# Interpreting 2020 NEC 705.12(B)(3)(2) ie 120% rule



## Eggyolk (Apr 30, 2021)

I need help understanding 2020 NEC 705.12(B)(3) in regards to a rather uncommon system setup comprising of PV generation and a battery system. 









*I want to run 2020 NEC 705.12 analysis on the 200A subpanel. *










Does a system such as this one qualify for option 2 above? [705.12(B)(3)(2)]. I'm confused because the "power source" battery is located on the opposite end of the 200A busbar. The primary overcurrent device would be the 150A breaker which feeds the subpanel. But what do I do with the PV coming from the dedicated PV panel? Obviously it can contribute current to the 200A subpanel. Questions below:

1. Can the two 20A PV breakers and the 150A subfeed breaker be additive to the total contribution of the overcurrent protective devices feeding the 200A subpanel?

[battery inverter current (125%)] + [OCPD's] <= [Busbbar rating(120%)]
[40A] + [20A + 20A + 150A] <= [240A]
230A <= 240A
2. OR does having the PV contributing to the 200A busbars disqualify from using the 120% rule and why? Does this only apply to two power sources (and not three power sources which is what we have in this example)? 

Thanks in advance for any insights.


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## HertzHound (Jan 22, 2019)

The panel on the left is called an AC combiner. It is combining the two inverter outputs. The reason they have to be combined is because there can only be one point of connection between the utility and another power source.
Is the inverter output leaving the combiner tied into the sub panel or the service? The one line looks like it’s taped? Without knowing all the numbers it should be connected to one or the other with a 40A breaker. 

I don’t know much about the battery systems. Is it some sort of power wall? It doesn’t feed into the utility system, so I do think it counts as another source as far as far as article 705 is concerned?

200A (sub panel) x 1.2 = 240A the solar back feed breaker can only be as big as 40A at the sub panel. I don’t know how big the main panel is, but the same 40A plus the breaker feeding it has to be less than 120% of the bus rating of that panel also. The 40A has to be figured into every panel all the way back to the service. In other words, if you back feed a 200A main lug panel first, and that gets feed from a 100A panel, the 100A panel is going to be exceeded by 20A. 
100A x 1.2 = 120A. (not to exceed) You would have exceeded it by 20A. because you have a 40A breaker for the solar


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## Eggyolk (Apr 30, 2021)

HertzHound said:


> _The panel on the left is called an AC combiner. It is combining the two inverter outputs. The reason they have to be combined is because there can only be one point of connection between the utility and another power source.
> Is the inverter output leaving the combiner tied into the sub panel or the service? The one line looks like it’s taped? Without knowing all the numbers it should be connected to one or the other with a 40A breaker._




This is somewhat hypothetical. Let's change the schematic so that the PV combiner panel is connected via subfeed lugs of the 200A subpanel. See pic below. Does it still require a dedicated 40A breaker?



HertzHound said:


> _I don’t know much about the battery systems. Is it some sort of power wall? It doesn’t feed into the utility system, so I do think it counts as another source as far as far as article 705 is concerned?_



Battery inverter is treated a generating source, same as a PV inverter. I have changed it to a PV source for simplicity, since the battery is irrelevant in this example.



HertzHound said:


> _200A (sub panel) x 1.2 = 240A the solar back feed breaker can only be as big as 40A at the sub panel. I don’t know how big the main panel is, but the same 40A plus the breaker feeding it has to be less than 120% of the bus rating of that panel also. _



I want to keep the focus on the 200A subpanel. My understanding is that the 150A breaker from the main panel is treated as the main overcurrent device in relation to the 200A subpanel. Therefore more than 40A of PV current is allowed because there is a 50A difference between busbar rating and 150A breaker (200A - 150A = 50A). This leads to 40A + 50A = 90A of of PV without violating the 120% rule.

However, you are only allowed to use the 120% rule if the power sources are located at the opposite end of the busbars. Does the schematic below still qualify since PV1 and PV2 and utility feed are on one side, and PV3 is on the opposite side?


HertzHound said:


> _The 40A has to be figured into every panel all the way back to the service. In other words, if you back feed a 200A main lug panel first, and that gets feed from a 100A panel, the 100A panel is going to be exceeded by 20A.
> 100A x 1.2 = 120A. (not to exceed) You would have exceeded it by 20A. because you have a 40A breaker for the solar
> _




I understand this, the rule has to be followed all the way up the chain or else it negates the protections.

*







*


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## HertzHound (Jan 22, 2019)

> This is somewhat hypothetical. Let's change the schematic so that the PV combiner panel is connected via subfeed lugs of the 200A subpanel. See pic below. Does it still require a dedicated 40A breaker?


I think it has to have a dedicated 40A breaker. Even if it was a tap on the feeder, it would still need breaker. I don’t think connecting to feed through lugs is the same as a feeder tap in this section. There are different rules for feeder taps and buss connections. 


*



705.12 Point of Connection.

Click to expand...

*


> The output of an interconnected electric power source shall be connected as specified in 705.12(A)or (B).
> 
> *(B)Load Side.*
> The output of an interconnected electric power source shall be permitted to be connected to the load side of the service disconnecting means of the other source(s) at any distribution equipment on the premises. Where distribution equipment, including switchgear, switchboards, or panelboards, is fed simultaneously by a primary source(s) of electricity and one or more other power source(s), and where this distribution equipment is capable of supplying multiple branch circuits or feeders, or both, the interconnecting provisions for other power sourcesshall comply with 705.12(B)(1) through (B)(5).
> ...


So looking at this as a buss type situation, there are four scenarios. The scenario where it wouldn’t matter where you landed PV 1,2 and PV 3 with the normal source is even more restrictive than if they were at opposite ends. So now it’s 125% of the PV plus the breaker feeding the panel can’t exceed the buss rating.
((40A + 40A) x 1.25 ) + 150A = 250A
That exceeds the 200A buss capacity.
I’m not sure PV3, the battery, is really the same as a PV inverter. If it’s a backup battery, it’s not pushing into the system unless normal power is off. So how could a buss be overloaded if it’s acting more like a small load when it charging, not trying to push back into the utility or the other branch loads? Although I didn’t see anything in the code, maybe you can omit the 40A of battery “Interconnected power production sources”?


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## HertzHound (Jan 22, 2019)

Here are the rules for buss bars for those following along. I think this scenario fits (a) best. 




> (3)_Busbars_. One of the methods that follows shall be used to determine the ratings of busbars in panelboards.
> (a) The sum of 125 percent of the power source(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed the ampacity of the busbar.
> Informational Note: This general rule assumes no limitation in the number of the loads or sources applied to busbars or their locations.
> (b) Where two sources, one a primary power source and the other another power source, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the power source(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the ampacity of the busbar. The busbar shall be sized for the loads connected in accordance with Article 220. A permanent warning
> ...


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