Assuming you are in the U.S., it sounds like you might be a little confused about how taxes work.
If you are referring to federal taxes, there is no '30%' bracket. As a single filer (and assuming the 115K is your net income after taking out payroll taxes and any deductible health insurance), then your marginal bracket is 24%. Also, you are forgetting the standard deduction of 12K.
Your taxable income is taxed in each bracket (10%, 12%, 22% 24%, etc) as you 'fill up' that bracket with income, and therefore contributing to your 401 reduces the amount of your income that is available to fill up the higher bracket(s).
Roughly, it looks like your taxable income without 401K contribution would be:
115K -12K (SD) = 103K taxable
(20,499 x .24) + (43,799 x .22) + (29,174 x .12) + (9,525 x .10) = ~4920 + 9636 + 3501 + 926 = 18,983 taxes owed
And with 401k contribution:
115K -12K (SD) – 18K (401K) = 85K taxable
(2,499 x .24) + (43,799 x .22) + (29,174 x .12) + (9,525 x .10) = ~600 + 9636 + 3501 + 926 = 14,633 taxes owed
You reduce taxes by ~$4,320 by maxing your 401k and reducing the amount of income in the 24% bracket. Taxable income in the lower brackets remains the same.