I plan on creating a new circuit and running 12/2 to my microwave for an dedicated circuit. The current outlet for the microwave is old wiring running through conduit. The wiring comes in through a junction box labeled A, then runs about 3 feet over before dropping down the wall labeled B, and that runs into the microwave outlet box. It looks like that box A is already pretty crowded so it doesn't seem like the best option to run the new microwave circuit wire through the conduit via box A.
I would think the easiest solution would be just to put in a second outlet box and then run the new wire directly to it? I would have two outlets in the cabinet above the microwave, one dedicated to the microwave. Also, how can I tell what the capacity is of the old light/junction box that is running conduit? Maybe the box isn't overly crowded.
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Best Answer
Here's the good news. You have figured out two methods of running your new circuit and both of them work. As a DIY'er probably the best method for you to take would be to run the 12/2 to a new box.
As an electrician I would look at trying to run the conductors in the conduit. The magic number is 40% fill for the conduit and boxes. So you would need to know the number and type of insulation of each existing conductor and the size and insulation of the new circuit. Once you have that information you would need to go to the Table section of the NEC and run the calculations to see if everything would work. Also you would have to determine how you were going to make the transition from the existing circuitry and feed over to the new receptacle. This seems to me to be a little overwhelming to someone with limited experience.
So I would suggest you use the old KISS (keep it simple stupid) method and just run a new circuit. The only reason I would think about using the conduit was if it became really difficult to run the NM.
Hope this helps and good luck.