I've gotten away with the type of configuration in question only when the existing main breaker was ..... grandfathered.
If the panel label doesn't specify the size, then next time just label the 90A breaker as 'existing' on your planset, and let it somehow be existing before the AHJ ever shows up to inspect.
Funny...ya know it's weird how often I run into 200 Amp MSP's, already "pre-configured" with (E) 2p175Amp main breakers. Somehow this seems to happen only when I am installing over 40 amps of PV.
I am really quite fortuitous.:roll:
What are the odds!
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On a serious note, and I know this is an old thread, I'd like to hear what was resolved.
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OP saw three options. I would add a fourth...... depending on total DC kW vs inverter rating.
Option 4: Get an inverter that is 16 A max. This means the 3800 W.
It is possible, given the 30A breaker mentioned, that th einverter chosen had been the ridiculously wastefully sized 4 kw or similar inverter. That is a hair over the 16 A output @ 125% = 20 A cutoff....yet requires a 2p30 pv breaker.
Also maybe a smaller inverter can be overloaded quite a bit with the existing kW.
It may be a minor loss in output and a major $$ savings.
Sometimes, I'd rather put 5 kW-STC, say, on a 3800 inv. than deal with a new service.....