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Tesla Changed Inverter Sizing @ Install

mstgkillr

Member
May 24, 2020
67
47
Cape Coral
Quick update...

I believe the different size inverters were an accident and not worth the hassle of making a big deal out of it, as long as the loads were distributed correctly, which after doing my own calculations I believe they were. If not an accident, maybe just an inventory issue. It could have been worse, they could have tried to squeak in Detlas. Unfortunately, they will have to update the signed and seal plans, which will probably delay the final inspection and PTO.

They paired the inverters with the panels/strings:
  • SE-3800H-US (3.8kW) inverter with one 13-panel string (4.42kW), a 1.16 ratio
  • SE-11400H-US (11.4kW) inverter with three strings consisting of 10, 12, and 15 panels (12.58 kW), a 1.10 ratio
Ideally, I would have preferred two SE7600H-US (7.6kW) inverters, with 25 panels on each one, so if one inverter goes down I only lose 50% production. Currently, I would lose either 26% or 74% of production.

I am waiting for an install similar to the OP. The drawing for my 17kw system calls for the following:

(1) Solar Edge SE7600H-US (7.6kW) inverter with 26 panels at 340W each
(1) Solar Edge SE7600H-US (7.6kW) inverter with 24 panels at 340W each

All panels are facing east and it should be no problem changing the number of panels per inverter. I do have some tree shading issues so most of the time I will not be getting max sun all day long depending on the season.

Is this the best configuration? I was thinking that it might be better to use the next size up inverter (SE-11400-US) with maybe 28 panels and the (SE7600H-US) with 22 panels. If there is a better configuration I'd like to make the change now while it's still in permitting. Thanks for any ideas.

I think your design is ideal!
 

Vines

Active Member
Jul 20, 2018
1,916
2,251
Silicon Valley, CA
This implies rather than spending money to replace my 11.4Km leave alone. And no reason to not do the second the same then?

THanks

120% rule and conductor sizing is the only other consideration to think on. NEC requires you to do 120% rule calculations as if all inverters are running at 125% of maximum output. If you have oversized inverters this will affect the calculations, perhaps a small negative effect depending on the situation.

So for instance a typical 400A busbar on a 400A service can take 64A of inverter output, which looks like 80A after NEC continuous duty multiplier.
80A is the maximum backfeed on a 400A busbar per the 120% rule, so if your PV were to be properly served by (2) 7600 Inverters, using (2) 11.4 inverters for the same application may require downsizing of the main breaker, which was not otherwise required.
 
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h2ofun

Active Member
Aug 11, 2020
2,661
491
auburn, ca
120% rule and conductor sizing is the only other consideration to think on. NEC requires you to do 120% rule calculations as if all inverters are running at 125% of maximum output. If you have oversized inverters this will affect the calculations, perhaps a small negative effect depending on the situation.

So for instance a typical 400A busbar on a 400A service can take 64A of inverter output, which looks like 80A after NEC continuous duty multiplier.
80A is the maximum backfeed on a 400A busbar per the 120% rule, so if your PV were to be properly served by (2) 7600 Inverters, using (2) 11.4 inverters for the same application may require downsizing of the main breaker, which was not otherwise required.
Thanks just sent this info to the engineer as a question
 

CrazyRabbit

Member
Apr 21, 2020
415
126
Fort Worth TX
i believe that is correct vines. which adds the expense of new main breakers.
what about line side vs load side solar connection? he could then connect ~400 amps of solar!
 

holeydonut

Supporting Member
Jun 27, 2020
1,856
1,237
East Bay NorCal
120% rule and conductor sizing is the only other consideration to think on. NEC requires you to do 120% rule calculations as if all inverters are running at 125% of maximum output. If you have oversized inverters this will affect the calculations, perhaps a small negative effect depending on the situation.

So for instance a typical 400A busbar on a 400A service can take 64A of inverter output, which looks like 80A after NEC continuous duty multiplier.
80A is the maximum backfeed on a 400A busbar per the 120% rule, so if your PV were to be properly served by (2) 7600 Inverters, using (2) 11.4 inverters for the same application may require downsizing of the main breaker, which was not otherwise required.


Question... PG&E insisted that the maximum backfeed calculation must also include the Powerwalls alongside the PV. I guess even though the Powerwalls are supposed to be configured as non-grid-export at all times, there's technically nothing physical actually stopping the Powerwalls from pushing power to the grid if the control software fails.

