My shop is very cold. Right now, two 110V 1500W heaters take about two hours to bring the temp from mid 50s to about 66-68 degrees. Looking for a 240V electric heater of about 4800-5000W to heat about 350 SF. Heaters spec to 400 SF. Plug-in units have the #6-30NEMA plug. Plug has two horizontal blades and the round ground. Specs on several heaters say to use a 30 amp double breaker; got that. First- I'm thinking this would take black/white.ground with the black going to one pole and the white to the other pole on the breaker. Ground is self explanatory. I have done electrical but want to make sure I'm on the right track. Not looked at the breakers as Lowe's is completely reworking the store and many aisles are blocked. Is there any way to determine on the breaker where the two wires attach or does it matter? Second- would 12/3 or 10/3 be best for this application? If I can get the shop warmed in a decent time, I can turn more or work on other projects that are on the list. Many thanks.