The bus, in this instance, has two line-side connections: the Poco and the sync'd inverter.
The load is invariant -- doesn't change -- whether the power is flowing via the inverter -- into the main bus -- and out and down to the dependent circuits ... or whether all of the power comes from the Poco. (Sundown)
The amperage seen by the bus simply can't exceed that flowing under sundown conditions. During noontime, even full amps -- from the PV inverter -- merely shove power backwards -- onto the bus. If the local load is way down, then the net result must be that power flows backwards -- up the line -- and into the Poco's grid.
In the most extreme case, that would be, say half of the bus capacity.
For PV inverters that are sync'd to the Poco frequency -- any breakdown in that synchronization must cause them to trip out. Other wise, arc welding would begin.
Sync'd inverters just can't over load the bus -- unless NEMA standards have really let us down.
(Exception: a seriously overdesigned PV array could over power the bussing. However, that level of overdesign would be wildly uneconomic -- crazy, in fact.)
