Not sure if Loren Thomas is still answering questions so I'll jump in.
@Paul Sandhu
If you perform the voltage drop equation using #12 AWG solid wire (VD=2KID/CM) you will drop approximately 5 volts over the 90 foot distance. If you home is 240v, that brings you down to 235v, which is about a 2% drop. According to NEC 210.19 (A) Informational note 4, branch circuits are allowed a 3% drop in voltage.
The simple answer: You can safely use #12AWG Wire for for your AC unit. Make sure you follow all code requirements when running the Romex and/or conduit.
@Erika G.
@Erika G.
Improper grounding is a little ambiguous, but I'm guess the previous owner replaced old 2-prong outlets with the new 3-prong outlets. This is fairly common. According to NEC 406.4(D)(2)(b) "a nongrounding type receptacle shall be permitted to be replaced with a ground-fault interrupter type receptacle(s). These receptacles shall be marked with "No Equipment Ground."...
The simple Answer: Replace the improperly grounded outlets with GFCI outlets. These can be purchased at any home improvement store. These outlets come with small stickers that read: "No Equipment Ground" Make sure you put that sticker on the trim plate. This is an easy task for a DIY homeowner. However, if you don't feel comfortable doing it, it shouldn't take an electrician long to do it for you.