Powerwall and Solar PV Install with Generation Meter Adapter Mismatch

[RoBoRaT]

My local PW and Solar PV system contractor submitted for permit to install, but was flagged down because of the Generation Meter Adapter vs Main Service Panel rating limitation per Southern California Edison:

Single Line Diagram Deficiency: Main Service Panel is 225A. For GMA MSP can be 200A or Less.
Please refer to section 5.4.1.1 of NEM Handbook.

The proposed system is as shown in the attached plan, basically:

A. 11.725 kW Solar PV (35 LG 335 panels) and 2 Powerwalls

a. 8.04 kW PV to SE7600 inverter on back up side with 2 Powerwalls, and Essential Load
b. 3.68 kW PV to SE3800 inverter with GMA

- Now, my installer wants to scrap the GMA, and reduce the solar system to 32 panels (10.72 kW) in ONE SE7600 inverter - oversizing DC/AC ratio of 1.41.

- Upgrading service to 400A is projected to cost upwards of $12K - so that is a 'no go'.

1. Are there any alternative solution to be able to keep the original plan?
Like, instead of upgrading to 400A service - can we "downgrade" to 200A to meet the GMA vs MSP limit without breaking 120% rule?

2. I asked to cut the number of panels to 30 (10.05 kW) in one SE7600 inverter (DC/AC ration 1.32) - is this better/worse than DC/AC ratio of 1.41?

All panels are on 2nd story roof oriented West at 260 degrees with 23 degree tilt, no shading, lots of sunshine with summer temp 90-110 and 30-60 dry winter in SoCal Inland Empire.


I do not know much about electrical systems at all, so any insight is welcome.
Thanks for any feedback.
RBRT

 

[wwhitney]

Your one-line shows an existing panel with a 225A bus and proposes to put a 175A main breaker in it. My main question is why SCE is calling this arrangement a 225A main panel, it would be more conventional to call it a 175A main panel.

If you are lucky, resolving this could be as simple as changing your one-line to call the panel "175A main panel: existing panel with new 175A main breaker" or something like that. Put the note about the 225A bus elsewhere. I.e. the SCE reviewer might have looked only at the text "Existing 225A panel" and stopped reading before getting to the part about the 175A main breaker.

A related point: what is the rating of your existing main breaker and of your meter? If those are both 200A, then that also shows that your panel should not be called a 225A panel.

Failing that, you could put a 150A main breaker in the main panel, if your load calculation for the house says that is adequate. Then you wouldn't need the GMA, and could land the extra solar in the main panel.

Cheers, Wayne

 

[Musterion]

@RoBoRaT:

Can I ask who is your installer? I looked over your plans and with a 225A main panel it doesn’t seem like you are pushing things by adding a 12 kW solar generator plus 2 powerwalls, such that it would require upgrade to 400A service. In my case (existing 15kW solar connected to main 200A panel via 80A breaker) and adding 4 powerwalls (30A breaker each), Tesla opted to intercept the load-side after the meter, send through backup gateway, and add a new 200A “generation panel” connected to Set #1 of load-side lugs on the gateway. My original main panel was re-connected to Set #2 of load-side lugs on the gateway. Here are some comments/questions:

1) As I understand it, the GMA program in SCE territory is optional and is for supply-side connections. It has some rules (https://www1.sce.com/wps/wcm/connec...n_Meter_Adapter_1478027630_AA.pdf?MOD=AJPERES) which include 200A max panel, which yours exceeds. So why not interconnect in different way without the meter, like load-side, as is done for part of your solar system design here?

2) That said, what is your design constraint or reasoning for having part of the solar connect to supply-side and the rest (plus batteries) to load side)? (i.e. why not all solar plus batteries on load side)? I think I am missing something about your constraints.

3) You asked about derating. Note they are already derating your main breaker from 225A to 175A and then adding a new 100A breaker for solar + powerwall on load side. They justify by showing generation output plus new main breaker (175A) is < 270A (120% rule based on 225A main panel rating).

4) Regarding the inverter sizing, the inverter you quote allows up to 155% DC/AC oversizing according to their spec sheet (https://www.solaredge.com/sites/default/files/inverter_dc_oversizing_guide.pdf). The higher this ratio the more clipping you will get on sunny days. The tradeoff is that on less sunny days the conversion may be more efficient. Perhaps someone else can comment on how significant this clipping will be for your solar specs. (Mine has nearly no clipping so I don’t have the experience here to comment on large oversizing).

Good luck.

 

[RoBoRaT]

@RoBoRaT:

Can I ask who is your installer? I looked over your plans and with a 225A main panel it doesn’t seem like you are pushing things by adding a 12 kW solar generator plus 2 powerwalls, such that it would require upgrade to 400A service. In my case (existing 15kW solar connected to main 200A panel via 80A breaker) and adding 4 powerwalls (30A breaker each), Tesla opted to intercept the load-side after the meter, send through backup gateway, and add a new 200A “generation panel” connected to Set #1 of load-side lugs on the gateway. My original main panel was re-connected to Set #2 of load-side lugs on the gateway. Here are some comments/questions....

Installer is Baker Electric Home Energy

I will ask if they can put all PV on the load side like you have/described. Thanks for the input...I hope that can be done like yours.

I dont know the another reason why GMA route is used other than to meet 120% rule not to exceed 270A with 225A BUS rated Main Panel as shown in the plan.

If putting all 35 panels to the load side is not feasible, I will do 30 panels (10.05 kW) on SE7600.

.

 

[RoBoRaT]

Your one-line shows an existing panel with a 225A bus and proposes to put a 175A main breaker in it. My main question is why SCE is calling this arrangement a 225A main panel, it would be more conventional to call it a 175A main panel. ....

