People get focused on the ampacity of the bus and forget about the temperature rise in the panel. Both have to be controlled. If we ignored the temperature rise then a PV breaker and main breaker at opposite ends of the bus could be rated for the full bus amperage and no point on the bus would be exposed to current in excess of the rating, assuming the branch CBs were less than or equal to the bus rating. Most people get this and don't understand why this would not be acceptable in the code. It would make PV system interconnection much easier.
But when we look at temperature rise we find the problem. If we have a 100A 240/120V panel with a 100A main CB and 100A of load then it can have up to 24kVA going through it. Some fraction of that power will be dissipated as heat inside the panel leading to temperature rise that has to be limited to protect the components. Now if we add a 100A PV CB to the panel with the same 100A load what happens? Nothing, 24kVA will still be the maximum power and the temperature rise will be the same. It's a zero sum game, 100A goes to the load so 100A must come from the sources, in this case from the PV system first and any extra needed comes from the main. In theory there may be a slightly a higher temperature rise since the whole bus length is being used but that's not much, most of the heating comes from the thermal elements in the over current protection devices.
The problem is that in a real panel the sum of the loads could be more than 100A even in a correctly designed system. Plug loads are variable and estimated to be low in the code but as anyone who has been in the lunch room when the microwave, toaster oven, and coffee maker are running and had the branch circuit break trip knows sometimes the actual loads can be higher that the circuits were designed for. So let's say there are 120A of load on the panel. With only the main CB it would trip and that would be it. With the addition of PV the loads could draw 120A even though at no point on the bus would the current be greater than 100A. The power through the panel would be 28.8kVA, 4.8kVA greater than the case without PV. Some fraction of that additional power will add to the temperature rise and could cause it to rise higher than the UL listing would allow for the panel resulting in operational problems or damage to components. The 120% allowance was added to the code to give some room to add PV while limiting the temperature rise. I'm not sure what testing went on to validate this, if any. The question then would be how high can we go while still staying in the limit of the temperature rise? I don't know if anyone has done this testing.
But when we look at temperature rise we find the problem. If we have a 100A 240/120V panel with a 100A main CB and 100A of load then it can have up to 24kVA going through it. Some fraction of that power will be dissipated as heat inside the panel leading to temperature rise that has to be limited to protect the components. Now if we add a 100A PV CB to the panel with the same 100A load what happens? Nothing, 24kVA will still be the maximum power and the temperature rise will be the same. It's a zero sum game, 100A goes to the load so 100A must come from the sources, in this case from the PV system first and any extra needed comes from the main. In theory there may be a slightly a higher temperature rise since the whole bus length is being used but that's not much, most of the heating comes from the thermal elements in the over current protection devices.
The problem is that in a real panel the sum of the loads could be more than 100A even in a correctly designed system. Plug loads are variable and estimated to be low in the code but as anyone who has been in the lunch room when the microwave, toaster oven, and coffee maker are running and had the branch circuit break trip knows sometimes the actual loads can be higher that the circuits were designed for. So let's say there are 120A of load on the panel. With only the main CB it would trip and that would be it. With the addition of PV the loads could draw 120A even though at no point on the bus would the current be greater than 100A. The power through the panel would be 28.8kVA, 4.8kVA greater than the case without PV. Some fraction of that additional power will add to the temperature rise and could cause it to rise higher than the UL listing would allow for the panel resulting in operational problems or damage to components. The 120% allowance was added to the code to give some room to add PV while limiting the temperature rise. I'm not sure what testing went on to validate this, if any. The question then would be how high can we go while still staying in the limit of the temperature rise? I don't know if anyone has done this testing.