The wire in question is that top wire. The black one with the red stripe.
It looks like feeder wire, probably stranded aluminum—though it could be copper. Judging by the scale of the breakers it looks to be AWG 6 or 4. See this chart for ampacity ratings.
If you inspect the jacket of the wire for up to 18 inches, you should be able to find some writing which gives the exact wire size (gauge number), temperature rating, and voltage rating of the insulator. Also, many cables have a distance number, usually in feet, from some origin (maybe the beginning of the 1000' spool the wire came from).