At times my demented self does not read things right and this may be one of them. As I read the question, it sounds as if the person is asking:
Why isn't the rating on the 200 amp panel such that if phases A, B, and C are loaded to 67 amps, and 3 times 67 is 201, therefore the panel would be fully loaded?
Answer:
A good question to ask, yet a rating based on this would create many problems due to the unbalnced loading that Mike mentioned. If panel ratings were such, what amperage would you build the panel bus to handle? 67 amps? If so, what would happen in a severly unbalanced system where phase A has 125 amps on it and phases B and C each have only 5 amps on them. The total amperage of the panel load is under the 200 amps, yet if this were the case the phase A bus would be carrying twice its rated current. This would be a big fire hazard due to misunderstanding of the rating.
Therefore to avoid misunderstanding, the ratings of each individual bus (phase A, B, and C) within the panel are used for rating the panel. Each bus must be able to handle the current the panel is rated for and keep within safety margins. If every bus can handle 200 amps safely we have a 200 amp panel, or if every bus can handle only 67 amps we would have a 60 amp panel (note: the manufacturer would have to round down to standard size).
