[HoosierDaddy]

OK , My NEC code chart says 55 amp @ 90 degrees calls for 8 AWG copper depending on distance of the run. That is at 86 degrees ambient. I guess your ambient temps may be higher so a larger wire size would certainly be warranted.
I would always follow local codes however. It never hurts to go larger but it gets difficult to get it to fit into the receptacle box. Thanks for the explanation on neutral size.