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
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