415(c) limit and catch up

Yes

In theory. In practice you may have some trouble with payroll taxes.

Aren't total annual additions limited to 100% of wages?  The $28,100 would exceed that limit.

To expand on Bri, Catch-ups don't count toward any applicable limit (which includes 415(c))

You still need to have compensation to make a 401(k) deferred contribution, that is you can't defer compensation you don't earn, but you can go over the 100% of pay limit or the 415 dollar limit if employer contributions push you over the regular limit but not past the catch-up limit plus the regular limit assuming 401(k) contributions are at least equal to the catch-up limit, the participant is old enough to qualify for catch-up, and they plan allows catch-ups.

On 1/5/2023 at 9:14 AM, Bri said:

The 6,500 in catchups don't count towards the 415(c) annual additions limit.  So that total is really just the 20,500 plus 1,100.

Well, this is on the right track but technically there is not 6,500 in catch-up. That's the limit but not in this scenario the amount to be treated as catch-up. Assuming the W-2 salary of $27,500 is also compensation for 415(c) purposes under the terms of the document, then the breakdown is: regular deferrals of 26,400, match of 1,100 for a total of 27,500 included in the annual additions testing, and 600 treated as catch-up. 

Nah, once you blow past 20,500 any deferrals start filling that catchup bucket.

Maximum annual addition here is $34,000, assuming they have at least $6,500 in deferrals.