Red wire 12V or 5V... How to tell?
I'm making a project for school that is supposed to mimic a wind turbine. So, it will use a case fan for a computer as the "turbine".


If you mean that you intend to blow air at the case fan and use it as a generator, I have bad news: that won't work.

Most modern computer fans use a brushless DC motor design. These motors cannot be "back-powered" to run as a generator, as they depend on electronic controls to drive the stator coils.

If you plan to simply run the fan, though, standard computer case fans are all designed to run on 12V. They may run slowly on a 5V input, though.