Table 310.15(B)(16) rules.
what is a good method to use to select wire size and type?
Start with Table 310.15(B)(16) for your minimum wire size.
Most wires use 75C thermal rating above 30A. 90C wire is limited to 75C because breakers and subpanel lugs are only rated for 75C. The only 60C wires are NM, UF and TW, and you won't use any of those in outdoor 70A+ feeder.
For 100A and assuming use of 75C wire, your minimum wire size is 1 AWG for aluminum, or 3 AWG copper if your last name is Kennecott.
And then, if the distance is over 180' (half that for 120V), then I bother to crunch the numbers on voltage drop. Find out what the appliances can tolerate, and run a few test passes through the Southwire voltage drop calc, to see if you can keep voltage drop down to 4-6% at the ACTUAL loads you will ACTUALLY be carrying (#1 mistake people make).
Once override enough stuff and "massage" Southwire's calc into using sensible values, its voltage drop calculations seem correct.
If ampacity is over 20A, I do a quick cost evaluation on aluminum wire to see if it makes sense. Mind you #2 aluminum wire (90A) is the same price as #10 copper wire (30A)... so if your goal is to run 30A to a garage it may make sense to run 90A instead, and be EV ready, for a nice bump in home value.
There is nothing wrong with aluminum wire if the terminations are approved for it, and with subpanels, they always are... heck the lugs are made of aluminum (because aluminum lugs are the universal donor: they take all wires). The crux of aluminum small wiring's historic trouble was hasty use on terminals not tested for aluminum, plus nobody was using torque screwdrivers back then, so torques were all over the map. It was a recipe for disaster, which got wrongly blamed on aluminum as the outsider.
Use of aluminum wire, even in small branch circuits, is 100% legal. It is wise in places with copper thieves.
If someone fears wires misbehaving, that is valid, but copper is no refuge: it misbehaves too. The refuge is in using metal junction boxes and if appropriate, metal conduit, so the boxes ground out and contain faults, instead of allowing the problems to fester, melting, breaching and eventually catching fire themselves (plastic boxes are made out of Petroleum for Pete's sake; adding borate only makes them resist self-ignition. They still burn when subject to flame.)
NEC Chapter 3 (the 300s) list every single cable type that is legal to use, and where you can use it.
Generally, cable type is a "push-pull" between the facts of your situation and the stocks available at your distributor. For instance a run that is both in outdoor conduit and also direct "staple to joists" crosses off a whole bunch of wires that are either not wet-rated (NM, SE-R) or not OK with stapling to joists (THHN, XHHW, MH feeder).
Example: 100A at 120feet distance:
Hold on. Do you have 100A of actual load? Because your subpanel and feeder must support 125% of that load. 125A requires 2/0 aluminum.
Or is 100A just an arbitrary number you picked out of the air, and you expect the actual loads to be far less?
This is yet another reason voltage drop calculators are wrong. People usually enter the breaker trip value, but if that were true, their breaker would need to be 125% of that!
the brand SouthWire has calculators that use the NEC tables and parameters to suggest a good size:
Keep in mind you are asking a cable salesman. This is akin to asking an Apple Store employee what kind of computer to buy.
Southwire's calculator also refuses to allow for 75C thermal rating on conductors, that's why I get the real number from Table 310.15(B)(16). A better calc would ask you about the thermal rating (and aluminum approval) of terminations on both ends, but they would only lose money if they made that change.
According to my understanding, for ampacities of 100A and more,
Remember how I showed a 90A AL run is the same price as a 30A Cu run? So basically any subpanel run benefits from aluminum, if nothing else by having more ampacity.
aluminum wires are the most reasonable choice. Thicker and more finicky to work with than copper, are however much cheaper.
"Finicky": What we actually discovered is that copper is finicky too, but we didn't see it due to confirmation bias. That's why 110.14 now requires torque screwdrivers on copper small branch circuit wiring.
And while they are thicker, they are half the weight for the same ampacity, and easier to bend since it's such a light metal.
It seems that 1/0 aluminum conductor is more than adequate for this application. This seems TOO GOOD TO BE TRUE. I was expecting a much larger wire for this power at this length. Can somebody confirm this is indeed a good enough size?
Huh? That seems super normal to me, but I do this a lot. You must be accustomed to calculations for low voltage (electronics, yard lighting) or for 120V circuits. Voltage drop by percentage is proportionately worse at lower voltages. For instance carrying 12V that same 120' will probably crash the voltage drop calc lol.
Like I say I don't even bother crunching the numbers til 180'. Code minimum size is fine below that. (1 AWG Al).
I this particular case, if the 1/0 size is confirmed, it's probably not worth doing so, but I am open to suggestion.
Use 1 AWG hots. Or use 2 AWG hots and downsize the feed breaker to 90A. 2 AWG is much more of a commodity, and for most people, "100A" is an arbitrary number anyway.
2 AWG is a commodity because 310.15(B)(7) gives a favorable derate to an entire service to a dwelling. It fits 100A service. Doesn't apply to you since your service is 200A.
I'm not a fan of downsizing the neutral in residential context. It doesn't save much money and gives the inspector something to red-flag.
Also, the neutral wire does help - remember when I said voltage drop is twice as bad for 120V? A full size neutral wire helps with the worst-case scenario when the only system load is ONE large 120V tool. (e.g. someone is trying to run a very long e.g. 400' subpanel run, and insists on #10 copper, disliking the price of #6 and not knowing about #2Al).
As far as the ground conductor, I have found another calculator... It seems that for a 100A panel, 8 AWG Copper or 6 AWG Aluminum should be enough. This is also very suspicious as I'd expect to be dependent on the run length.
THAT only applies if you use the minimum legal wire. You are always allowed to use larger wires than required, but if you up-size the hot wires, you MUST also up-size the ground in proportion. NEC 250.122(B).
If you take a voluntary bump from #1 to #1/0 you MUST bump ground from #6Al to #5Al (which means #4Al). Mandatory!