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Expanding My 6.0kW System - 4.08kW + 2x PW

wwhitney

Member
Nov 2, 2017
981
1,342
Berkeley, CA
Some local jurisdictions including CPA were until recently ruling that each Powerwall needs a bladed lockable disconnect and citing 2017 NEC 705.22 as the reason.

This interpretation is still being used in at least one other jurisdiction, for any ESS more than 5' from the breaker it is served by. I am working on writing a white paper to describe this question, and get various smart people's opinion on the matter while I have access to a bunch of smart inspectors at the IAEI meeting.
It's not going to change any AHJ's behavior, but there's nothing in 705.22 requires a bladed disconnect versus a circuit breaker, and nothing in 705.22 refers to a 5' requirement.

Cheers, Wayne
 

jsurpless

Member
Feb 20, 2020
107
10
Massachusetts
It is reasonable to put a single lockable AC disconnect in to do all ESS and PV disconnection at once. The issue with a large firm like Tesla is the designers want to make it easy across the country with various codes. The approach Tesla is taking is to just throw up a bunch of disconnects and it will never be wrong, but there are also more correct approaches, which use less equipment.

Make your request known to Tesla that you want as few disconnects as the code allows. If you aren't satisfied with the response, try a smaller installer. They will cater to your local market, and charge you more likely for the personal service.
I have informed Tesla of my desire and they have told me that they are adhering to the local code. Per my discussion with the town wiring inspector, they are not correct but they are unyielding in their position. Honestly, I am not sure how true this is or how good of a job they are doing.

They moved the PowerWalls to my garage without even consulting me as they believed they couldn't be in the basement. When I informed them that the basement was not in living space and that there was sufficient space, they said OK but just today I received a nearly $1K increase for "conduit run". I requested an explanation for this as I have already accepted a design with the panels on the garage roof and the remaining equipment on the opposite side of the house.
 

Vines

Active Member
Jul 20, 2018
2,078
2,495
Silicon Valley, CA
I have informed Tesla of my desire and they have told me that they are adhering to the local code. Per my discussion with the town wiring inspector, they are not correct but they are unyielding in their position. Honestly, I am not sure how true this is or how good of a job they are doing.

They moved the PowerWalls to my garage without even consulting me as they believed they couldn't be in the basement. When I informed them that the basement was not in living space and that there was sufficient space, they said OK but just today I received a nearly $1K increase for "conduit run". I requested an explanation for this as I have already accepted a design with the panels on the garage roof and the remaining equipment on the opposite side of the house.

The current code enforcement is pretty messy so I can see them taking the conservative approach with regard to disconnects. There are other things to consider besides just is a living space, such as whether 100% of the walls and ceiling are currently finished with sheetrock or not.
 

h2ofun

Active Member
Aug 11, 2020
3,046
661
auburn, ca
The current code enforcement is pretty messy so I can see them taking the conservative approach with regard to disconnects. There are other things to consider besides just is a living space, such as whether 100% of the walls and ceiling are currently finished with sheetrock or not.
It was SO SO nice working with 3rd party suppliers that provided real time personal service. Worth every extra dollar.
 
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jsurpless

Member
Feb 20, 2020
107
10
Massachusetts
The current code enforcement is pretty messy so I can see them taking the conservative approach with regard to disconnects. There are other things to consider besides just is a living space, such as whether 100% of the walls and ceiling are currently finished with sheetrock or not.
The basement is completely unfinished - my biggest frustration is them just doing things without consulting me.
It was SO SO nice working with 3rd party suppliers that provided real time personal service. Worth every extra dollar.
I am starting to lean in this direction
 
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Vines

Active Member
Jul 20, 2018
2,078
2,495
Silicon Valley, CA
The basement is completely unfinished - my biggest frustration is them just doing things without consulting me.

I am starting to lean in this direction
The basement would have had to be fully finished to install the ESS in that location, at least around here. Not sure what Mass. is doing with code but I understand them to be forward looking. No surprise the ESS cannot be in an unfinished basement, same here as of July 1 2021 in most jurisdictions.
 

jsurpless

Member
Feb 20, 2020
107
10
Massachusetts
The basement would have had to be fully finished to install the ESS in that location, at least around here. Not sure what Mass. is doing with code but I understand them to be forward looking. No surprise the ESS cannot be in an unfinished basement, same here as of July 1 2021 in most jurisdictions.
Interestingly, I think it's the exact opposite in MA - my mom has a completely unfinished basement and just had hers installed last October.

