Wow, spent the afternoon searching the archives re 100A subpanel. Bazillions of posts, opinions, and interpretations, but no consensus, but the safe way(s) to go are either:

1) Install 90A cb on feeder panel for #4 CU;
or
2) Use #3 CU.

(BTW, I plan on THHN).

Question about option #2- the neutral & grounding bars on the main panel are only rated for up to a #4 CU. What do I do with my grounded & grounding wires if I use #3?

Yes, I've removed the bonding jumper from my sub. Will make sure that neutral bars get only grounded conductors, and grounding bars get only grounding counductors.

Thanks!

Check the manufacturers catalog. Sometimes they make an auxilliary lug that will fasten into a couple of the neutral bus holes. I know that GE and Square D make these for their panels.

Tom