I've just added a 100A subpanel to a house with a 200A main panel, and would like to know if I did everything correctly and up to code before getting it inspected (I'm in WA state USA -- 2020 NEC).
What I installed
It's a Square D HOM2448L125PQGC 125A 24-48 panel with main lugs (but no main breaker). IIUC, you don't need an explicit disconnect (whether main breaker or otherwise) on a subpanel if it's right next to the main panel that feeds it.
Per #3 / #5 THHN and 1-1/4" metal flex conduit for feeding 100A subpanel?, I used 1-1-1-3 AL SER for the feeder.
Wiring the subpanel
I drilled 1 1/4" holes through the studs (1.5" from the edge) for the SER to go to the correct stud bay. This part of the garage is unfinished and it's unlikely that drywall will be added in the future.
I used 1 1/4" knockouts and NM cable clamps + bushings on both panels for the feeder. I didn't staple the SER cable since it wasn't close to any of the vertical studs, but I did tighten the clamps enough to provide strain relief. I used Noalox on the lugs and the tips of the stripped Al conductors before tightening.
Most importantly, I used a torque screwdriver to tighten all lugs to the spec (50 in-lb) per the breaker and bus bars.
I installed one of the provided ground bars on the right of the subpanel. Since this is a subpanel, I didn't attach the screw that bonds the neutral bus to the panel / grounding system.
I didn't install any additional ground rods since it's right next to the main panel, so the subpanel's ground bar is only connected to the main panel's neutral / ground bars (which are bonded there).
I also moved two 15A 120V AFCI circuits to the subpanel to make space for the 100A subpanel breaker. I used 2x 14/2 runs of Romex, with 1/2" knockouts and NM clamps on both panels but no bushing (there wasn't enough thread after adding the locknut to securely attach the bushing). IIUC, bushings are optional for that small size.
I did a load calculation using the Mike Holt app (which uses the NEC optional method), and got a total load of 122A (requiring a 125A breaker). The house currently has 200A service. Happy to share details if relevant.
I'm not sure if the 100A subpanel counts towards the actual load calculation, since it's not really adding any load itself. I used a 100A breaker because it could take 1 AWG wires and seemed like a nice round number here. But let me know if I'm mistaken and need to change the subpanel breaker size.