Hi retro hail I am an electrician. I would recommend you getting qualified electrician for your answers obviously, but looks like you need at least a 60 amp feed number 6/3 awg for your garage. Do not use existing feed wire as it's only 30 amps and won't handle your compressor and heater at the same time. Your electrician will do a load calculation to see amperage you will need. Sent from my iPad using Tapatalk