Make sure the terminals in the Chiller are rated for 75° C if you plan on using #8 copper wire. Most equipment of this nature are only rated for 60° C and that would call for a #6. As for the line side of the disconnect (assume it is separate from the Chiller) it probably would be rated 75° and #8 would be fine. The 80a. overcurrent device is proper in either case.
