When I have to use 2d or 3d modeling?

Hi Everybody!

I´m Wagner, from Brazil and I have a question, a simple question I think!
How I decide if my modeling will be in 2d or in 3d?
I have Heat Transfer and AC/DC model and I work calculating and projecting Electrical Equipament, like Transformers, Reactor, Harmonic Filters and more...
Someone here have this knowledge?

Thank you for your time.

Best Regards,

Wagner.