My main service panel is 125amp and I have grid tied solar with net metering agreement. With 120% rule, I believe I am out of compliance with my backfeed. 125amp bus bar x1.2 minus 125amp breaker = 25amps of solar. With my 21 panel enphase system, I have 34 amps of backfeed. I know I can either derate my main breaker to 100amp (125amp x 1.2 - 100amp breaker = 50 amps) or get a bigger panel bus bar to solve that problem. What I’m unclear on is how to add an off-grid system that will supplement my current system while not backfeeding the grid. At the same time, I’d like the ability to use my current panels (in addition to the new ones), should the grid shut down. My main panel is already maxed out and I will be trying to add an EV to the mix as well! Upgrading to a 200amp service isn’t an option either, so I think I’m stuck with an independent off grid system that can use grid power if necessary, but not backfeed. Also something that will help lower the draw from the grid
Off grid solutions for grid tied system with maxed out main service panel
- Thread starter Treepin
- Start date
Hmmm, not sure you can "get away" with not upgrading your main panel/service to 200A, but maybe...
If you just wanted to add an "off-grid" system (eg no sell-back), that is one with no grid connection, should be able to do that. However you will need a 240v spot in your main panel to add any new inverter to the grid, and it sounds like you don't have room for that in your current panel right now. But if you did add another inverter and moved some of the circuits currently in your main panel to a new sub-panel off the new inverter, then you would likely have room to put a 240v feed to the new inverter in the main panel. There are quite a few inverters which can be setup for no grid power back, which you would want to do. There has been some discussion about how this is not perfect with some of these inverters, but given that you already have a back feed agreement (I assume you do) then the occasional trickle back from the new inverter in no back feed mode won't be an issue. This second inverter will power the circuits you move over as well as allowing for more PV panels and a battery to be added. And potentially give you the space to put in a feed for an EV charger circuit. If you connect the new inverter to the grid in the main panel, then you could charge its batts from the grid and power loads from the grid too without back feeding, with the right inverter choice.
The biggest issue I suspect may be that you will still be limited to that 125A service. With enough PVs (and batts if you are installing those) that may not be an issue. But for extended cloudy periods, or when the grid goes down for an extended period, you may need to limit your power consumption and juggle loads to get through. Also, only the circuits in the new sub-panel will be backed up during grid outages.
None of this really addresses the issue you mentioned of going over the 20% rule on the buss of your current panel. I am actually surprised your PoCo and AHJ did not flag that during your inspections. In reality, it is not likely to really exceed that, especially if you move a bunch of circuits to a new sub-panel/inverter and do not back feed from the new inverter; but still is a possibility. Nor does it address being able to use your existing PVs when the grid is down.
If you sent the AC from your current micro inverters into the new inverter's AC input instead of the main panel (need one that takes that along with a grid connection), that should allow you to use the old PVs output when the grid is down (need to double check that on the new inverter choice). This is a common solution to adding a batt to existing micro inverter systems and getting some backup for grid down. It would not address the 20% rule stuff, since then the new inverter would still need to back feed the grid and if you added more panels you will have potentially more energy to back feed. Of course, if you moved most/all the main panel circuits over to a big sub-panel on the new inverter it would likely solve the 20% problem (in reality but likely not per code), but you would probably have way more loads on that new inverter than it could handle when the grid is down, so you'd have to juggle loads... Or perhaps if your new inverter was one of those which had connections for critical loads and less important loads it could be made to work (eg two sub-panels off of the new inverter).
Anyway at first glance, these were my thoughts of your options. Others may have other/better ideas.
If you just wanted to add an "off-grid" system (eg no sell-back), that is one with no grid connection, should be able to do that. However you will need a 240v spot in your main panel to add any new inverter to the grid, and it sounds like you don't have room for that in your current panel right now. But if you did add another inverter and moved some of the circuits currently in your main panel to a new sub-panel off the new inverter, then you would likely have room to put a 240v feed to the new inverter in the main panel. There are quite a few inverters which can be setup for no grid power back, which you would want to do. There has been some discussion about how this is not perfect with some of these inverters, but given that you already have a back feed agreement (I assume you do) then the occasional trickle back from the new inverter in no back feed mode won't be an issue. This second inverter will power the circuits you move over as well as allowing for more PV panels and a battery to be added. And potentially give you the space to put in a feed for an EV charger circuit. If you connect the new inverter to the grid in the main panel, then you could charge its batts from the grid and power loads from the grid too without back feeding, with the right inverter choice.
The biggest issue I suspect may be that you will still be limited to that 125A service. With enough PVs (and batts if you are installing those) that may not be an issue. But for extended cloudy periods, or when the grid goes down for an extended period, you may need to limit your power consumption and juggle loads to get through. Also, only the circuits in the new sub-panel will be backed up during grid outages.
None of this really addresses the issue you mentioned of going over the 20% rule on the buss of your current panel. I am actually surprised your PoCo and AHJ did not flag that during your inspections. In reality, it is not likely to really exceed that, especially if you move a bunch of circuits to a new sub-panel/inverter and do not back feed from the new inverter; but still is a possibility. Nor does it address being able to use your existing PVs when the grid is down.
If you sent the AC from your current micro inverters into the new inverter's AC input instead of the main panel (need one that takes that along with a grid connection), that should allow you to use the old PVs output when the grid is down (need to double check that on the new inverter choice). This is a common solution to adding a batt to existing micro inverter systems and getting some backup for grid down. It would not address the 20% rule stuff, since then the new inverter would still need to back feed the grid and if you added more panels you will have potentially more energy to back feed. Of course, if you moved most/all the main panel circuits over to a big sub-panel on the new inverter it would likely solve the 20% problem (in reality but likely not per code), but you would probably have way more loads on that new inverter than it could handle when the grid is down, so you'd have to juggle loads... Or perhaps if your new inverter was one of those which had connections for critical loads and less important loads it could be made to work (eg two sub-panels off of the new inverter).