Also, regarding the various shading potential on that roof, would it be wise to use power optimizers? My existing solar system uses them, even though there is no shading issue (aside from clouds).
 

Vines

Active Member
Jul 20, 2018
2,078
2,495
Silicon Valley, CA
Interestingly, I think it's the exact opposite in MA - my mom has a completely unfinished basement and just had hers installed last October.

Also, regarding the various shading potential on that roof, would it be wise to use power optimizers? My existing solar system uses them, even though there is no shading issue (aside from clouds).
Power optimizers are probably not going to bring a benefit commensurate with the cost.
 

jsurpless

Member
Feb 20, 2020
107
10
Massachusetts
My understanding that the Tesla 3.8 kW inverter having 2 MPPTs means that it's capable of supporting two separate strings of panels.

Assuming that this is correct, what do folks think of insisting that Tesla put the potentially shaded panels on a separate string from the ones that have no risk of shade?
 

jsurpless

Member
Feb 20, 2020
107
10
Massachusetts
I spoke with Tesla more about my shading concerns and was told that the inverter having two MPPTs meant that it was capable of managing the solar panels more efficiently and compensating for my shading concerns. All this despite the fact that all 12 panels are wired in series with each other.

Thoughts?
 

wwhitney

Member
Nov 2, 2017
981
1,342
Berkeley, CA
I spoke with Tesla more about my shading concerns and was told that the inverter having two MPPTs meant that it was capable of managing the solar panels more efficiently and compensating for my shading concerns. All this despite the fact that all 12 panels are wired in series with each other.
Negative.

If the panels are wired into two separate series strings, and both strings are brought to the inverter and attached to the two separate MPPT inputs, then the inverter can do that. So putting the sometimes-shaded panels on one string and the unshaded panels on another string would work, if that is possible while complying with the minimum string voltage.

Cheers, Wayne
 

jsurpless

Member
Feb 20, 2020
107
10
Massachusetts
Negative.

If the panels are wired into two separate series strings, and both strings are brought to the inverter and attached to the two separate MPPT inputs, then the inverter can do that. So putting the sometimes-shaded panels on one string and the unshaded panels on another string would work, if that is possible while complying with the minimum string voltage.

Cheers, Wayne
OK, so I was correct... the key thing for me to understand is that each MPPT is basically a string connection input. The inverter is capable of operating each string as optimally as its component panels allows independently of whatever other MPPT/strings are connected.
 

jsurpless

Member
Feb 20, 2020
107
10
Massachusetts
Negative.

If the panels are wired into two separate series strings, and both strings are brought to the inverter and attached to the two separate MPPT inputs, then the inverter can do that. So putting the sometimes-shaded panels on one string and the unshaded panels on another string would work, if that is possible while complying with the minimum string voltage.

Cheers, Wayne
The more that I think about what I was told by Tesla, the more I am not sure what to make of it.

I specifically asked if my understanding of how MPPTs and strings worked was correct, in that it was as you described here. He replied that their "proprietary technology makes it work better than a traditional string inverter".

I see two possibilities - either the project advisor doesn't understand how their technology is being applied OR I was deliberately misled.

Not sure what, if anything, to do about this. Thoughts?
 

yblaser

Member
Aug 4, 2018
87
61
South Bay Los Angeles
I don't have any real world experience here so take this is you may. I don't think a 2nd string is really going to help unless you can make a string of panels that are uniformly shaded throughout most of the day. I think a shorter string with a higher percentage of shaded panels is more likely to affect production of the unshaded panels in the same string. On a single 12 panel string the shaded panels will be bypassed and should not affect the other panels' production.
 

jsurpless

Member
Feb 20, 2020
107
10
Massachusetts
I don't have any real world experience here so take this is you may. I don't think a 2nd string is really going to help unless you can make a string of panels that are uniformly shaded throughout most of the day. I think a shorter string with a higher percentage of shaded panels is more likely to affect production of the unshaded panels in the same string. On a single 12 panel string the shaded panels will be bypassed and should not affect the other panels' production.
How are the shaded panels bypassed? Is that a function of MPPT that I'm not understanding? Thanks
 

wwhitney

Member
Nov 2, 2017
981
1,342
Berkeley, CA
How are the shaded panels bypassed? Is that a function of MPPT that I'm not understanding? Thanks
Look up bypass diodes.

