Single phase inverter output circuit balancing amps to three phase

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Anode

Member
Location
Washington, USA
I know there have been countless threads regarding this, but perhaps this one is unique, and I figured I would start a new one.

Interesting question here regarding balancing the back feed for an existing system that is being expanded, where we would be adding another single phase inverter.

The situation:
There is an existing solar installation (we did not install) which has one single phase SMA 6000W inverter with 29A max cont. output. That circuit lands in a 3p 125 Amp load center with a 40Amp 2p breaker, the load center has no other breakers and is designated presumably to be a panel for future inverter output circuits.

From there, the feeders to that panel come from a 3p breaker in MDP, which is also 40Amp.

The Add-on installation
This particular project is specified that it must use a 3.8 Fronius inverter, which has a 18.3 Amp output.

Questions
If the inverter output circuit for the existing installation is landing on B and C phase, then regardless of whether the Fronius inverter lands on A-C or A-B, then we would end up with an array of 18.3, 29, 47.3 Amp on that breaker at MDP.

Two part question, first - does it even matter that we would have a back feed breaker with an array of amperage on the three phases in an MDP where imbalance is already present from loads in that panel? If it does matter, what code could you point me to as governing that situation, and what would be a good solution.

Second question, Is it feasible that we could switch the L1 and L2 outputs for each inverter on two breakers?
In other words, if we took C phase of the existing L2 inverter output circuit landing on the existing 40A 2P breaker, and instead put L2 of the new inverter (18.3 A) in its place, and put the L1 of the new inverter output circuit (18.3 A) on C phase of the new OCPD 2p breaker, that would make the fronius inverter output circuit all landing on C phase, but on two separate breakers. Then we could take the 29A output that was on breaker 1 c phase, and just land it on A phase making a more evenly distributed back feed. This would result in phase A being 29A, phase B 29A, phase C 36.6A. Is there any reason why this wouldn't work?


I didn't mention it, but this is a 3p 120/208Y configuration, if it wasn't assumed in my description.

**Edit - changed the output of the breaker @ 208 for the fronius inverter, originally posted 15.9A, is actually 18.3A @ 208
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I know there have been countless threads regarding this, but perhaps this one is unique, and I figured I would start a new one.

Interesting question here regarding balancing the back feed for an existing system that is being expanded, where we would be adding another single phase inverter.

The situation:
There is an existing solar installation (we did not install) which has one single phase SMA 6000W inverter with 29A max cont. output. That circuit lands in a 3p 125 Amp load center with a 40Amp 2p breaker, the load center has no other breakers and is designated presumably to be a panel for future inverter output circuits.

From there, the feeders to that panel come from a 3p breaker in MDP, which is also 40Amp.

The Add-on installation
This particular project is specified that it must use a 3.8 Fronius inverter, which has a 18.3 Amp output.

Questions
If the inverter output circuit for the existing installation is landing on B and C phase, then regardless of whether the Fronius inverter lands on A-C or A-B, then we would end up with an array of 18.3, 29, 47.3 Amp on that breaker at MDP.

Two part question, first - does it even matter that we would have a back feed breaker with an array of amperage on the three phases in an MDP where imbalance is already present from loads in that panel? If it does matter, what code could you point me to as governing that situation, and what would be a good solution.

Second question, Is it feasible that we could switch the L1 and L2 outputs for each inverter on two breakers?
In other words, if we took C phase of the existing L2 inverter output circuit landing on the existing 40A 2P breaker, and instead put L2 of the new inverter (18.3 A) in its place, and put the L1 of the new inverter output circuit (18.3 A) on C phase of the new OCPD 2p breaker, that would make the fronius inverter output circuit all landing on C phase, but on two separate breakers. Then we could take the 29A output that was on breaker 1 c phase, and just land it on A phase making a more evenly distributed back feed. This would result in phase A being 29A, phase B 29A, phase C 36.6A. Is there any reason why this wouldn't work?


I didn't mention it, but this is a 3p 120/208Y configuration, if it wasn't assumed in my description.

