The 120% rule applies to situations where the current to the bus or conductor from each source is limited and so the bus/conductor is protected. Adding the two current sources would allow a load to draw more current and defeat the protection.
Service conductors are NOT protected, so adding more current from PV will not make things worse. So no 120% rule.
That lack of protection is why service conductors are not allowed to run very far inside.
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For what its worth, here is a bit on supply side connections by the Washington State Labor and Industries:
https://www.lni.wa.gov/TradesLicensi...ts/Elc1410.pdf
Ethan Brush - East West Electric. NY, WA. MA
"You can't generalize"
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