How do I calculate 120% rule with a Main Lug and sub-panels?

[dapug]

I'm installing a 15kW system using IQ7X micro inverters. If anyone is familiar with electrical, I'd love some tips to get my design right.

Here is my existing system:
Screen Shot 2019-10-01 at 9.19.23 AM.png

Here is my proposed design:
Screen Shot 2019-10-01 at 9.19.35 AM.png

In this case, it's not really a tie-in to the main house panel (2), but rather, the service lug on it's own branch, independent from the house load.

My calculations (if correct?):
125A bus (box 3) * 1.2 - 70A Main feed (box 1) = 80A max PV size. My PV is 62.5A, so a 70A breaker is shown in box 3, and is less than 80. Furthermore... the TOTAL of all breakers in box 3 = 150A, which is 120% of the 125A bus in that box.

Given that this sub--panel (3) is on the house, it doesn't have a main breaker. But I assumed I still have to use the value of its feed breaker from box 1 in the calculation. I haven't seen much detail about that scenario online, so that is my main question.

Questions:
-Did I follow the 120% rule correctly, or miss the mark?
-Is a 70A breaker in panel 1 correct?

If I got this wrong, I have another design that adds a dedicated panel for PV. The main thing I'd like to avoid is a new service/meter main panel (1). If I have to upgrade panel 3, or must add a dedicated new panel, I'd rather go that route.

[foo1bar]

70A doesn't sound right for your PV breaker if it's a 15kW system.

Are you planning to replace the wires from the main to your HOMC12UC panel? If not, seems like you're replacing a 60A breaker with a larger one and therefore won't have the correct protection for those wires.

[dapug]

Good point. I think the wires to that panel were oversized when put in but I’ll verify.

15,000/240v = 62.5A. So 70A breaker too small?

[foo1bar]

Good point. I think the wires to that panel were oversized when put in but I’ll verify.

15,000/240v = 62.5A. So 70A breaker too small?

I think so

http://www.smainverted.com/properly-sizing-a-pv-inverter-breaker/

What's the documents for your inverter say?

BTW - have you checked if it's OK to downsize the breaker feeding panel 4?
I don't know what the rules are for minimum size of feeds to panels in a detached garage, but there probably is one.

[ButchDeal]

15,000/240v = 62.5A. So 70A breaker too small?

What inverter(s) are you using? The breakers are NOT based on the size of the array but the size of the inverters. 15kW is going to be two inverters, likely two 7.6kW inverters (40a each) but could be smaller if the array is shadowed or on multiple azimuths.

[dapug]

41 Enphase IQ7X on 360W panels comes out to 14.7kW on configurators I’ve used, such as solardesigntool. I rounded up to 15kW, and I suppose I could put in an 80A to back feed it if 70A is pushing it at 61-62A max continuous.

Another option I have is to put a new 125A sub panel (5) off that main (1), backfeed solar into that with 80A, but then it would need to also feed the attached garage (3) at 60A since said new panel would be using the only other available slot in panel 1. Still leaves me the question of what size breaker to use in panel 1 (a 125A box I don’t want to upgrade unless absolutely necessary).

Screen Shot 2019-10-02 at 8.53.09 AM.png