Does anyone know the size (AWG,Guage) of the OEM charging wire from the alternator to the battery on a 2017 Hyundai Accent SE sedan?

 

Most vehicles use a 10 gauge stranded copper wire.

Vehicles factory wired with 100 amp output use a 10 ga wire.

There is a little more to know here.
!0 gauge is considered the size for a 30 amp circuit, BUT the alternator is supposed to replenish power taken from the battery when starting and maintain NORMAL vehicle usage when running. That is why we are warned to not use the alternator to charge a discharged battery because the load would exceed the rated capacity of the 10 Ga wire. A battery charger should be used to do that.
There is no harm using a larger gauge wire but it just is a little more difficult to work with. Vehicles with constant high loads do use a larger gauge wire ,ie. 8 or 6 gauge.