New PV deployment - Main and Sub Interconnects 120% and 125%

LATTC Student

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Location
SoCal
Occupation
Student
Just deployed a small Enphase 1.5KW system on top of my shed at home in SoCal as Phase I of a larger deployment ~6KW.

FWIW: Currently using (5) IQ7+ (1.21 Max A/ea) and (5) 355W JA Solar panels. I have the Enphase Combiner Box 3.

Main panel is GE 100A/125A (breaker/Bus rating) and shed subpanel is the same rating (GE Gold Load Center 100A/125A). I have the main lug of the Combiner box connected to a 50A breaker in the bottom left slot in the subpanel (a throw switch shutoff box in between the two boxes). The subpanel main lug is fed from the Main Panel at my primary dwelling. The breaker, feeding the sub, at the main panel is also a 50A breaker (making it a 50A subpanel I guess). This breaker is almost right next to the main 100A service disconnect breaker - near the top of L2.

So I have two questions. If I understand the basic calc of the 120% - since my main and sub are 100/125, then i should be able to have a 50A "inverter" breaker at the sub (125*1.2-100=50A). Is it still required that I keep that 50A breaker at the bottom of the subpanel? Additionally, I am guessing that I would need to move my 50A breaker at the main panel to the bottom most right slot, correct?

I figure I have actual max current rating right now (@125%) from PV of ~7.5A ((5*1.21)*125%). Eventually that will quadruple to ~30A max (with a little room ~20A for growth). I saw a post elsewhere that the PV inverter breaker sizing had to be under the service breaker rating plus the 125% PV max A. So that would mean 107.5A (100A+7.5A) is under the 150A (125A*1.2) rating? I could scale to ~33 panels theoretically (100+(33*1.21)*125%) Seems like two different rules (120%/125%). Not sure which applies

To summarize the two main questions:
1) Do I need to move that main breaker (50A feeding sub) to the bottom most slot on the right bottom of main?
2) Is it ok to use a 50A PV "inverter" breaker in the sub if the main AND sub are both 100A/125A service/bus rated?


Note: I have the Sense consumption CT's in my main panel and at my sub. The (total) main peak KWh never goes over 5.5KWh (~45A) even with HVAC, etc. . .. The Sub never peaks over 1KW (7.7A) though that load is continuous.
 
Last edited:

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
The locations of the breakers do not matter because your total utility side current plus 125% of inverter output does not exceed your busbar ratings. If you were to expand the solar system to output more than 20A (25A after mutliplying by 125%), then you'd be required to move the subpanel breaker to the opposite end of the main panel.

Yes, you are allowed up to 40A of PV - 50A breaker - because that complies on both panels. Technically the breaker size is irrelevant. What matters is 125% of inverter nameplate continuous output current.
 

LATTC Student

Member
Location
SoCal
Occupation
Student
The locations of the breakers do not matter because your total utility side current plus 125% of inverter output does not exceed your busbar ratings. If you were to expand the solar system to output more than 20A (25A after mutliplying by 125%), then you'd be required to move the subpanel breaker to the opposite end of the main panel.

Yes, you are allowed up to 40A of PV - 50A breaker - because that complies on both panels. Technically the breaker size is irrelevant. What matters is 125% of inverter nameplate continuous output current.
Awesome Jagged Ben and greetings to my Northern Brother! Thank you, it is perfectly clear to me now as it applies to my current (and future) install(s).
 

LATTC Student

Member
Location
SoCal
Occupation
Student
The locations of the breakers do not matter because your total utility side current plus 125% of inverter output does not exceed your busbar ratings. If you were to expand the solar system to output more than 20A (25A after mutliplying by 125%), then you'd be required to move the subpanel breaker to the opposite end of the main panel.

Yes, you are allowed up to 40A of PV - 50A breaker - because that complies on both panels. Technically the breaker size is irrelevant. What matters is 125% of inverter nameplate continuous output current.
So it appears our Phase II will happen sooner rather than later. I purchased 8 more panels today for a total of 13. These are each running an IQ7+ microinverter (@1.21A Max each). The manuf designed 13 as max per breaker to be just below a 16A "continuous" (*125%=20A OCPD). Still under the two requirements you mentioned above. Great!

However, the second part of this Phase II expansion includes the purchase and installation of their Encharge 10 battery (10KWh, 3.84KVA, but more importantly 15.99A Max). The manuf combiner box (PV Panel) we have is designed to handle just one of these batteries on a dedicated 20A circuit (again they designed this battery just under 16A "continuous"). So now I really will hit the 40A max. The panels and the breaker sizing are fine at as they are, as you mentioned, but I would need to move the subpanel feeder breaker to the bottom now, correct?
 

LATTC Student

Member
Location
SoCal
Occupation
Student
I think I may have the answer after looking at your answer (and the code) again:

From Jade Learning (2014 NEC) -

For interconnection to a busbar, there are 3 new options that will be commonly encountered:
Option one, 705.12(D)(2)(3)(a) : 125% of the inverter output circuit current plus the rating of the OCPD protecting the busbar cannot be greater than the ampacity of the busbar. When using this option, there is no restriction on where the backfed PV breaker must be located in the panelboard.
Option two, 705.12(D)(2)(3)(b) : 125% of the inverter output circuit current plus the rating of the OCPD protecting the busbar cannot be greater than 120% of the ampacity of the busbar. This method can only be used where a service main breaker is at one end of the busbar and the PV breaker is at the opposite end
Option three, 705.12(D)(2)(3)(c) : First, the sum of the ampere ratings of all OCPD’s on the panelboard, both load and supply devices, excluding the rating of the OCPD protecting the busbar, shall not exceed the ampacity of the busbar. In this method, we are simply adding the face value of the OCPD’s in the panelboard excluding the main. Second, the rating of the OCPD protecting the busbar shall not exceed the rating of the busbar. If this method is used then there is no restriction on where the backfed PV breaker must be located in the panelboard.

Doing the math, looks like I fall onto option 2, Here are the calcs:

Option 1: 125%*32A+100A = 140A. The Busbar Amp rating (BAR) is 125A. 140A>125A So this is not an option for me
Option 2: 125%*32A+100A = 140A. (BAR)*1.2 = 125A*120%= 150A. 140A<150A so I can have both PV/battery circuits if I move the breaker per the code705.12(D)(2)(3)b
Option 3: Forget it! Too much to calc.
 
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