@Martin__Schmid, I'm not sure I'm following what exactly your question is in your first response. But I'll throw in my 2¢ (in more detail than we've discussed in the past) on this issue and maybe it'll help.
You are correct that someone could put everything on phase A and B and nothing on phase C. I want to point out, though that this isn't necessarily bad design - it could be design constraint. Suppose the designer must (for a some legitimate (albeit unfortunate) reason) add load to an existing single-phase panel that's fed from a three-phase panel; you could end up with that scenario and not be able to do anything about it. There are lots of reasons that certain scenarios can occur that aren't necessarily bad design. We don't live in a perfect world and we're not perfect ourselves.
It's important to note that 'total panel kVA' or 'total panel Amps' is mere shorthand. We don't really care about the total current on a panel. We care about the total demand current per phase. To say that a panel "has 160 amps demand on it" is just a shortcut. Sure, if the panel is perfectly balanced panel, this would mean 160 demand amps per phase. But such a panel is rare, as connected load per load classification must be perfectly balanced.
Both companies at which I've been employed calculate demand per phase (in Excel, for AutoCAD jobs) for this reason. In fact one of those companies refused to do panel schedules in Revit for years precisely because it wouldn't do this calc. Admittedly, in new construction it's rarely an issue because we have control over the entire design. But when adding load to an existing system, you really have to pay attention to this when the panel is heavily loaded. I have on more than one occasion had to move load from one phase to another because one phase was overloaded in demand - but not necessarily connected - load.
Example with made-up numbers to suit my purpose: Suppose I have an existing 200A 208/120V panel with the following known demand (however I obtain the information):
Phase A: 22 kVA / 183 A
Phase B: 21 kVA / 175 A
Phase C: 22 kVA / 183 A
Total: 64 kVA / 178 A
Now suppose I need to add a 1.7 kVA single-phase lighting load to that panel. That's about 14 A connected 18 A demand. Adding 18 A to 178A - pretty close but doable and I've done it before when I didn't have a better option. But I cannot add this load to phases A or C. It would overload those phases in demand, but not in connected. Revit only gives you that bottom line in demand. It doesn't tell you which phase will support the load. Phase B isn't necessarily the lowest phase in connected load.
You ask how the calc is to be done, but you seem to answer your own question because you do calc it. Below is the panel schedule calcs (in Excel) from one company I've worked at:
This is from a real new-construction job that I designed. Demand load is calculated per-phase. Notice that the lowest phase connected isn't the lowest phase demand. In fact the phases are exactly reversed when comparing relative load connected vs demand. If I relied on the per-phase information that Revit gives me (connected only) to determine which phase were most lightly-loaded, I'd actually be picking the most heavily-loaded. This demand information is mapped in Excel to whatever upstream panel is feeding it and the calcs are redone. The panel total kVA and amps are just shorthand and mostly meaningless without also having the per-phase information.
I've seen @oduday's Revit solution suggested on other forums and blogs when I've researched this issue. It may get past the permit reviewers, but it's not an accurate calc. I think Revit should be able to calc this correctly - firms have been doing it for a long time in Excel.
Hopefully that answered your question and gave you useful info without being too long-winded (or with too many math errors) ![]()