I would be going the other way myself, and preparing to run the house off the river! :) River ain't 12 cents a KWH.
Wire ampacity
See Table 310.15(B)(16) 75C column for the ampacity of wires.
- #3 aluminum doesn't exist but would be 75A if it existed
- #2 aluminum is 90A
- #1 aluminum is 100A
- #3 copper is 100A
Some people are drawn to a smaller, simpler table that calls out #2AL for 100A, but that table is wrong for this. It's for whole services. Since you have 200A service, it's telling you that you can use 4/0 for the service wire and any feeder off that service. "that's not a lot of help" lol.
4-wire feeder (unless running 120V)
You need to run 4 wires with a ground that is separate from neutral, unless you're only running 120V down there. If you learned electricity a long time ago you might've learned something else, but this was made a requirement at least 15 years ago. John Ward nicely describes what happens when you combine neutral and ground, in this video. Basically you don't have ground at all at that point. And that's worse than it sounds.
If you're thinking "local ground rods are just as good", no actually. Local ground rods deal with lightning and ESD, and they're important to protect you from voltage gradients across the ground (e.g. from lightning). But they don't help with returning fault current, which is what you need or this'll happen.
Ground wire needs to be #6 aluminum or #8 copper. Copper can be bare.
Separate neutral and ground in the subpanel
Subpanel neutral and ground must be separated. That means an accessory ground bar may be needed. They're only a few bucks and the panel labeling will recommend ones that dock right up to pre-tapped mount points.
Any neutral-ground bonding screw or strap must be removed. Follow panel instructions there.
Cannot reverse the line to backfeed the house
It might be tempting to rig up something to feed the house off the river generator. And of course then, the temptation is to try to run the 4 wires "in reverse". That is a serious code violation that could get a lineman killed, or worse, get kicked off the utility grid for backfeeding! And yeah, smart meters let them catch you.
An interlock or transfer switch is mandatory to guarantee that can't ever happen. It's not enough to promise to be clever.
The straightforward way, unfortunately is to run a redundant set of wires. If your generator is 48kW this would be 4/0 wires. It's not really practical for a generator to feed 2 panels unless the transfer switch switches neutral.
The high-end way to do that, used on Tesla Powerwalls and the like, is an automatic disconnect switch with electrical handshaking, so the generator will refuse to come up unless the disconnect has been pulled. This can be accomplished with small signal wires instead of three 4/0 wires.
The cheapest way involves what I call "permanent rewiring, temporarily". In which you permanently rewire the house so that connecting to the utility is now impossible, and then wire up the generator. Once the power outage is over, you permanently wire it back.
