Sub Panel and 120% Rule

[csoc64]

Have an installation requiring a Fronius 10.0-1 UNI Advanced solar inverter, which calculates out to a 55A breaker. I have access to a 200A sub-panel bus fed with 100A breaker at the 200A main panel. 120% rule tells me that the sub-panel can take 200 * 1.20 - 100 = 140 amps of PV, which is ok for my 55A load. However, am I also interpreting the rule correctly in that I have to base my available capacity by the main panel bus bar / breaker rating, which would be 200 * 1.2 - 200 = 40A? So am I correct in saying that I am limited to 40A without doing a supply side tap ahead of the service.

 

[ggunn]

Have an installation requiring a Fronius 10.0-1 UNI Advanced solar inverter, which calculates out to a 55A breaker. I have access to a 200A sub-panel bus fed with 100A breaker at the 200A main panel. 120% rule tells me that the sub-panel can take 200 * 1.20 - 100 = 140 amps of PV, which is ok for my 55A load. However, am I also interpreting the rule correctly in that I have to base my available capacity by the main panel bus bar / breaker rating, which would be 200 * 1.2 - 200 = 40A? So am I correct in saying that I am limited to 40A without doing a supply side tap ahead of the service.

Yes. You are limited to a 40A inverter breaker (or a 40A sum of inverter breakers) feeding a main panel with a 200A main breaker and 200A busbars no matter how it/they get(s) there. All the breakers in between are irrelevant to the 120% rule calculation in the main panel.

 

[PWDickerson]

If a load calc for the home is 175 amps or less, it is usually easiest to reduce the main breaker to 175 amps, which allows a maximum backfeed of 65 amps in the main panel. We do this frequently.