jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
I don't understand how the 200A breaker would solve the problem of the 193.5A operational current being greater than the 180A ampacity of the 4/0 Al cable.
In real life there is no such operational current. (In fact 166A is not a number that goes into this.)
Simply put, 310.15(B)(7) allows you to protect 4/0 Al cable with a 200A OCPD for applicable dwellings. (We haven't gone into why it allows that in this thread. That's another topic.) Discussion of ampacities can end there. 4/0 AL SER, 200A breaker, it's okay. You can draw 200A on it until the breaker trips, NEC says it's okay.
The problem with your backup loads feeder is that the additional sources defeat the 200A OCPD that is upstream of the switch. Article 705 steps in and says you either upsize the feeder or protect it with an additional breaker at the switch. Now that you have a breaker there, that breaker can be 200A for the same reason that the one at the service can be. 310.15(B)(7) is back in force now that you've satisfied 705.