Use the savings on aluminum to buy stuff that matters.
Copper wire is much stiffer than aluminum because it is a much denser metal (3.5x density). As such, aluminum wire of a given ampacity is easier to work with than copper. Don't use MH feeder, use aluminum individual wires. Available same as copper, as THHN, XHHW or USE/RHW.
Aluminum heavy feeder was always reliable, even the brittle old AA-1350 alloy, because it is put on terminals properly rated for aluminum (made of aluminum actually) and the lug is torqued to spec with a torque wrench. Those rules were not followed in the 1960s when they tried using it on small branch circuit wiring, hence the bad reputation that does not apply to heavy feeders.
The torque thing was actually discovered due to failing copper connections. But it settled the remaining questions about why aluminum small branch circuits failed, because nobody was using torque screwdrivers back then.
Copper does not buy you any safety margin, it's just waste.
Panel ampacity does not need to match feeder.
My car has 85 mph tires. The sheriff sternly explained to me that's a safety absolute limit, not a recommendation!
Likewise, subpanel rating is a safety limit - you shouldn't precisely match it to feeder amps, because that would unfairly limit your selection of panels. That GE there seems like a weird choice, like someone was trying to get the maximum number of breaker spaces in a 100A panel. Forget that.
Pick any panel >= 100A that has the number of spaces you want. If it's a 200A panel that's fine.
"But I have a secret plan, I want to get main and supply breaker the same size so the nearby breaker trips first!" Forget that -- neither breaker is ever going to trip. ...... OK fine then, use a main lug panel of the same make, and backfeed its "main breaker". That way the 2 breakers are identical. If the wrong one always trips, swap them LOL. But it'll never happen. Really.
Note that the "main breaker" is only there to be a disconnect switch, which is only required because it is an outbuilding. The amps don't matter. A 200A breaker is fine here. The disconnect does not need to be outside.
Grounding and bonding
A subpanel in an outbuilding needs both ground wires and ground rods.
The feeder ground wire must be:
- For 61-100A, #8 copper or #6 insulated aluminum.
- For 101-200A, #6 copper or #4 insulated aluminum.
- or IMC or RMC conduit, which provides the ground until it rusts out.
- Larger is always allowed.
The ideal ground rod is an UFER ground cast into the concrete at pour time. But of course you didn't think to ask. Absent that, you can use 1 ground rod if it passes a very, very costly impedance test.... or 2 ground rods and you can skip the test. If you want the ground rods to actually do their job, the farther apart the better, ideally catty-corners of the building. NEC slumlord bare minimum is 6' apart.
Neturals and grounds are separate everywhere except the Neutral-Ground Equipotential Bond in the main panel. In the main panel, neutral lands on the neutral bar, and ground lands on the ground bar. Since the N-G bond is in the main panel, it is not a code violation to stick grounds on the neutral bar, but I consider it inelegant and it wastes neutral bar spaces.
Heavy-up the electrical service?
The other big question we are discussing in the house right now - should we take this moment to hire an electrician to upgrade our main panel to 200a prior to connecting the sub panel? This is an older 1930s home that we bought last year and clearly this panel is pretty full
"How can I be overdrawn? I still have checks in my checkbook!"
See how one doesn't have anything to do with the other? Same with "breaker spaces" vs "load in the panel".
Running out of breaker spaces is always bad. Every house would be better off with more spaces in the panel. A breaker does not add one iota of load to your panel.
What adds loads is Loads, and those are determined by a NEC Article 220 Load Calculation. Generally they take 3 VA per square foot of finished house space, plus 1500 VA for each kitchen and laundry room receptacle circuit, plus nameplate loads for dryer and water heater, a wacky formula for range, then it gets some fudge factors to allow for load diversity. It's not what you'd expect but it's the required method.
Subpanels don't count in a Load Calculation, the loads do. Though each subpanel needs its own Load Calculation to assure the feeder and supply breaker are appropriate for its loads.
Do you need a service upgrade? The Load Calc will tell - you might need one already - people are supposed to re-do the Load Calc when adding loads to a house, but they never do LOL.
But Real Soon Now we're going to get some advanced load shedding technologies that will make service upgrades unnecessary in most cases. Historically loads have been dumb - you would have to downgrade your A/C unit because you never knew if the water heater would choose that moment to kick on.
But this is the 21st Century, for Pete's sake! You'd think the water heater could coordinate with the dryer and A/C so we don't have to worry about them all kicking on at the same time. And now, automation is appearing that does exactly that, and also coordinates with EVs. It's stuff we could've done in 2000. It's happening now because of the pressure to get away from fossil fuels and fully electrify, with EV charging being the "killer app".
Right now the technology is being done piecemeal (or you can get a fully integrated, all-singing all-dancing panel called the Span at $7000 are you kidding me?) but in the next few years that will improve by leaps and bounds. Eaton and Square D is where the action is. Eaton will bring it to Siemens and GE by way of their "Eaton CL" line of breakers made for those competitor panels.