TLDR: your panel manufacturer will offer a neutral lug kit that attaches to 2 slots on the neutral bar. 2/0 is a lot to attach to a 125A breaker and I advise revisiting the size choice.
Wow, you don't need anything like that much power.
I am adding an EV charging 125A sub panel in the garage
Wow, you don't need all that for EV charging. We see people spend crazy amounts of money putting in huge charging rigs, overloading their house - when it's just not necessary.
People get confused by the travel unit that comes with the EV. That's for "opportunity charging" on the road. This here (at 26:14) is the textbook, correct use of it. Notice how Grey has been traveling all day, and the battery is empty, and must be full by morning, because y'know, travel. That's why a travel unit needs 50A and a home unit doesn't need anywhere near that.
If you want to use the travel unit for home charging, you can just get a dongle plug for whichever amp circuit makes sense at home.
And then people think if they get two EVs, they'll need two of them. No, that actually works even better.
Power Sharing allows two EV wall units to share a single power feed. Say you have two EVs. You never know which one will need a big charge, so you install Power Sharing wall units sharing as little as 30A (giving 200 miles in a 10-13 hour overnight charge, that starts when the FIRST car plugs in). The 30A is split until the lightly used car is full, then the hungry car gets all of it. This happens automatically. The Tesla Wall Connectors just do this right out of the box, and they make a "every other brand of car" version of that, so you can even mix and match.
Go for it if you got it, but you probably don't.
The electric service to your house has limits. 125A is a huge impact no matter how you count it. What your house can support is determined by a NEC Article 220 Load Calculation. That generally pops out a number like 155A on a 200A service, and then you know you can have up to a 45A circuit for EV charging (300 miles in 10-13 hours). Again, bonkers for a single car, but would make sense Power Sharing 2-3 cars.
The feeder size
I hope that a more realistic view of EV charging will have tempered your EV power demands because now it simplifies a bunch of your problems - not least, how to get the wires onto a 90-125A breaker, their support of 2/0 is hit and miss.
- If 90A feeder will suffice, #2 aluminum won't be a problem, and it's a commodity.
- For 100A, #1 aluminum.
- For 120A, #1/0 aluminum - you can only plan loads of 120A, but you can "round up" to the next available breaker size of 125A.
- I can't recommend 2/0 because some 125A breakers are not approved for it. Absolutely do not break off wire strands.
- I'm not listing copper because aluminum is proven reliable at these power levels, copper is obscenely costly and I'd rather you spend that coin intelligently, and 2/0 copper would be 175A wire anyway - gross overkill.
The 125A panel
Many smaller panels have a bus rating of 125A. You probably know this, but that's an absolute maximum redline - like the 112 MPH speed rating on your car tires. You don't have to drive 112 MPH, and you don't have to feed 125A to that panel. Any amount less is fine! In fact, spaces are cheap, getting more is always a good idea. So a 200A-rated panel may be a better choice, even if you're only feeding it 50 or 90 amps.