Ecnerwal wrote the posting I would've written! Consider that an endorsement.
Don't rush off and buy new wire. I hate when people do that: Harper's Rule: buy the wire last. Normally, that's because you learn more stuff talking to people, but in this case it is to learn more stuff as you dismantle the installation.
Learning part A is to disassemble those gobbed up boogers of wire splices, to see what's actually under all that tape. Fair chance that's the problem: the tape makes them functionally uninspectable. Once exposed, you could try carefully insulating each exposed wire, then throw the power back on and see if hot is making it here. Wouldn't that be nice! Change those out to modern Polaris connectors. Split-bolt and gobs of tape was the state-of-the-art back then, but Polaris connectors are now readily available and they're inspectable!
If you're replacing all the wires, get enough wire to bypass (run through) this box. Avoid unnecessary splices. You'll know why when you price Polaris connectors.
Learning part B is you need to actually pull the wires out of the conduit to even know what the score is. That will tell us several things.
First: you want to do a careful physical inspection of the old wires. #1 if they're tip top, you can reuse them! #2 the wire condition is a Rosetta Stone for conduit conditions. If you find one particular zone where 2 wires have caked-on scale, that tells you exactly where the pipe is rusting out. Segue to:
Second: Is the conduit actually in good physical shape? The proof of the pudding is to pull the old wires out. If they can pull reasonably, that's a good indication the conduit is OK and you'll be able to pull new wire in. If not, it doesn't matter which new wires you buy, because you will not be able to pull them in without destroying them. Problems in the pipe will be revealed by either the wire being immovable, or the wire revealing damage either from the collapse or from being dragged past the collapse.
The cure is to, unfortunately, dig up the yard and replace pipes. Since FMC and thin-wall EMT is illegal for direct burial, most likely this is IMC or RMC which are entitled to being buried at 6" of cover. That means they may not be so bad to dig up - at that depth there's no need to call 1-800-DIG-SAFE or whatever your area calls it, and you can excavate with hand tools (a shovel hit will not faze RMC, but obviously a digger hit will destroy it.)
If you want an honest 100A, you need #1 aluminum or #3 copper. If 90A is plenty, #2 aluminum is popular, meaning it's at a pricing and availability "sweet spot" - cheaper and available in more types. Keep an open mind while shopping.
Because of the way aluminum wire is priced, I wouldn't go any lower than 90A. The next size down is only 65A and barely cheaper.
90A is plenty for dual EV charging at 60A per car, due to "Share2" sharing technology, which allows the EVSE's to coordinate to keep their total ampacity down to a target figure such as 80A.
The Rule of Six
Your mini-split installation violated Code by adding a 7th breaker. Code requires that a subpanel in an outbuilding have either:
- 6 or fewer hand throws to shut off all breakers, and all must be right next to each other. ("Rule of Six"). ...OR...
- A main disconnect.
Once you have both poles working, you can position 120V breakers right next to each other and install an approved "Handle-Tie" between them. This makes them count as 1 hand throw. (it might also cause them to trip together, so choose wisely - not "table saw" and "lights" for instance.)
Otherwise you can retrofit a main disconnect. You do this by installing a main breaker of appropriate size, and a hold-down kit for said main breaker for that panel, and then connecting the supply wires into the now-main breaker instead of the main lugs. This will use up 2 of your 12 spaces.
Alien breakers must go!
This is a GE panel. I recognize the unique cruciform bus stabs (that support their "thin" breakers), but also the model number of the box.
Yet most of your breakers are a dog's breakfast of random other manufacturers. "Why not? They fit." Because the bus stabs are different shapes, and they don't make proper contact, and will arc and burn up a bus stab if not the garage. Thus, the only breakers permittable in this panel are:
- General Electric THQL (full width)
- General Electric THQP (half width, not handle-tie-able)
- Eaton CL (which are UL-Classfied for this panel, and feature a distinctive circular UL-Classified logo).
The Eaton CL is hard to find. I suspect those Eaton breakers are Eaton Type BR/Type C, because many installers have not gotten the above memo (or deny inconvenient truths).
The GE half-width breakers will allow you to put more circuits in this panel; however they are not available in AFCI, GFCI or smart breakers.
Wait. What??? Smart breakers??? Yeah. They're in the pipeline, be here Real Soon Now. They will let you do things like let you power things that otherwise wouldn't fit on the service, e.g. interrupt the water heater while both A/C's are cycled on, or go after advantageous rate plans from the power company, or have an EVSE adjust EV charge rate on the fly to keep the house within limits.
If the old RMC or IMC proves out, there's no need for a ground wire for that section. The conduit covers the need. If you do need or want a ground wire, 100A requires #8 copper or #6 aluminum.