120% panelboard question

[ggunn]

Not sure which method you're referring to. But something like that only makes sense where a panel would presently comply with both the "120% rule" and the "sum of all breakers rule". Unless you can only use the latter rule, and the sum would exceed the rating, adding a load doesn't matter.

It seems unlikely to me that a panel would comply with both rules, since in order to comply with the 120% rule the breaker protecting the busbar must be part of the calculation.

 

[jaggedben]

It seems unlikely to me that a panel would comply with both rules, since in order to comply with the 120% rule the breaker protecting the busbar must be part of the calculation.

But I've seen it more than once.
Resi customer has a 100A service disco feeding an intererior sub. They want to add a car charger, so they replace the disco with a 200A panel. So the 200A panel has a 100 sub breaker and a 40A or 50A charger breaker. I come along and want to put in solar that doesn't require larger than a 40A breaker. So I can use either rule. 120% rule is probably better because they may be planning to change the furnace to a heat pump and dryer and water heater to to electric as well.

 

[Dennis Alwon]

Not sure which method you're referring to. But something like that only makes sense where a panel would presently comply with both the "120% rule" and the "sum of all breakers rule". Unless you can only use the latter rule, and the sum would exceed the rating, adding a load doesn't matter.

I was referring to putting the overcurrent protective device at the top when the calculation would allow it.

 

[SKSolar]

Discussion question:
In selecting a PV backfeed breaker, is there a specific feature that has to be different from a circuit load breaker consider the circuit breaker feeds the load only where as the the PV breaker feeds the load busbar? Since PV source is also AC current (via a combiner box ), I believe it also feed the combiner box also when PV is not producing power at night??? Correct?

 

[Carultch]

Discussion question:
In selecting a PV backfeed breaker, is there a specific feature that has to be different from a circuit load breaker consider the circuit breaker feeds the load only where as the the PV breaker feeds the load busbar? Since PV source is also AC current (via a combiner box ), I believe it also feed the combiner box also when PV is not producing power at night??? Correct?

It has to be bidirectional, but it's common that most standard breakers made today will have this rating. You do have a burden of proof to confirm the breaker is bidirectional, by looking for a lack of line and load markings. Unfortunately, this is like proving a negative, since manufacturers don't do a good job of documenting this. Generally speaking, for most standard breakers made today that don't have additional features, they are bidirectional.

There is a very small amount of current that travels back to the inverter during the nighttime, while the inverter is in standby, waiting for a viable source of sunlight to turn on. This is insignificant for a standard grid-tied inverter, that has no other destination for that power.

 

[jaggedben]

Discussion question:
In selecting a PV backfeed breaker, is there a specific feature that has to be different from a circuit load breaker consider the circuit breaker feeds the load only where as the the PV breaker feeds the load busbar? Since PV source is also AC current (via a combiner box ), I believe it also feed the combiner box also when PV is not producing power at night??? Correct?

This is why we don't use AFCI or GFCI breakers, as they are not certified to provide their special protections with power flowing in either direction.
So with typical breakers today, as Carultch said, it is in a way the opposite of the PV breaker needing any special feature. It needs to not have any special features that only work in one direction.