The 120% rule when PV inverter size is larger than the main breaker?

[Matu]

I'm currently working a project with service coming from a Siemens I-T-E FC-II switchgear rated at 1600A. The switchgear feeds a couple of sub panels, one of which is an 225A busbar rated MLO fed by a 125A main breaker in said switchgear. Using two SolarEdge 43.2kUS inverters, we have 300A of PV to interconnect.

One option I'm looking at is a load side tap on the busbar of the switch gear (probably have to confirm with Siemens first). However, this may not be possible.
My other option is performing a line side tap on a transfer switch (400A rated with a 400A busbar) but I don't think this is allowable anyway.

My third option is upgrading one of the 225A MLO to a 400A busbar to allow interconnection with the 300A of PV. This is obviously more costly, but something that can be considered. My question is whether this is allowable, as it would pass the 120% rule but would be feeding a 125A main breaker with 300A of PV. We've upgraded panels before, but never one where the main would be smaller than the inverter. If we went with this option and it was allowable, what additional considerations/adjustments should be made (Upgrading feeder conductors, etc.)?

 

[tortuga]

You cant add a section to the switchgear and land another 400A bucket?

 

[Carultch]

My third option is upgrading one of the 225A MLO to a 400A busbar to allow interconnection with the 300A of PV. This is obviously more costly, but something that can be considered. My question is whether this is allowable, as it would pass the 120% rule but would be feeding a 125A main breaker with 300A of PV.

Your path has to be capable of supporting the full load of the inverter output current, without depending on local load to consume it. Any OCPD in the path between the inverter(s) and the service point, has to be rated at least 125% of the inverter output current to meet the NEC. In your proposed solution, the 225A breaker would trip when the system is at the full 240A capacity and the local load in that panel is temporarily zero. That is, unless you had a power control system that would allow you to take credit for the new 705.13.

The most extreme case scenario where you could use the 120% rule in a manner such as this, is when the main supply breaker is equal to the 125% of the inverter output current. Essentially, when both the inverter source and the main supply breakers are equal.

 

[Matu]

Carultch, that makes sense, thank you.

Tortuga, I'm not so sure. I'm going out to the site today with an electrician to get some more insight on this so hopefully this'll work out