I find very good method of determining conductor ampacity, pretty much exactly the way I did it earlier, but that is all it has, and my 4/0 would be fine when it comes to determining minimum ampacity. It does not mention overcurrent protection, but I have come to the conclusion that overcurrent protection and determining ampacity are not the same thing and I simply missed the fact that overcurrent protection of this conductor that otherwise has enough ampacity for the load just doesn't work with the selected conductor. I'm guessing it doesn't work out that way too often, but at same time I wonder how many times I may have missed something similar.
As far as the motor calculation in the other thread, I did not find anything that really specifically addressed the problem I was having there. The book I have did mention same thing NEC mentions about minimum ampacity, but this book does not have any specific examples or instructions of selecting ampacity for motor circuits, just a general ampacity selection section that kind of combines the general requirements from 210, 215, 230 and 310 for ampacity selection. I still concede to what was said in that thread about what I was doing wrong in that particular calculation.
I appreciate the input though, I have learned from this.
Your wrong and right....
According to Larry T. Smith's 2008 National Electrical Study guide, he states from his 5 step method that you compare steps 1 & 2 and the larger of the two is the correct conductor.
STEP 1: Calculate the load at 125% of continuous load, and 100% of the noncontinuous load.
STEP 2: Select the conductor from the temperature column that corresponds to the equipment rating.
note 1: if derating is not involved your DONE.
note 2: Derating is a completely a new calculation, start over. Use 100% of the load, and select the conductor at its maximum operating temperature. For example, THHN is rated at 90c. You are permitted to use the 90c column for derating.
STEP 3: Adjacent conductor adjustment
STEP 4: Ambient temperature correction
STEP 5: Go to the applicable Table in Article 310 and select the conductor. Compare this conductor with the one selected in Step 1 and 2. The larger of the two is the correct conductor.
So 4/0 would not be minimum ampacity. 250KCMIL would be the minimum if you compared the two, since it is the largest of the two.