Minimum Sized PV system to Upgrade a 125a panel

[cebury]

I know there are several reasons to upgrade a 100 or 125a panel to 200a. But limiting the reasons to support a solar PV install, I'd like help with my own education/understanding of the process used.

QUESTION: What is the maximum size PV system that can be added w/out upgrading my current panel?
From what I understand, the answer depends on several things like total kW of the system, efficiency, inverter output rating, wiring etc. But in reality for a homeowner who is hiring a professional installer to match components up correctly, in such a way that nuisance breaker trips are rare, and it will pass inspection/code compliance in the US (or specifically CA), most of those details can ignored by the homeowner trying to get this answer?

Under this very broad assumption, does that reduce the main consideration to: 1) what size/spec inverter is installed and 2) their current electrical panel spec?


EXAMPLE 1:
A 125amp rated panel means 150amp (maximum for 120% rule) can supply the bus?
Assuming room in said panel, currently installed with (only one) supply to a 100amp main circuit beaker, one additional 50a breaker can be added for back-feed supply from the inverter (Solar PV system)?

A 50a double-pole breaker can then support an inverter rated for max AC output of 40amps @ 240? A Sunnyboy 8000 or smaller (or pretty much all major 6k & 7k inverters) would comply? SunnyBoy 6000 = 25a max@240, 7k=29a, 8k=32a

ANSWER? : Any sized PV system proposed by an installer which includes an inverter of 8k or less would fit?

That just doesn't seem right such a large system would be supported by such a small panel. I must be wrong somewhere here so please correct my limited understanding of this topic.


EXAMPLE 2:
A 200a panel with 200a main breaker installed: Adding a 40amp breaker allows for at most 32A (@240) back-feed from the inverter, i.e. A SunnyBoy 8000 would also be in compliance.




PICS FOR EXAMPLE 1:
125amp rated electrical panel:



It looks like this (below), with the panel cover removed.

Pay no attention to the poorly installed blue donut looking CTs.
The main supply breaker is located just below the center. Notice there is only one more spot to add a double-pole (likely double-throw, ie DPDT) circuit breaker but down at the very bottom there is room for a single-pole.


Below is the POCO supply side.

Pay no attention to the small red-black wires connected to the black clamp looking thing as they shouldn't be there anyway.


Below is the 100amp main circuit-breaker to limit the supply fed-to/drawn-by the system.





I understand that physical "room" must also be available in the panel. Room for 1) additional circuit breaker, 2) for extra cabling inside the panel, 3) room (i.e. access holes) to allow into the panel. Once the labor required is significant to work around a 125a panel, you might as well apply that labor to upgrade to a 200a panel. I also understand there are options to reduce the incoming supply from the POCO, to make more supply available to the breaker for solar (if the home electrical demand can support the reduction).

The visual picture alone makes me want to upgrade it to 200amp, if done by a nice organized anal-retentive electrician.

[Naptown]

Lets take the first example first the 125A rated bus bars with the 100A main.
Yes you can feed 50A into this and here's why.
You can feed those bus bars with a maximum of 120% of their rating or 150 A total.
Because the main is 25A shy of the rating of the bus bars you also get that 25A to use. So a total of 50A can feed that panel. The 100 from the main and 50 from the PV.

In the 200A example the bus bars are rated for 200A with a 200A main. So only 40A can be fed into that panel with the 20% rule. Now replace the 200A main with a 150A main and you can now put 90A into the panel with PV as you gained another 50A by derating the main.
The main rating is not what the calculations are based on it is the rating of the bus bars. You can go to 120% of their rating with the combination of the limiting factors being a combination of the full rated output of the inverter and the capacity of the main breaker.

Lastly the PV breaker needs to be installed at the end of the bus opposite the main location. For example if the main feeds the bus bars at the top the PV breaker needs to be at the bottom of the bus bars.

[cebury]

OK thanks for the confirmation!

You can go to 120% of their rating with the combination of the limiting factors being a combination of the full rated output of the inverter and the capacity of the main breaker.

Isn't there code limiting the (PV back-fed) supply to 80% of the breaker rating? So in the 125amp case, the additional PV 50amp CB, an inspector would only allow a GT inverter with hard-limits of 40amp (50ax.80) continuous AC at 240v?

IIRC the de-rating/limit was to avoid nuisance tripping b/c the stated CB rating was in-lab, open-air setting; the real-world, closed-box panel are only guaranteed not to trip at 80% continuous of their stated rating. I think there are (expensive?) 100% rated CBs that could handle the full rated load w/out tripping in a closed panel.

So for the 200amp with 40amp room on the bus, it must be 32amp max inverter. I recall reading that in an example online from John Wiles.

The whole reason I'm doing this "backwards" is just to have an easy way to answer the question for friends/family "do I have to upgrade my panel?" I'm hoping to only examine 1) their panel bus rating, 2)the main breaker, & 3) the output ratings for the proposed inverter.

Lastly the PV breaker needs to be installed at the end of the bus opposite the main location. For example if the main feeds the bus bars at the top the PV breaker needs to be at the bottom of the bus bars.

Interesting. The panel above has the main connected to the bus in the very center. There isn't any excess wiring to easily relocate to the top/bottom. Wonder what an installer will do if the panel isn't upgraded.

[cebury]

In the panel above it is labeled "RATINGS: 125 Amps. Maximum". It does not explicitly state "bus bar rating". Is it always assumed so?


Thanks again.

[Naptown]

yes it is the bus bar rating That panel was not designed with a main breaker they are back feeding a 100A breaker. now if that breaker has line and load marked on it then it is not approved for back feeding.

[Naptown]

So are the small red wires feeding the grow lights in the basement?

[cebury]

So are the small red wires feeding the grow lights in the basement?

LOL. Legal in CA, pass the dutchie man. But no... it's a CT on my side of the meter, despite what it looks like.