Main panel load calcs question ..

[Another C10]

lets say a 30A 240v breaker from a solar array breakered into a utility main panel which Id think is allowing for 7200 w of "bonus" consumption ,

Question is, is there a percentage of that 7200w that can be deducted from the overall load calculation of a main panel regarding the determined utility consumption.

Im basically providing a load calc for the city but wondered if I should add the solar provision, to bring my utility amps down a tad.


I may of complicated the question too much.

 

[jaggedben]

No, you can't do that. An intermittent interactive source cannot be counted upon to reduce the load. There is no provision in the code which supports this, and at least one section (705.12(B)(3)(2) in 2020 NEC) that requires a panel busbar to be sized to article 220 regardless.

I suspect even a major cogen plant wouldn't get this kind of consideration unless there's a means (such as load shedding) to automatically reduce the load when the additional source isn't available. Every power source has downtime.

 

[Another C10]

I suspect even a major cogen plant wouldn't get this kind of consideration unless there's a means (such as load shedding)

thank you, l suppose this does make sense. no such thing as "bonus" potential. If the power can be used , it will.

 

[topgone]

thank you, l suppose this does make sense. no such thing as "bonus" potential. If the power can be used , it will.

Maybe, the "bonus" mentioned refers to the possible lowering of the main circuit breaker rating to comply with the 120% rule in Art. 705.12(B)(3)(2) - utility and the solar inverter output are located at the opposite ends of the panelboard busbars (load side connection of the PV input.
In your case, if your 30A PV inverter output is to be connected to a 100A panelboard with a 100A main circuit breaker, the rule states that 125% of the PV current plus the main circuit breaker rating shall be less than 120% of the panelboard busbar rating. Computing for the reduced main circuit breaker size will be Main CB = 1.2 x 100 -1.25 x 30A, the main circuit breaker must be less than 82.5A ~ 80A (instead of 100A).

 

[jaggedben]

I don't think that's what he had in mind, plus lowering a main breaker on a single family dwelling to less than 100A would be a code violation.

 

[Solar Guy]

Solar output circuits must be oversized at 125% to start with, so the most you can get out of a 30A backfeed (if it was sized correctly) is 24A, or 5760W. The requirement for locating the backfeed opposite the main is so that power supplied form each end of the bus will be drained off by the various branch breakers so that the bus is never overloaded at any point. This allows the sum of the main breaker and the backfeed ratings to equal up to 120% of the bus rating, but no more. The "Bonus" comes in lowering the utility consumption (and its bill) while the sun is up, not in anything cheap you can do to the panel. After all, the whole load is carried by the utility after sundown. Mr. Utility is your battery.