We are putting a large 2 story addition on the back of our house. I am installing 2 sub-panels; 1) a 60A panel upstairs for a few general lighting circuits and a 20A 2-pole circuit for a jet tub, and 2) a 125A panel downstairs to power general lighting, kitchen appliances (electric wall oven @ 40A but an all gas range), outside lighting, and 2 AC compressors @ 30A each. For the upstairs panel I ran #6-3 nm-b. Both panels will be fed from an existing 200A main panel with plenty of room for additional circuits. The run from the main to the upstairs panel is about 80 ft and the run for the downstairs panel is about 70 ft. I'm looking for advice/suggestions as to what to use for the supply run for the downstairs panel. From the charts it looks like I need either #1 or #1/0 depending on the conductor and insulation types. I am hoping to use a flexible conduit because the run between the panels is through a basement and a crawl space and is anything but straight and unobstructed. Any advice on the type of materials to use would be greatly appreciated. Also, if there are any red flags in my description please feel free to point them out.

I would use #2 Aluminum 4 wire SER Cable, and use a 2 pole 100 amp breaker to feed the downstairs panel.

A local code may require to sleeve the conduit as it passes through floors .

Read Article 338 of the:

https://www.askmehelpdesk.com/electrical-lighting/reading-nfpa-70-etc-national-electric-code-online-270277.html

To learn how to treat and install. See the attached manufacturers instructions for terminating aluminum wire.

At the new panel, keep the neutral insulated form the equipment grounds, use a separate accessory equipment ground bar bolted to the metal panel box using machine screws for all green and bare equipment ground wires.

Thanks, You threw a bunch of new stuff at me. Let me do some research based on your info and I will see if I need to ask for more help.