Yblaser has a good point, if the shading is just from the higher roof next to the solar panels, then a single string may perform fine. When the panels on the right are totally shaded, they will just drop out of production with a small negative impact on the production of the unshaded panels.

If you split the long string into two strings, with all the sometimes-shaded panels in one string, that could be worse. A string needs a certain number of unshaded panels in it to produce; if the sometimes-shaded string drops below that, any remaining unshaded panels in that string will stop producing.

I think a scenario where separate strings and MPPT inputs for panels on the same roof plane would be useful would be when, say, half of the panels are sometime subject to 50% shading. Then I think a separate MPPT input for the 50% shaded panels could more optimally generate energy from them. Not sure.

@nwdiver has thought more about this more than I have.

Cheers, Wayne
 
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yblaser

Member
Aug 4, 2018
87
61
South Bay Los Angeles
Each panel has one or more bypass diodes (I don't know how many the Tesla panels have). The string is wired in series so each panel needs to carry the same amount of current as all the other panels. The maximum amount of current the panel can support is a function of the illumination of the panel. If you are trying to "push" more current through the panel than it can generate you get a reverse voltage on the panel and the current will flow through the bypass diode instead. The bypassed panel will then not produce any power. If you didn't have a bypass diode on the panel you would actually start to dissipate power into the panel and it could catch fire so they are a safety device too.

The MPPT has some ability to compensate for small differences in illumination or panel performance by adjusting the voltage/current of the string to maximize the power out of the string. That is it could pick a point were one or more panels are producing slightly less than their optimal output power but by keeping other panels from being bypassed the string as a whole produces its optimal output power given the conditions.

This all just theoretical knowledge and I don't have any real world experience here so I could always be missing some of the finer details.
 

nwdiver

Well-Known Member
Feb 17, 2013
8,354
11,418
United States
How are the shaded panels bypassed? Is that a function of MPPT that I'm not understanding? Thanks

Every panel has 3 bypass diodes or 6 if it's a newer 'half-cell' with a line in the middle. Each panel is 3 cell groups separated by a bypass diode that acts like a check valve. If the cell group is in full sun voltage rises so 'pressure' is higher downstream and 'holds' the bypass diode closed so current can't flow. If a few cells are shaded voltage across the cell group actually drops since the shaded cells are now a load. This allows the bypass diode to 'open' and current to flow bypassing the shaded section.

Having multiple MPPTs helps because if you have parallel strings on a single MPPT and one string has shade you get uneven strings since the shaded sections are basically 'deleted' from the string by the bypass diodes so you can lose production from unshaded panels. With each string on its own MPPT you only lose the shaded sections very similar if not identical to if you had micro inverters or optimizers.
Screen Shot 2021-08-28 at 10.59.08 AM.png
 

jsurpless

Member
Feb 20, 2020
107
10
Massachusetts
This is all very interesting and I think that I follow the basic concept in that if a panel is producing insufficient power, its bypass diode(s) will activate so that upstream power can be diverted around the low-performing cells. This is good to know.

Yblaser has a good point, if the shading is just from the higher roof next to the solar panels, then a single string may perform fine. When the panels on the right are totally shaded, they will just drop out of production with a small negative impact on the production of the unshaded panels.
It's not clear to me just how much of the array will encroach into the shade caused by the higher roof. I do know that the (4x3) array is 17 x 13.5 ft and that the garage itself is 24 ft wide. I'm guesstimating that the shade is at most 25% of the roof so my feeling is that most of the array should be unaffected by the roof.

My concern has been focused on what effect the tree will have. It sounds like it shouldn't have as much impact as I had feared, particularly if I am proactive in pruning.

Many thanks for the explanations!
 

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