As you are adding more sources to a busbar, the busbar can heat up and was designed to only take so much current. Once you exceed the busbar rating by 100%, then that last 20% for the 120% rule has to be on the opposite end of the busbar, to make sure there are no hot spots.

People can connect more loads than the busbar is rated for and the protective mechanism is the main breaker for the busbar. If we add another source, such as solar, we have to be careful about the breaker placement and rating.

We have many different rules and some are for connecting to feeders too. We can over stress feeders or busbars if we do not follow the special rules in 705.12.

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Let's say we have a 200A MDP with 200A Main Breaker.

If you have only one source then whenever it exceeds 200A the main breaker will trip but, if we have the utility and solar feeding to the same busbar and for example you are taking 150A from the utility and 80A from solar (using 100A breaker for solar) non of the breakers will trip and you are applying more than 200A to the busbar and in this case it will cause a disaster

Yes, you understand the concept of adding a source.

If you add the sources at opposite ends of the busbar, you are allowed to use the 120% rule, so you can exceed the busbar rating, so that the main breaker plus 120% of the inverter current do not exceed the busbar rating, then you are good.

Since they are on opposite sides of the busbar, then there will be no case where you could have over 200A on the busbar in the same place on the busbar. There would be some place in the middle of the busbar with zero amps if you had maximum loads and maximum solar just below the point of tripping the main and solar breaker on a sunny day.

There is another way, called the Sum Rule, where as long as the sum of all breakers, excluding the main do not exceed the busbar, then you cannot overload the busbar.

This is all found in 705.12.

Thanks