 55 amps, however I can only use #8 THHN at 50 amps because the device is rated 75 Deg C.
You mention a disconnect with a temp rating of 60 Deg C.. Then the same wire #8 THHN, which has a max rating of 55 amps at 90 Deg C, and I can rate it at 50 amps if I connect to the range outlet, can only be rated at 60 D C if I connect to the disconnect you have.
Look at page 56 of the following Square D brochure: http://ecatalog.squared.com/pubs/Ele...0110PL9401.pdf
This is for a popular 60 amp disconnect, note the rating is a max of 75 Deg C. see footnote under Terminal Data.
OK now let me really confuse you.
Next is the max rating of certain cable assemblies. If you are using Romex cable, AKA NM-B. See Section 334.80.
Even thou NM-B has THHN wire and is rated at 90 Deg C, Sec 334.80 limits the ampacity of NM-B to the 60 Deg C Column of Table 310.16, which, of course, is 40 Amps.
So this is why you get all different answers. Clear as mud. At least to many electricians.
Oh, the attic you mention? Now refer to the Correction Factors at the bottom of Table 310.16. If the area a wire/cable will be installed in will be a high temperature, now apply these multipliers to lower the ampacity of a wire/insulation. If the rating of a certain wire ends up lower, you may need to increase the wire size.
If, for example, the #8 THHN wire you protect with a 40 amp breaker because of the 60 De C terminals at the disconnect must run through an area that the temperature will reach 160 Deg F, ( an exaggeration), multiply 40 amps by .41 and now the amp rating of this wire is only 16.4 amps!
I hope I have been able to clear this up for you without adding more confusion.
So to finally answer your question, no,#8 THHN cannot be used on a 50 amp circuit when using a disconnect that is only 60 Deg C because of Section 110.14 (1) (a) (2).