New 7.9 kW System

[Ampster]

We recently purchased a new home in Kenwood CA and the first priority was putting solar on the roof. My house faces Southeast with ocassional shading from some trees and a chimney. I was fortunate to find a local resource that connected me to someone who would draw a set of plans to submit to Sonoma County. We settled on a 7.9 kW system since that was the maximum we could fit on that face of the roof. I found a reasonable price on Mission 345 Watt panels from EcoDirect. Conveniently they had a supplier with a nearby warehouse. The balance of the system I purchased from CED Greentech. We used IronRidge SR1000 rails and their standoffs.
My permit from Sonoma County includes a new main service panel and two subpanels. One of those subpanels will become my essenial loads panel when I reinstall the Outback Skybox I removed from our prior home. The main service panel is a 100 Amp Zinsco panel that is a known fire hazard. Initially I had planned on a new 200 Amp service panel but susequently decided a like for like replacement would be more expedient after consulting my electrician. A like for like replacement allows me to use a 125 Amp service panel in the same location as the original Zinsco. A rule from PG&E would have required me to locate an upgraded panel 36 inches away from an existing natural gas riser. That would have required a new service drop and possibly a transformer upgrade. Those were expenses and delay causing contingencies which I wanted to avoid.
The panel installation was completed last week and I am still waiting for PG&E to pick a date for my electician to do the service panel swap out. I have already wired the two subpanels.

[J.P.M.]

$$$ ?

[Ampster]

$$$ ?

I think it is going to come in at $2.50 per Watt. The only remaining expenses are service panel change out, which I estimate at $2,000 and a few hundred dollars for PGE application. my last bill was $270 and 1.0 mWhrs.

[solarix]

What inverter are you planning to use? 7.9kW of power is too much backfeed for a 125A main panel. The 20% backfeed rule in the code limits the solar inverter to 4.8kW on a 125A panel. A small panel like that is a real detriment to the future sales value of your house. Do whatever it takes to upgrade to 200A. You won't be sorry. And do it as part of the solar project and you can claim the 26% tax credit on that too.

[J.P.M.]

I think it is going to come in at $2.50 per Watt. The only remaining expenses are service panel change out, which I estimate at $2,000 and a few hundred dollars for PGE application. my last bill was $270 and 1.0 mWhrs.

Thank you.

[Ampster]

What inverter are you planning to use? 7.9kW of power is too much backfeed for a 125A main panel. The 20% backfeed rule in the code limits the solar inverter to 4.8kW on a 125A panel. A small panel like that is a real detriment to the future sales value of your house. .........

I am using Enphase microinverters. The inverter capacity is 6.6 kW and I am backfeeding to a subpanel with a 200 Amp bus bar. I am well within the 240 Amps of breakers allowed by the rule as I understand it. That subpanel will probably be fed by a 60 or 80 Amp breaker.
I am not worried about the resale value being affected by a new 125 Amp service panel. The location and neighborhood are the biggest factors affecting value. It is a significant improvement over the 100 Amp Zinsco panel that is there now. My biggest load is a 50 Amp dual electric oven and that goes to another 200 Amp rated subpanel.

[solarix]

Its not enough for the just the subpanel to be 200A. Everything upstream of the backfed solar breaker needs to be 200A to meet code if you have 6.6kW of inverter output.
What happens when the utility is feeding 125A into the main breaker, and then your solar is backfeeding 28A (+25%) into that same panel? (you melt the 125A bus) Of course, typically you will have loads in the subpanel using up the solar power - but the code is based a worst case condition. Even if your particular arrangement will never have a problem like this, the code is clear on backfeeding no more than 20% of panel capacity.

[Ampster]

Its not enough for the just the subpanel to be 200A. Everything upstream of the backfed solar breaker needs to be 200A to meet code if you have 6.6kW of inverter output......

My interpretation and my electrician's interpretation of the rule is that the sum of the breakers cannot exceed 120% of the busbar capacity. I don't think I have exceeded that anywhere in my install. I clarified my original comment by explaining that the solar subpanel will be protected by a 60 Amp breaker. Therefore I don't see how the 125 Amp main service panel busbar could be overloaded. My understanding of the 120% rule is that the sum of the breakers in the main service panel will not be in excess of 120% of 125 Amps. The other subpanel will be fed by and 80 Amp breaker. Therfore the sum of the breakers in the 125 Amp main panel will be 140 Amps which is within the 150 Amp limit of that service panel busbar. If that is a problem I can always get a main panel with a larger busbar and use a 125 Amp main breaker.
If there is anything I can explain in more detail that would help clarify the 120% busbar rule, please let me know.

