In most cases, the 2012 child tax credit amount is limited to the amount of tax liability on your return. In other words, if your credit is bigger than your tax liability, your tax liability is just reduced to zero, and the rest of the credit is lost.
In certain cases, though, you can get a child tax credit refund when the credit exceeds your tax liability. This means that you would get a refund of the difference between your tax credit and what you owe in taxes. This refundable child tax credit is called the Additional Child Tax Credit, which you calculate on Form 8812.
For families with three qualifying children or fewer, this tax credit is refundable to the extent of 15 percent of taxable earned income in excess of $3,000 for 2010. For families with four or more qualifying children, the refundable amount may be the excess of the family's Social Security tax over the earned income credit, or the extent of 15 percent of taxable earned income in excess of $3,000, whichever is greater.