This message was posted on behalf of Intel Corporation
In regard to your question, the thing is that there is no information about unreleased products, in order to answer your inquiry we would need to verify the technical product specification for each NUC: NUC7i7 and NUC7i5, and that document is not available yet as you can confirm on the following link:
Let me apologize for any inconvenience.
In regard to Intel® NUC6i5, that one will work with an external power adapter that uses voltage of 12-19 V DC depending on the type of power brick used, so it should work with an external power adapter that meets those requirements, as you can confirm on the following link, page 51:
Any further questions, or if you need more information, please let me know.