Curious about box fill
Ok so this is a two part question. First off someone tell me if I'm interpreting the box fill calculation correctly in this scenario.
All of these are #12 THHN conductors in a j-box
1 Hot, 1 Neutral, 1 Ground enter box from a conduit
2 Hot, 2 Neutral, 2 Ground spliced to corresponding feeds and exit into two different conduits
If I'm reading box fill correctly then I've got 6 Hots x 2.25cu in = 13.5 + 6 Neutrals x 2.25cu in = 13.5 + 1 Ground * 2.25cu in = 2.25 for a total of 29.25 cu in.
Is that calculation correct?
If it is correct then here's the second question. I understand the purpose of the box fill limit to control the potential heat in the box. However the calculation doesn't make sense to me. Since all the conductors are on the same circuit the most current they can carry is governed by the breaker they're on so generated heat should also be limited by that which means (to me at least) that the 3 hots should count as 1 hot and the 3 neutrals should count as 1 neutral since they will all carry the same current. I realize this all goes out the window if you have different circuits coming in.
Is my logic all screwed up? The fact that you ignore pigtails in a box calculation also implies that my logic is correct.
This all comes up by the way because I am cleaning up some old wiring a previous owner did and I found a box that seems to violate box fill so while I was looking it up I started thinking it all through.