So I'm in the process of finishing a new garage and I want to make sure I have everything right for my heater. I've picked out a 12kW electric heater that is rated to use 52 Amps continuous. I was planning on running 6 gauge THHN wire in 3/4" aluminum flex conduit with a 70 Amp breaker and a 8 gauge ground.

Does anyone see an issue with this? I think this covers the 80% derating for continous loads. Am I correct that I need a 8 gauge ground and not a 10? Thanks

By my numbers you're gtg.

.8 X 65 amps (THHN 75 Degree rating) = 52

OK to upsize breaker to next standard size per 240.4(b).  You'll need to verify that the breaker and heater are rated for 75 degree connections or you'll have to upsize to #4 AWG wire.

Yes, you'll need a #8 ground.