**Edit - changed the output of the breaker @ 208 for the fronius inverter, originally posted 15.9A, is actually 18.3A @ 208
I am having trouble visualizing what you are describing, but generally if a single phase inverter can put out 208V and can operate without a neutral, you can use it to feed any two phases of a 208 3P service. If you have multiple inverters you can move them around however you want to reduce imbalance.

As to the amount of imbalance you can have, that is an AHJ decision, not a code issue.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I am having trouble visualizing what you are describing, but generally if a single phase inverter can put out 208V and can operate without a neutral, you can use it to feed any two phases of a 208 3P service. If you have multiple inverters you can move them around however you want to reduce imbalance.

As to the amount of imbalance you can have, that is an AHJ decision, not a code issue.

Actually, I don't know for sure that operating without a neutral is a requirement, but it seems weird to me that L-L would be 208V and L-N would be 120V.
 

Anode

Member
Location
Washington, USA
Understood with the AHJ and unbalanced back feed, I was just wondering if there was anything with respect to unbalanced amperage back feed, like there is for voltage in 705.100.

With regard to the second part of the question, more of a theoretical/hypothetical...

sketch.jpg

Pictures are worth a thousand words, I drew it a little different than I explained I think, but I think you get what I am shooting at.

Is it possible to land the second inverter both on A phase? I am thinking it is not.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Understood with the AHJ and unbalanced back feed, I was just wondering if there was anything with respect to unbalanced amperage back feed, like there is for voltage in 705.100.

With regard to the second part of the question, more of a theoretical/hypothetical...



Pictures are worth a thousand words, I drew it a little different than I explained I think, but I think you get what I am shooting at.

Is it possible to land the second inverter both on A phase? I am thinking it is not.
And you are thinking correctly. The inverter is pushing current against voltage in a line to line configuration, not pushing each line with respect to neutral.

So your drawing number two is just shorting the inverter output, resulting in no grid being sensed resulting in no power (current) being produced. The inverter will just shut down.
Note that when, as in drawing 1, you have two inverters on different line to line phases each connection one output terminal on the B line the current on B will be somewhat less than the sum of the two inverter currents because of the phase angle between the two contributions.
 

Anode

Member
Location
Washington, USA
And you are thinking correctly. The inverter is pushing current against voltage in a line to line configuration, not pushing each line with respect to neutral.

So your drawing number two is just shorting the inverter output, resulting in no grid being sensed resulting in no power (current) being produced. The inverter will just shut down.
Note that when, as in drawing 1, you have two inverters on different line to line phases each connection one output terminal on the B line the current on B will be somewhat less than the sum of the two inverter currents because of the phase angle between the two contributions.

Thank you for the confirmation.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Questions
If the inverter output circuit for the existing installation is landing on B and C phase, then regardless of whether the Fronius inverter lands on A-C or A-B, then we would end up with an array of 18.3, 29, 47.3 Amp on that breaker at MDP.

Those amps are not actually correct because peak currents do not add simply in a 3-phase configuration. The math is frankly over my head but it is thoroughly explained here:
http://solarprofessional.com/articl...ng-3-phase-line-currents?v=disable_pagination


Two part question, first - does it even matter that we would have a back feed breaker with an array of amperage on the three phases in an MDP where imbalance is already present from loads in that panel? If it does matter, what code could you point me to as governing that situation, and what would be a good solution.

Limits on phase imbalance are typically imposed by the utility. At those power levels I doubt they'll object but I am not them.

Second question, Is it feasible that we could switch the L1 and L2 outputs for each inverter on two breakers?
In other words, if we took C phase of the existing L2 inverter output circuit landing on the existing 40A 2P breaker, and instead put L2 of the new inverter (18.3 A) in its place, and put the L1 of the new inverter output circuit (18.3 A) on C phase of the new OCPD 2p breaker, that would make the fronius inverter output circuit all landing on C phase, but on two separate breakers. Then we could take the 29A output that was on breaker 1 c phase, and just land it on A phase making a more evenly distributed back feed. This would result in phase A being 29A, phase B 29A, phase C 36.6A. Is there any reason why this wouldn't work?

