Electrical Theory of 120% rule

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jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
How so? Power factor is indeed a vagary that muddies the water, but the distribution of electricity, or power or energy if you like, is better explained by Kirchoff than by a "preference" of a load to take power from one source over another. Moving backfed and load breakers around in a panel has no effect on from where a load breaker gets its power. It does have an effect on the current density in regions of the busbar, which is the reasoning behind the 120% rule's requirement that breakers feeding a busbar must be at opposite ends of the bar.

I think we're engaging in semantics here...

When explaining the concept to customers I always say that the power is used 'first by the house and any excess goes to the grid'. I don't think this is at all misleading or physically inaccurate and it's not helpful to reference Kirchoff with lay people. I agree that the word 'preference' is inappropriate, and that particularly if you don't have energy backfeeding on a given feeder or service conductor then it's useless to try to parse where the energy on the load side is coming from.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I think we're engaging in semantics here...

When explaining the concept to customers I always say that the power is used 'first by the house and any excess goes to the grid'. I don't think this is at all misleading or physically inaccurate and it's not helpful to reference Kirchoff with lay people. I agree that the word 'preference' is inappropriate, and that particularly if you don't have energy backfeeding on a given feeder or service conductor then it's useless to try to parse where the energy on the load side is coming from.
You are correct, sir.

Peace,
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Then explain why if you have equal pv power generated to power consumed the meter has no register of power used from the grid.
If you draw out the circuit and apply Kirchoff's Law to the nodes it becomes apparent. My point was that moving breakers around in a panel has no effect on the result at the meter, i.e., putting the backfed breaker at the opposite end of the busbar from the main does not make the house more able to use the power from the PV system than would putting it at the same end as the main. Doing that does potentially subject the busbar to a hot spot, however, where the current in part of the busbar exceeds its rating. Hence that part of the 120% rule.
 
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