Wayne,

The existing Main Breaker is 200A; not sure about the meter. Several pages in the plan called the Main Panel rated at 225A.

I like the idea of derating the Main Breaker to 150A.
This is the resulting calculation:

By putting the SE3800 inverter (16A Output) to the 200A Protected Load panel and derating the Main Breaker to 150A

Then:

(32A SE7600 inverter + 41.66A 2 Powerwalls + 16A SE3800 inverter ) x 125% + (150A Main Breaker) = 262 A

and: 120% of BUS 225A = 270A

so: 262 A < 270 A

Will this work? Any problem derating the Main Breaker to 150A?

Attached is the house load...

 

[wwhitney]

Yes, the problem is that your load calculation came out to 173 amps, so a 150 amp main breaker wouldn't be sufficient. [It practice, it probably would work, as the NEC is very conservative, but per the rules it won't work.] So scratch that idea.

Do you have an "all-in-one" where a single box has your meter, main breaker, and many/all of your load breakers? If so, your options are pretty limited. Intercepting the wiring between the meter and the main breaker is generally not possible on all-in-one service panels.

Cheers, Wayne

 

[RoBoRaT]

Yes, the problem is that your load calculation came out to 173 amps, so a 150 amp main breaker wouldn't be sufficient. [It practice, it probably would work, as the NEC is very conservative, but per the rules it won't work.] So scratch that idea.

Do you have an "all-in-one" where a single box has your meter, main breaker, and many/all of your load breakers? If so, your options are pretty limited. Intercepting the wiring between the meter and the main breaker is generally not possible on all-in-one service panels.

Cheers, Wayne

No, I don't think I have an all in one.

But, if I remove the 50A NEMA 14-50 outlet (I had installed as back up for my Wall Connector) in the Garage - the total load will be 146.4A.
Is this OK then?

 

[wwhitney]

In my case (existing 15kW solar connected to main 200A panel via 80A breaker)

How was that NEC compliant? If your main panel had a 200A bus, a 200A main breaker, 15kW of inverter backfeed, and multiple load breakers, it doesn't meet the 120% rule or any of the other options.

and adding 4 powerwalls (30A breaker each), Tesla opted to intercept the load-side after the meter, send through backup gateway, and add a new 200A “generation panel” connected to Set #1 of load-side lugs on the gateway. My original main panel was re-connected to Set #2 of load-side lugs on the gateway.

Presumably your solar connection was moved to the generation panel, and the backup gateway has a 200A main breaker? If so, that's a good solution.

So why not interconnect in different way without the meter, like load-side, as is done for part of your solar system design here?

11.7 kW of solar is more generation than 2 Powerwalls can handle, so with a blackout, full solar production, and minimal loads, the Powerwalls might have to shut of all the solar. Moving a little of it outside of the Backup Gateway avoids this problem, and it helps with the 120% rule, too.

Cheers, Wayne

 

[wwhitney]

No, I don't think I have an all in one.

In that case, an install like Musterion's seems like a good option.

But, if I remove the 50A NEMA 14-50 outlet (I had installed as back up for my Wall Connector) in the Garage - the total load will be 146.4A.
Is this OK then?

That 50A/240V load has a demand factor of 40% applied, so removing it would only reduce the calculated load by 20A, putting you at 153A. You'd need to tweak things a little more to get down to 150A.

Cheers, Wayne

 

[RoBoRaT]

In that case, an install like Musterion's seems like a good option.


That 50A/240V load has a demand factor of 40% applied, so removing it would only reduce the calculated load by 20A, putting you at 153A. You'd need to tweak things a little more to get down to 150A.

Cheers, Wayne

Yup, I see 153A after getting the calculations right.
OK, I will have to see what can cut down.

Can we increase (derate from 225A)) the Main Breaker to 160A vs 150A?

 

[RoBoRaT]

Yup, I see 153A after getting the calculations right.
OK, I will have to see what can cut down.

Can we increase (derate from 225A)) the Main Breaker to 160A vs 150A?

I just realized 160A will exceed 120% rule.

So, how about using 155A main breaker?
@ 155A main breaker: 267.5A < 270A

 

[Vines]

Main breakers only come in 150-175 and 200A sizes. There is no middle ground

 

[RoBoRaT]

Main breakers only come in 150-175 and 200A sizes. There is no middle ground

I see; then 150A it is.

By reducing the Wall Connector to 50A and removing the 50A NEMA 14-50 the load went down to: 143.05 A

Sub-Total Load: 24993.0 VA

First 10K: 10000.0 VA
Remaining X 40%: 5997.2 VA
Heating/AC: 6336.0 VA
Wall Connector (50A): 12000.0 VA

Total Load: 34333.2 VA = 143.05 A

Is this calculation correct? I am using calculators from the Googlelator

So, I can ask for derate the Main Panel to 150A.

Any other potential show stopper for this set-up?

 

[Musterion]

How was that NEC compliant? If your main panel had a 200A bus, a 200A main breaker, 15kW of inverter backfeed, and multiple load breakers, it doesn't meet the 120% rule or any of the other options.

Only 40A of additional load on that panel. Is feedthru panel at service entrance feeding other building.

Presumably your solar connection was moved to the generation panel, and the backup gateway has a 200A main breaker? If so, that's a good solution.

Correct, generation panel had relocated solar circuit plus powerwalls. I still think this is best option for OP.

 

[RoBoRaT]

Thanks all for your insight. I will ask my installer to derate my MP to 150A - and to be able to keep 11.7 kW Solar split to two inverters (24 LG335 panels/SE7600 and 11 panels/SE3800).

I would have to remove the 50A NEMA 14-50 and derate my HPWC to 50A to decrease the load to 143A.

Again, thanks all!
RBRT