I can't make head nor tail of what your thinking here. Each inverter must be on a breaker on two poles. As the others have said, this doesn't work.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Those amps are not actually correct because peak currents do not add simply in a 3-phase configuration. The math is frankly over my head but it is thoroughly explained here:
http://solarprofessional.com/articl...ng-3-phase-line-currents?v=disable_pagination
The math is not that bad, just a little tedious. Multiplication, addition, and a square root at the end. Three times to get all three phase currents. You need to do it to correctly size your conductors and OCPD on the output of your AC combiner. Size them for the highest phase current.

Alternatively, you can assume a three phase inverter three times the size of your largest single phase inverter, calculate what the current would be, and size your conductors and OCPD accordingly, but you might be wasting money.

And yes, what the others are telling you is correct; you cannot land both inverter outputs on a single phase. That would be a dead short for the inverter and it would never power up.
 
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Smart $

Esteemed Member
Location
Ohio
The math is not that bad, just a little tedious. ...
An Excel (or any functional spreadsheet) template will reduce the tediousness.

Line currents calculate to 18.3A and 29A on the solo lines... 41.3A on the common line.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
An Excel (or any functional spreadsheet) template will reduce the tediousness.
Except that Excel itself is tedious. :D

If it's something he has to every once in a while, that's practical, but if it's just this once, it would be a lot faster to just work it out on paper than to enter the formulas in Excel, especially if he's not already adept at the program.
 
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jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I'd probably be inclined to create the excel sheet the first time around so I didn't have to remember the math. :D

Also I regard the kind of setup as a little passe given today's three phase inverters. I've actually never had to do it kinda hope to never have to.
 

Smart $

Esteemed Member
Location
Ohio
Except that Excel itself is tedious. :D

If it's something he has to every once in a while, that's practical, but if it's just this once, it would be a lot faster to just work it out on paper than to enter the formulas in Excel, especially if he's not already adept at the program.
Excel isn't much more tedious than using a scientific calculator to do the math. Probably even less so. If you do it just once with Excel with the intent of setting it up to do at least one more time in the future, you have a template that all you have to do is plug in the different variables.

If not adept at Excel, there is more help on the 'net for it than these fora are to electrical tradespersons. I have used http://www.excelforum.com/ on several occasions. If someone asked, I would probably even put together a basic template for them. I've already shared a more complex version on here...
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Excel isn't much more tedious than using a scientific calculator to do the math. Probably even less so. If you do it just once with Excel with the intent of setting it up to do at least one more time in the future, you have a template that all you have to do is plug in the different variables.

If not adept at Excel, there is more help on the 'net for it than these fora are to electrical tradespersons. I have used http://www.excelforum.com/ on several occasions. If someone asked, I would probably even put together a basic template for them. I've already shared a more complex version on here...
Of course all of this is correct; I confess to having been a bit facetious. I do that sometimes.

That said, though, some things like this I prefer doing on paper with a simple calculator so that I can watch and understand what is going on with the equations. I do all my stringing, maximum voltage, conductor sizing, and voltage drop calcs by hand.
 
Actually, I don't know for sure that operating without a neutral is a requirement, but it seems weird to me that L-L would be 208V and L-N would be 120V.

I have never installed a single phase inverter on a three phase system. Are most Ok with this? Is there any special configuration required to tell the inverter to "expect" the 120 degree phase angle between the N and two lines?
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I have never installed a single phase inverter on a three phase system. Are most Ok with this? Is there any special configuration required to tell the inverter to "expect" the 120 degree phase angle between the N and two lines?


Depends on the inverter. Many use the phase angle to determine if they are supposed to be on a 240 or 208V window. Others need to have some dip switches set to tell them the grid configuration. Single phase on a wye system is usually not a problem. Connections without neutrals are a bit harder to find support for.
 
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