zanydroid
Solar Wizard
In most situations the calculations will show that there is no problem, but you might find busbar or feeder segments that can be hotspots.Yes to both.
The math seems trivial, even unnecessary. But then, my logic is surely flawed. If the inverter is feeding the grid, then the house is using less than what the inverter is putting out. Since that is much less than what the system can handle, there is no issue. And if the inverter is not feeding the grid, then either it is night, thus no issue, or the house is using both from the grid and the inverter. But the current from the inverter is going to the house and doesn't ever travel to the service lines. So as long as my tap to the inverter and main panel is sized correctly, thats all I should have to worry about. Please set me straight.
Very rarely will there be a risk to service lines. In fact I don’t think the NEC actually polices that. The POCO might though. You would have to export over 320A to cause that kind of problem. My house has almost 80A of grid tie on 100A service (I need the huge system to overcome trees so it doesn’t actually produce that much energy, but POCO still treats it as a big system).
So let’s say you have a 100A service and 100A panel. You backfeed 40A at the middle of this Panel into a 50A breaker. At the bottom of the panel is a plug on lug (no overcurrent protection) that daisy chains to a second subpanel, with no additional overcurrent protection. This wire is 100A rated.
Without the solar, that setup is safe since it’s not possible to send more than 100A through that lug to the subpanel.
With the solar you can pull 140A-150A into the lug wire and subpanel without blowing any breakers.
To make this safe the plug on lug needs to be replaced with a 100A breaker (there may be other ways to address this too, for instance plug on lug, feeder, and subpanel can be upsized to 150A.