How to use NTSC Xbox in Europe
Solution 1:
If you want to know the voltages accepted by you Xbox unit, just look at the sticker right next to the power port. It will list the required amps as well as frequency.
The voltages should also be listed in the Xbox manual.
Solution 2:
NTSC and PAL are analogue TV standards and you don't have to worry here - all modern TVs (actually even most of the old CRT ones) have no problem between understanding those two.
When it comes to the power - well from what I've read it seems that XBox power supply is not as universal as for example PS4 one - while the latter one would accept 100-240V, the former is either built only for 110 V or 230 V. Read PRECISELY what is written on the power supply.
If you can, just buy an European power brick for your XBox.