I have a 100 AMP main panel. What to run #8-3 to garage for a sub panel. Will put 40 amp 2 pole in main. #6 conductor from sub to new ground rod. Will be using for lights, construction heater(20 amps) until I put in radiant heat and for general woodworking.
Is my main 100 amp panel large enough?

If your main panel is in the house, then you do not run a separate ground.

If you only have 100amps total to the house and then you pull 40 off to service the basement, is there going to be enough left for the house while you are working in the basement?

How do I figure that out?