[solarix]

I think you are mixing code clauses. If you are going by 3(a), the sum of all the breakers is limited to 125A, If you are using 3(b), the breaker feeding the solar subpanel cannot be more than 20% of 125A.
From your description, you have an unusual implementation that the code does not address well. I agree you won't overload the busses, but your design is not code. Been through this stuff many times with my local inspector and this situation will not fly. Maybe your inspectors are less strict....

705.12(D)(2)
(1) Feeders. Where the inverter output connection is made to a feeder at a location other than the opposite end of the feeder from the primary source overcurrent device, that portion of the feeder on the load side of the inverter output connection shall be protected by one of the following:
(a) The feeder ampacity shall be not less than the sum of the primary source overcurrent device and 125 percent of the inverter output circuit current.
(b) An overcurrent device on the load side of the inverter connection shall be rated not greater than the ampacity of the feeder.
(2) Taps. In systems where inverter output connections are made at feeders, any taps shall be sized based on the sum of 125 percent of the inverter(s) output circuit current and the rating of the overcurrent device protecting the feeder conductors as calculated in 240.21(B).
(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 inverter(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 utility and the other an inverter, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the inverter(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

[solardreamer]

I think you are mixing code clauses. If you are going by 3(a), the sum of all the breakers is limited to 125A, If you are using 3(b), the breaker feeding the solar subpanel cannot be more than 20% of 125A.
From your description, you have an unusual implementation that the code does not address well. I agree you won't overload the busses, but your design is not code. Been through this stuff many times with my local inspector and this situation will not fly. Maybe your inspectors are less strict....

705.12(D)(2)
(1) Feeders. Where the inverter output connection is made to a feeder at a location other than the opposite end of the feeder from the primary source overcurrent device, that portion of the feeder on the load side of the inverter output connection shall be protected by one of the following:
(a) The feeder ampacity shall be not less than the sum of the primary source overcurrent device and 125 percent of the inverter output circuit current.
(b) An overcurrent device on the load side of the inverter connection shall be rated not greater than the ampacity of the feeder.
(2) Taps. In systems where inverter output connections are made at feeders, any taps shall be sized based on the sum of 125 percent of the inverter(s) output circuit current and the rating of the overcurrent device protecting the feeder conductors as calculated in 240.21(B).
(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 inverter(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 utility and the other an inverter, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the inverter(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

Assuming 3(b) with a 125A main panel busbar and 100A main breaker, are you saying the max PV breaker is 50A?

If so, that's news to me as my understanding from my electrician has been similar to Ampster's for the 120% rule.

[solarix]

Yes, if the main breaker is 100A, then you get 25A for the reduced main breaker and 20% of 125 for 25A more equals 50A for the solar subpanel.

[Ampster]

Yes, if the main breaker is 100A, then you get 25A for the reduced main breaker and 20% of 125 for 25A more equals 50A for the solar subpanel.

My main breaker will be 125 Amps. Either way I can live with a 50 Amp breaker on the solar subpanel. Most of those circuits are 15 Amp lighting circuits and a few plugs for critical loads.
The second phase of the project, once I get my PTO, will be to add my Outback Skybox to the solar subpanel. It has 60 Amp Breakers internally but I can easily use a 50 Amp breaker in the panel. My biggest concern at that point is the AC coupled limit on the Skybox which is right around 7kW. If needed I can reroute one of the Enphase strings to the other sub panel or the main service panel.

I did not explain my other reason for abandoning the idea of a 200 Amp service panel. PGE has a new rule that the service panel needs to be 36 inches horizontally from the natural gas riser which is now directly below my service panel. I can do a "like for like" replacement without invoking that rule. I had come to terms with installing a new service panel 36 inches further away from the service drop. Unfortunately it might trigger a transformer upgrade, and cause delays because a new service drop would have to be run from the pole. I took the most efficient route to getting a timely PTO. My long term goal is to be more self sufficient and I am not concerned with the limitations of a 125 Amp service panel. I can charge my two EVs (not simultaneously) with that limitation..

[foo1bar]

My main breaker will be 125 Amps. Either way I can live with a 50 Amp breaker on the solar subpanel. Most of those circuits are 15 Amp lighting circuits and a few plugs for critical loads.

So, 125A main breaker, a 125A busbar, a 50A breaker to solar subpanel, and numerous breakers for other branch circuits.

That does not sound code compliant to me.

Is the 50A breaker on a separate connection to the meter from the 125A busbar?
I know there are some panels that have a dedicated breaker spot for solar backfeed. The conductor splits and feeds the main breaker and the backfeed breaker in parallel - so the main bus (off the main breaker) is all loads - no backfeed supplies there.
That's the only way I see it being code compliant.