So, if the Powerwalls' internal controls fail and they start grid-exporting at 100%, then your home could have lots of amps backfeeding the grid from the PV at the same time the Powerwalls decide to discharge to the grid. And of course this would blow up the 120 percent rule since you'd backfeed a ton of amps onto the busbar.

Is there actually any documentation out there to explain why this scenario cannot happen?
 

Vines

Active Member
Jul 20, 2018
1,916
2,251
Silicon Valley, CA
Question... PG&E insisted that the maximum backfeed calculation must also include the Powerwalls alongside the PV. I guess even though the Powerwalls are supposed to be configured as non-grid-export at all times, there's technically nothing physical actually stopping the Powerwalls from pushing power to the grid if the control software fails.

So, if the Powerwalls' internal controls fail and they start grid-exporting at 100%, then your home could have lots of amps backfeeding the grid from the PV at the same time the Powerwalls decide to discharge to the grid. And of course this would blow up the 120 percent rule since you'd backfeed a ton of amps onto the busbar.

Is there actually any documentation out there to explain why this scenario cannot happen?

When UL listed it as a Power Control Device this should preclude this sort of backfeed issue you mention, as long as the export control limits are set. In some cases this is not required, in others it is. This is not a simple discussion, and has many dimensions.

There is a white paper that Tesla put out on this, to satisfy AHJ who are concerned. So far when I have passed it along to AHJ they have agreed with the results.
 
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Vines

Active Member
Jul 20, 2018
1,916
2,251
Silicon Valley, CA
i believe that is correct vines. which adds the expense of new main breakers.
what about line side vs load side solar connection? he could then connect ~400 amps of solar!

Practically most 400A services are 2x200 so when you backup the 200A MB only without distribution you avoid the 120% issues in the main panel. Line side tap is a possibility depending on service and the AHJ.
 

CrazyRabbit

Member
Apr 21, 2020
415
126
Fort Worth TX
vines, what do u mean service? i get the AHJ!

could i install a 200 amp panel and load it up with 200 amps of solar generation breakers all on a line connection? or does 120% rule still apply?

I believe i can load a panel, without generation, up with more the then the bus bar ratting... but if i want to install generation on top of and existing panel i'm restricted to sum of breakers...

the Enphase Combiner 3 is limited to 80 amps of solar generation, but has a 125 bus bar. why can't i but 115 amps of generation into that box? is it because it is a plastic housing??
 
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Vines

Active Member
Jul 20, 2018
1,916
2,251
Silicon Valley, CA
Most 400A main service panels are actually a 200A distribution bus, with 200A main breaker, along with a 200A main breaker with no distribution. This second main breaker usually feeds some subpanel in the house. This configuration is commonly referred to as a 2x200. Apologies if I confused by mistakenly calling it the service.

Crazy you can load up a 200A panel with (10) 20A of PV breakers, in that case it would comply with the 100% rule.
 

h2ofun

Active Member
Aug 11, 2020
2,661
491
auburn, ca
Practically most 400A services are 2x200 so when you backup the 200A MB only without distribution you avoid the 120% issues in the main panel. Line side tap is a possibility depending on service and the AHJ.
So, since my 400 amp service has 2 200 amp breakers, which feed 2 200 amp subpanels, if I connect a 11.4K inverter to each GW, is there an issue?
 

CrazyRabbit

Member
Apr 21, 2020
415
126
Fort Worth TX
Crazy you can load up a 200A panel with (10) 20A of PV breakers, in that case it would comply with the 100% rule.
that is what i would think. now for the Enphase Combiner 3 is limited to 80 amp total of breakers for solar generation, but has a 125 bus bar. why can't i but 115 amps of generation into that box? is it because it is a plastic housing??

my meter base is 320 amp, with two 200 amp main panels. my solar has (temporarily, waiting for powerwalls) been connected to my meter base by triple lugging it.
 
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Vines

Active Member
Jul 20, 2018
1,916
2,251
Silicon Valley, CA
Because t
that is what i would think. now for the Enphase Combiner 3 is limited to 80 amp total of breakers for solar generation, but has a 125 bus bar. why can't i but 115 amps of generation into that box? is it because it is a plastic housing??

