So I have 3 questions based on the above...
1. In either case you pointed out, do you take into the consideration the bus rating of the sub-panel? or the calculation is always based on the bus of the main panel only?
2. Not sure i understand the difference. is the only difference the way i add up the breakers? In point 1, i just add the breakers. In point 2, i combine the inverters and calculate the full sized breaker.
3. Is this in the newer codes? I did not come across this in the 2008 NEC, where there's a difference if the sub-panel is a mix of sources and loads or not.
1. Draw an imaginary box around the panelboard you are studying. Ignore all other panels, until we get to them. Determine its main supply OCPD trip rating and call it M, whether inside the panel, or external. Examples of an external main supply OCPD would include (but not limited to) the following: a fused disconnect, an enclosed circuit breaker, the breaker in a meter/main combination device, the breaker in a higher ranking panel that supplies an MLO subpanel. Then determine the busbar rating (which we'll call B), and the total rating of the interconnection circuit breaker(s) that we'll call I.
NEC2014 and later, we no longer use breaker ratings to determine I, but rather we use the calculation that sizes that breaker prior to rounding up to a breaker rating. An example where this makes a difference, is given a 600A main panelboard with a 600A main breaker. Instead of limiting you to a 110A breaker (88A of current), it now allows up to 96A of current, and it is OK that it is interconnected on a 125A breaker. Even though 125A+600A exceeds 120% of 600A, it isn't the 125A that matters. It's the 125% of 96A that matters instead.
Your condition is as follows, to use the 120% rule, assuming you feed the panel on opposite ends of the busbar:
I + M <= 1.2*B
Once the first panelboard at the point of interconnection passes, move on to studying the higher ranking panelboards, until you get to the service point. Each panelboard from point of interconnection to the service point must qualify under one of the busbar protection rules in 705.12. You do not necessarily need to select the same rule each time.
2. The difference is that when you mix sources and loads, you have more total amps among all the breakers. The reason we don't get to use a 200% rule for feeding panelboard busbars from opposite ends, as Kirchhoff's current law alone would imply, is that it is an industry compromise to limit you to 120% of the busbar rating. The underlying issue is that the 200A panelboard is not tested for 400A worth of current among all the breakers, even when no more than 200A are present at any given cross section of the busbar. The industry compromise is that it is OK to have 20% extra current that is potentially feeding the breakers, provided the second source is strategically located to not add up with the main supply.
When you only have inverter sources in a panelboard, these devices don't draw current, they supply current. This is what installers would call an AC combining panelboard. You aren't at the point of interconnection yet, where there is a mixture of sources and loads. The most the current can add up to, both on the busbar and among the branch breakers, is the sum of the inverter output current ratings. You'd still apply the 125% factor for continuous loads when sizing OCPD's along the main supply, and that OCPD would still have to protect the busbar. But you don't get the situation of current travelling in both directions on the busbar, and feeding all the branch breakers with more than the busbar rating worth of current.
Suppose you use an AC combiner to add up two 14A inverters, each behind a 20A breaker. If both those 20A breakers are in a mixed use panelboard, you'd use 40A as the input for your 705.12 calculation. However, if you first combine them in a dedicated load center, then you get to interconnect them at a single 35A breaker. Now you get to use 35A in your calculation as your inverter interconnection breaker, instead of 40A. This is per the wording of the 120% rule in 2011 and earlier.
3. Article 705 is about interconnected power sources. In a dedicated AC combining panelboard, you aren't interconnected yet. You only have one type of power source and you are paralleling your inverter outputs, for an aggregate feed to the general electrical distribution of the service. The total amps on a dedicated AC combining panelboard is not going to exceed the sum of the inverter output currents, because there aren't any loads to also draw current. It is a continuous load, so you still have to apply the 125% factor for sizing the bus and main breaker, but that's really all it will be. You aren't going to have load branch breakers that add up to far greater than the main supply or busbar rating, as is common for load panelboards in general.
Load panelboards in general are routinely filled with loads that can add up to a lot more than the main supply. The underlying assumption is that it is an unlikely scenario that they all draw their full load at once. If they do, the main supply will trip before the busbar gets overloaded. If you add a second source to the panelboard, now that second source can support the main supply in supplying these loads, and potential overload of the panelboard becomes a blindspot to the main breaker. That's why we are restricted in the 705.12 rules, when mixing sources and loads in the same panelboard.
...... nah, I prefer to ridicule NYC and its red tape.