A 100A main, 125A busbar and 50A backfeed could be compliant under the 120% rule. But you're saying 125A main breaker, so you're not doing that.

[Ampster]

So, 125A main breaker, a 125A busbar, a 50A breaker to solar subpanel, and numerous breakers for other branch circuits.

That does not sound code compliant to me.

I don't have the main service panel yet so I don't know what the busbar rating is. I can always put in a panel with a 200 Amp busbar and downsize the breaker to 125 Amps if there is an issue. What part of the above is not code compliant and why? solarix said it would work. Are you and he making the same assumptions? My two subpanels have 200 Amp busbars but they are going to be fed with breakers less than the busbar rating and sized to not exceed the 120% rule on the main service panel.

A 100A main, 125A busbar and 50A backfeed could be compliant under the 120% rule. But you're saying 125A main breaker, so you're not doing that.

As I mentioned above my electrician is installing the main service panel and he thinks he can install a 125 Amp panel in place of the existing 100 Amp panel. He did a similar changeout for my sister. The conductors from the weather head were increased to accommodate the larger capacity breaker but PGE did not change the the size of the service drop. That system had no solar and was approved by Sonoma County. I will check the busbar rating tomorrow when I am there meeting the inspector for some corrections regarding grounding the water heater.

[foo1bar]

I don't have the main service panel yet so I don't know what the busbar rating is. I can always put in a panel with a 200 Amp busbar and downsize the breaker to 125 Amps if there is an issue. What part of the above is not code compliant and why?

Busbar is 125A
125A * 120% = 150A.
If the main breaker feeding that is 125A and the backfeed breaker is 50A, that is a total of 175A.
175A is more than 150A, therefore not code compliant.

And I thought you could not do a 200A busbar panel, because you are trying to do a like-for-like swap of the panel and a 200A busbar panel (even with a 125A main or 100A main) wouldn't meet PGE's rules for that.

FWIW, I would probably put in the 200A main panel - even if it meant that I had to relocate the main panel and pay a few hundred to PG&E for the upgraded 200A service

solarix said it would work.

He did not.
He said *IF* it's a 100A main breaker with 125A busbar.
In that case it would be
125A * 120% = 150A
100A main and 50A backfeed is total of 150A
150A is equal to (or less than ) the 150A from the first step, therefore it's OK.

Are you and he making the same assumptions? My two subpanels have 200 Amp busbars but they are going to be fed with breakers less than the busbar rating and sized to not exceed the 120% rule on the main service panel.

What are the subpanels?
Do you only have a main breaker, a breaker for the subpanel (for the branch circuits) and a breaker for the backfeed?
IF that's the situation, then IF all the breakers on the busbar sum up to less than (or equal to) the main breaker, I think you would be compliant.
BUT that option for being code compliant wouldn't be using the 120% * main breaker - it would be just: Is main_breaker >= (sum_of_all_other_breakers)? (And more importantly busbar >= sum_of_all_other_breakers)
This approach is the "(3)(c)" section in the code.

Basically there are 3 options for having it be code compliant and using the busbar, spelled out in 705.12(D)(2).
You can do 3a, 3b, OR 3c.
But you can't use part of the phrasing from 3b and part from 3c.
The AHJ is the real authority though - so if they say it's OK, they have final say.
I think they'd look at it and if it's not meeting 3a, 3b, nor 3c it's not passing inspection (or plan review if they do a plan review beforehand)
And right now I believe what you've described doesn't meet the 3a, 3b, nor 3c.
But I may not have understood your description correctly.


So - back to the 200A panel.
I would upgrade because
A> You effectively get a 26% (solar tax credit) discount on changes like this that you can show are related to installing solar.
B> a 200A service is worth a slight amount more when it's time for resale.
C> 200A service is larger conductors to your house (I used to have the lights dim a little when my AC kicked in)

The downside is that the 200A panels may cost a tiny bit more. (Sometimes they don't - economies of scale and all)
The cost for PG&E is probably only a bit more. If they want to charge you a significant amount to upgrade the transformer, I'd probably stick with a smaller service, but still upgrade the panel.
The current main panel location can still be used as a big junction box, so you wouldn't need to pull new wires inside the walls - just run a conduit with some wires from the new main panel to the box where the old main panel was (which now has no breakers - just a bunch of wirenuts connecting the wires from the new breakers to the wires going to the outlets/switches/etc.)

It sounds like you have overhead service - so having wiring capable of 200A up to the weatherhead probably isn't going to be that costly for you. Not like if you have to trench in a new larger conduit if you had underground service.