Most people will have tax liability greater than zero, and even if they have less than $7500 in federal tax liability, they will still get to claim a portion of it. Obviously there are some retirees and other situations where people may have very little federal tax liabilities.
If you make $46,550 using 2014 tax tables and have no other deductions of any kind then your tax liability is exactly $7500 and you will get to claim the full $7500 tax credit.
If your "Taxable Income" is $46,550 you might have that liability.
Working down a form 1040 (long form) the labels are
7 - Wages, salaries, tips, etc (think gross income)
a bunch of other possible sources of income
22 - Total income
37 - Adjusted Gross Income
40 - Itemized or Standard Deduction
42 - Exemptions
43 - Taxable income
For anyone with simple taxes 7, 22, 37 will be the same but 40 will be $12,400 for a married couple with no kids and no itemized deductions and line 42 will be $7,900.
meaning for that simple married family of two they'd need line 37 to be
$66,850 to get to line 43 being your $46,550 number from the tax table.
for a single person with no dependents or spouse it'd be a lower number but I'm guessing a couple with no kids is likely the closest to simple taxes around the range we want for an example.
(numbers from 2014 tax form, not meant to be tax advice)