Sole Prop Contributions to DB plan

If I understand correctly, a one-person owner of a business with a DB plan can ultimately contribute more than their W-2 to a DB plan.

Can a Sole Proprietor (with no W-2 income) do that as well?  Seems the subtraction on Schedule C would make it impossible.

The required contribution might be more than the Schedule C income, but deduction is limited by the Sch C.

Normally you'd have a non-deductible contribution subject to the excise tax but the excise tax is waived for a Sole Prop whose MRC would drive his income negative but you still have a non-deductible contribution to the Plan.

Correct - there is a circular function to split the total earned income between ER Contribution and plan compensation.  Assuming you need some amount of compensation to produce the benefit that is creating the contribution requirement, it w/b rare that the entire Sch C amount goes to deduction, but it is certainly possible if the high 3 is already established, or if you have a fixed dollar formula in a cash balance plan.  This could easily create a 415 limit problem.

There is also a special adjustment for 1/2 of the SE tax, and you have comp limits that can also come into play, so be careful.  

Just to be clear regarding the Sched C income that the deduction would be limited to - we're referring to the income resulting from that circular calculation (i.e., after the net Sched C income is reduced by the contribution and the 50% SE tax), and not the actual net Sched C income reported on the Sched C, right?