Theory Behind the 120% rule

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McT

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I understand how to apply the 120% rule, but I really don't understand the theory behind it. Does anyone have any links to white papers or other information on the topic? To me it seems like as long as the PV system doesn't exceed the capacity of the bus bars, it shouldn't matter since the load on the panel determines the current draw across the bus. For example, if I have a 3000A panel, but no loads, the ammeter on that panel will read 0A. If I have 100A worth of load, 100A will be used. If I'm supplying power from two sources to the same bus will not the one current flow always go to the load? If I don't have more than 3000A worth of load will I never see more than 3000A on that bus? I'm just trying to wrap my head around this one. Any technical information would be appreciated.
 
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GoldDigger

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McT said:
I understand how to apply the 120% rule, but I really don't understand the theory behind it. Does anyone have any links to white papers or other information on the topic? To me it seems like as long as the PV system doesn't exceed the capacity of the bus bars, it shouldn't matter since the load on the panel determines the current draw across the bus. For example, if I have a 3000A panel, but no loads, the ammeter on that panel will read 0A. If I have 100A worth of load, 100A will be used. If I'm supplying power from two sources to the same bus will not the one current flow always go to the load? If I don't have more than 3000A worth of load will I never see more than 3000A on that bus? I'm just trying to wrap my head around this one. Any technical information would be appreciated.
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You are right as far as your analysis goes. But there is no requirement that the sum of the load breakers in the panel be less than the size of the main breaker. That means that if for some unspecified reason (either coincidence or fault) the load on the panel is greater than 3000A, the main breaker will not longer be able to protect the bus if another 1000A is coming from the PV system.

The application of the 120% rule to panels which do not have, and do not even have space for, actual loads is not justified. And some argue that it does not in fact apply under those circumstances. But if there are open spaces in the panel, or the total of load breakers with all replaced with the maximum size the panel will hold, then the Code says 120% rule must apply. (Note that it is not that specific in its reasoning, it just says that if any loads are supplied, the rule applies.)
 
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jaggedben

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As far as published materials, I think this article by John Wiles is a good overview of the types of issues the Code Making Panel thinks about regarding this subject:

http://www.iaei.org/blogpost/927663...ve-PV-Inverter-Connections-690-64-B--705-12-D

Beyond that, you could look at the ROPs from the last few code cycles to see what the substantiation has been for the proposals that have become part of the code.

One of the key issues, apparently, is that load centers are not tested for this sort of thing. They are tested by passing the max rated current from the supply feed to a breaker right next to it, to see if they withstand that temperature without deforming. They are not tested to see what happens if, for example, a PV inverter is simultaneously delivering the same amount of current to another load at the other end of the busbar. Basically the CMP seems to have said "yes, we understand that theoretically no point on the busbar shouldn't ever see more than its max rated current if fed only from both ends. But the max rated current isn't based on that scenario, so we have to be much more conservative." Maybe if UL standards or testing procedures were changed, the 120% value could also change. Good luck wishing for that.

Also, the 2014 code will completely rewrite the rules. IIRC, it will include a provision that allows more than 120% of the bus bar rating if the loads are less than 100%.
 
Sierrasparky

Sierrasparky

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I do not see what the big issue is there.

lets just say this is a home with a 100 amp panel. It is the middle of the summer hot as hell. You are having a pool party. All the lights are on indoors the electric range and ovens are going. The Ac has been going all day without rest.

You install 40 amps worth of PV.
Now as unreasonable as it may be, you could potentially utilize more than 100 amps of power. Do you really think that the buss in the panel can sustain handling 110 amps or 120 amps or more with out problems.

From anther post it was discussed that no point on the buss would see too mucsh. So now one would need to be concerned as to where the inputs were placed in a panel. The rule is conservative based on the many variables. Better safe than sorry.
 
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GoldDigger

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GoldDigger said:
The only thing I have a problem with at first glance is his statement that the 100A main breaker will trip "in a short time" if 120A is pulled from it.
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In reality a randomly chosen breaker from a lot could easily carry up to about 140 amps indefinitely. Or it could trip on as little as 120 amps but only after tens of minutes. (Time/current curve figures taken from a "typical" molded case breaker.)
 
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