The additional child tax credit is a refundable tax credit for people who have a qualifying child and did not receive the full amount of the child tax credit. Because this credit is refundable, it is possible to get a refund even if you do not have any tax liability.

In order to receive the additional child tax credit, you first must qualify for the child tax credit. If the amount of your child tax credit is limited by your income tax liability, you may be able to claim the additional child tax credit if your earned income is less than the limits for the year.
 
The limits are:

• Married filing jointly – $110,000
• Single, head of household, or qualifying widow(er) – $75,000
• Married filing separately – $55,000

If you have 3 or more qualifying children, you may be able to claim the additional child tax credit up to the amount of social security taxes that you paid for the year. If you are eligible to receive the earned income tax credit, the maximum amount of additional child tax credit under this method is the total amount of social security taxes less the amount of the earned income credit for which you are eligible.

The amount of the additional child tax credit is determined on Form 8812.

For more information see IRS Publication 972.