Because the manufacturer says so, that's how they designed and rated it. It might be like you say something in the UL listing that requires a metal housing or a problem with the number of slots, or wire bend radius problems vs wire entry locations. I do not know the specifics. If you want 115A of PV just use a 125A subpanel.
 

charlesj

Active Member
Oct 22, 2019
1,191
253
Monterey, CA
Enphase also published some findings:
Technical Brief: Why Is My PV Module Rating Larger Than My Inverter Rating? (EN-US)

Since Enphase probably has an interest in people over-sizing their inverters (make more money selling oversized gear), it's notable that they instead show how a 1.2 to 1.4 DC/AC ratio is still optimal.
Enphase sells inverters mostly and now batteries. Yes, they do have Longi panels that are pre-fitted with inverters but they are not in the installation business. They sell to installers and some homeowners like myself for self installations.
So, an installer is the one who has the real financial incentive to install whatever, not Enphase.

I am of the opinion that a system is designed to ones needs, and power company's limitations on generation.
I am also of the opinion that oversizing panels to inverters will generate more power on the long run each day.
 

h2ofun

Active Member
Aug 11, 2020
2,661
491
auburn, ca
You need to share some SLD plans then we can discuss if it works.
Is this what you are asking. This was my first pass and has changed, but no update yet. The solar would be split into both GW's.
The generator connects into both ATS's. The AC disconnects are not shown since they are still trying to agree on where and how many are needed. My house service will be both 200amp breakers now. The sub panels are all 200 amp panels. Does this help at all?
 

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Vines

Active Member
Jul 20, 2018
1,916
2,251
Silicon Valley, CA
Is this what you are asking. This was my first pass and has changed, but no update yet. The solar would be split into both GW's.
The generator connects into both ATS's. The AC disconnects are not shown since they are still trying to agree on where and how many are needed. My house service will be both 200amp breakers now. The sub panels are all 200 amp panels. Does this help at all?

As far as the 120% rule at the main panel you have no concerns, since there is no busbar, hopefully the diagram is accurate. It appears quite complete, so I'd be surprised if that was omitted.

However you do need a 125A or 200A main breaker inside the backup loads panel for system B.

Also, as it stands system B existing 125A subpanel is not 120% rule compliant, though it may be 100% compliant, that is not clear to me from this. One solution is to land the 11.4 inverter for system B inside the dedicated internal subpanel.

Edit: Even with the 200A subpanel it is not 120% compliant, but a 225A subpanel would make it so.
 

h2ofun

Active Member
Aug 11, 2020
2,661
491
auburn, ca
As far as the 120% rule at the main panel you have no concerns, since there is no busbar, hopefully the diagram is accurate. It appears quite complete, so I'd be surprised if that was omitted.

However you do need a 125A or 200A main breaker inside the backup loads panel for system B.

Also, as it stands system B existing 125A subpanel is not 120% rule compliant, though it may be 100% compliant, that is not clear to me from this. One solution is to land the 11.4 inverter for system B inside the dedicated internal subpanel.

Edit: Even with the 200A subpanel it is not 120% compliant, but a 225A subpanel would make it so.
System B is where there are some mistakes. The main breaker will be put back to a 200 amp since with the GW, that would be code.
With the generation panel, code made me put in a 125 breaker. The output of the B GW goes to the garage existing 200 amp panel. Wording is wrong on drawing. The 11.4 inverterw would land is the GW's for both A and B. So, would these changes make this code legal?
 

Vines

Active Member
Jul 20, 2018
1,916
2,251
Silicon Valley, CA
System B is where there are some mistakes. The main breaker will be put back to a 200 amp since with the GW, that would be code.
With the generation panel, code made me put in a 125 breaker. The output of the B GW goes to the garage existing 200 amp panel. Wording is wrong on drawing. The 11.4 inverterw would land is the GW's for both A and B. So, would these changes make this code legal?

There is no reason the GW2 cannot be fed with a 125A breaker. Why run larger wire when the backup subpanel is only 125A

If all the PV is landing in the GW2 then there are no other 120% concerns.
 

uuidgen

New Member
Dec 31, 2020
3
0
Illinois
I also had my inverter downsized during the install last week, but just noticed right now.

It's a fairly small 5.4kW system, and the plans and interconnect application showed a SE7600H-US inverter, but what actually got installed is a SE5000H-US inverter. The smaller size can certainly handle the load in the winter with my panel placement, but perhaps not in the peak generation time in the summer.

Notified Tesla via tech support (since my project advisor is hard to get a timely response from) and asked for it to be fixed to the specified inverter. Inspection was already complete (they didn't catch it, I did afterwards) but ComEd hasn't granted PTO yet.

We'll see what happens - I do expect the inverter to get swapped out given it doesn't match what was on the permit.
 

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