25 ft. 6/3 UF-B Wire

Short answer: It depends on the area. If you are going to be burying cable in "high use" areas (across a driveway, parked vehicle area, etc. it is 24 inches. That applies to direct burial and conduit. If your path will be under "only dwelling related purposes" i.e. yard space between two buildings that see no heavy vehicle traffic, then you are looking at 18 inches for either.

UF-B is not certified for UV resistance-it is certified for direct burial. Though it does have some degree of UV resistance owing to its' gray sheath. This will not work for a weather head installation. You will want to use a service entrance wire rated for overhead and tension relief. You will want to consult NFPA 70 aka The National Electrical Code and your local Authority Having Jurisdiction (Building Department) if you live in a codes governed area (not rural).

The cable diameter in mils. is 400 x 1090.

Based on 60C you would realize about 5.9% voltage drop. NEC allows for a maximum of 5% drop for feeder and branch circuits. If you could, I would consider increasing voltage to 220VAC which would give you about a 3% voltage drop across length of run.

yes, you basically always-at least in anything bought at HD-have an additional bare ground in UF or NM cable. So 6/3 is actually 4 conductors (3 insulated and 1 bare ground) within the sheath. If you are ever in doubt look on the label below the big "6/3" it should say "with ground" underneath.

#10 ground

What service you need would depend on the loads of those devices. First question to ask is how much load are you going to need. If you are set on 60amp from your main, you have some options. When you say 60amp breaker-you are either running a single pole or double pole 60amp off the main. The single pole off the main will provide 60amp&120v. A double pole will provide 60amp&120v/240v. Double pole breaker usually takes up 2 spaces in main panel. Not sure which you are adding. 6-3UF cable is for 120/240V since it has 4 total conductors (2 #6 hots, 1 #6 neutral, 1 #10 ground). If you want to run only 60a&120v on this wire you can just not use 1 conductor but it would be cheaper to buy 6-2UF cable. Now to get a little more complicated, if you run the full 6-3 to panel you may be able to split the hots depending on your panel and run 60amps per leg. That would give you more circuit options and actually 120amps of total current-60 per leg. So you could feed a 50a hot tub on one leg and the rest of the equipment on the other or something similar. For this you would get a 100a or 125a panel As for wire size, if you are installing a 60amp service to the other building, you should be fine with #6 although at that length you are pushing into #4 territory. #6 should work but to keep voltage drop <5% you technically should use #4. Going "by the book" you would use #4 over 100ft but in practical use it would probably not be an issue. If you get a permit/inspection the inspector may require you to use #4.

Yes, 6-3 has 4 total conductors. It has x3 #6 conductors and x1 #10 ground conductor.

You are going to realize about a 10.3% loss over 100 feet one way. For your charge controller to properly sense it needs no more that a 5% loss over run one way. Using 1 AWG will get you to 3.2%. You could, theoretically, place 2 conductors in parallel and that will get you to 5%. Theoretically. Actual application goes against NEC. UF-B grade wire does not need a conduit. If you could up your PV voltage to 48VDC using 6 AWG will get you right at 5%. Are you sure about 30ADC? that sounds like 95% efficiency on a PV panel...

#10 ground