It might help you to understand it if you look at form 1040. I'll use 2017 form 1040 lines since it's a little more straightforward. Your tax liability is on line 44. That is what must be paid either from non refundable tax credits or withholding. The only way to get the credits is to pay tax with them first before using withholding. Non refundable credits are shown on lines 48 through 54. They are subtracted from taxes in the order listed on the form. Child tax credit is on line 52 and the EV credit is on line 54. So child tax credit will be deducted first from the tax on line 44. Whatever amount of tax is left can be used for EV credit. Since you will only have $3,095 of tax left to offset, that is what limits the amount of EV credit you can get because credits can only reduce your tax to $0. The total credits are on line 55. The credits are subtracted from the tax and entered on line 63 which in your case is $0. This shows that the credits paid your taxes. All the withholding on line 64 is not needed because the taxes were paid by your credits and you get a refund of all the withholding as shown on line 75.
Link to 2017 form 1040: <a rel="nofollow" target="_blank" href="
https://www.irs.gov/pub/irs-pdf/f1040.pdf">https://www.irs.gov/pub/irs-pdf/f1040.pdf</a>
