Byron:
Backfeed BETWEEN the twin 200 Amp C/Bs;
using the tap rules and an adjacent j-box -- suitably large.
You'd end up with twin splice blocks ( Ilsco ? ) and very short 3/0 THHN jumpers interposed between the splice blocks and the circuit breakers.
During the night, the current would shunt around exactly like your current set up.
During daylight, your inverter output(s) would flow out to both panels -- from this center point -- until the meter ran backwards.
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Such a scheme entirely removes the bus calculation from the design. You'll never have so much PV sourced current that it stresses that top copper bus.
http://www.ilsco.com//productcatego...31RZYOsRgUwvNMGDmVHPiZnijY7FLyC%2bjmo%3d