Anyway at first glance, these were my thoughts of your options. Others may have other/better ideas.
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1201
Net zero
I think this one is easy.
Add a critical loads panel and move a calculated amount of loads from your main panel to the new panel.
You'll buy a hybrid inverter that will feed the critical loads panel from grid, pv or battery.
Set the inverter to zero export and you're done
Add a critical loads panel and move a calculated amount of loads from your main panel to the new panel.
You'll buy a hybrid inverter that will feed the critical loads panel from grid, pv or battery.
Set the inverter to zero export and you're done
Yes, but it does not address his desire to be able to use the existing PVs with micro inverters when the grid is down. Nor the 20% rule issue.
1201
Net zero
Does the 120% rule count if op will not backfeed the grid with the new inverter?
As far as using existing pv he needs an inverter capable of ac coupling
This is along the track of what I was thinking. 120% rule aside, when creating a critical loads panel and utilizing an inverter capable of ac coupling… can it be configured in a way where off grid system prioritizes solar, battery, and grid last? I assume so. And if grid goes down, with ac coupling, does the original system PV feed the critical load panel as well the new system PV?
1201
Net zero
Correct- the off grid system(in your case would be a hybrid system) can and usually prioritizes solar. Can be set to use grid or battery second.
With ac coupling, if the grid is up your backup panel and main panel are both fed. When the grid is down only the backup panel will have power
can you clarify the second part. I can see how my critical load panel will be available when the grid goes down, but what is confusing is how both sets of solar panels play into that. With one system having micro inverters and grid tied and the off grid that’s where I get confused.
1201
Net zero
Ac coupling means the micro inverter output will be connected to your hybrid inverter. So all your pv will pass through the hybrid inverter.
Ok, so the grid tied system acts as ac from the grid, right? And not towards the PV limit of the hybrid inverter?
zanydroid
Solar Wizard
Search for my numerous posts on sum of breakers/hawaiian tie in for how the main panel can be refactored to go well above 120% rule. I have 90A of backfeed breakers on 100A service/100A bus bar installed as of last year
As well, PCS (power control system) limiting is another legal way to back off the backfeed.
As well, PCS (power control system) limiting is another legal way to back off the backfeed.
GXMnow
Solar Wizard
- Joined
- Jul 17, 2020
- Messages
- 3,394
This is very similar to the issue I am having.
My main panel is only a 100 amp from 1969. I have just a 20 amp back feed breaker in that panel. But it now goes to the grid AC1 side of a Schneider XW-Pro battery inverter. The output of the Schneider goes to a 100 amp backup loads panel with a 40 amp breaker. My Enphase system also goes into that backup loads panel with a 30 amp breaker. I have not expanded it et, it is still just 16 iQ7 inverters which maxes out at 16 amps, which works on a 20 amp breaker, but I am hoping to add another 2,000 watts of panels. That will max out the 30 amp breaker with up to 24 amps of back feed current.
The only issue is when the battery on the XW becomes full. I will need to add a relay to turn off the extra AC coupled solar so I do not back feed more than 16 amps to the main panel.
I already added 2,000 watts of extra panels, but they are DC coupled right to the batteries. Their power cannot go to the grid directly. They just help charge to give me more night time power. I have found a 90 amp main breaker for my panel, so I can safely go to a 30 amp back feed, but I would need to have the power company pull the meter to swap the main breaker. And I really don't want them to change my billing with additional solar. As far as they know, I still only have the original Enphase system. I back feed less during the day, and use very little at night, so I am sure they can figure it out, but it all fits within my NEM 2.0 agreement of 16 amps max back feed and no more than 900 KWHs per month of energy export.
My main panel is only a 100 amp from 1969. I have just a 20 amp back feed breaker in that panel. But it now goes to the grid AC1 side of a Schneider XW-Pro battery inverter. The output of the Schneider goes to a 100 amp backup loads panel with a 40 amp breaker. My Enphase system also goes into that backup loads panel with a 30 amp breaker. I have not expanded it et, it is still just 16 iQ7 inverters which maxes out at 16 amps, which works on a 20 amp breaker, but I am hoping to add another 2,000 watts of panels. That will max out the 30 amp breaker with up to 24 amps of back feed current.
The only issue is when the battery on the XW becomes full. I will need to add a relay to turn off the extra AC coupled solar so I do not back feed more than 16 amps to the main panel.
I already added 2,000 watts of extra panels, but they are DC coupled right to the batteries. Their power cannot go to the grid directly. They just help charge to give me more night time power. I have found a 90 amp main breaker for my panel, so I can safely go to a 30 amp back feed, but I would need to have the power company pull the meter to swap the main breaker. And I really don't want them to change my billing with additional solar. As far as they know, I still only have the original Enphase system. I back feed less during the day, and use very little at night, so I am sure they can figure it out, but it all fits within my NEM 2.0 agreement of 16 amps max back feed and no more than 900 KWHs per month of energy export.
If the OP is going to keep using all the current PV panels, and add more possibly, the 20% issue cannot be solved as long as he is using the current main panel as the main, short of not sending power back to the grid or setting the new AC coupled inverter to only send back a limited amount of power (possible but then why add more panels if you're already over the 20% limit).
A possible solution might be to keep the existing main panel but make it the load panel for this AC coupled inverter. Put in a new panel with very few slots (or maybe none, just pass-thru). Put a 125A main breaker in it but choose one with a 200A buss. Not necessary to increase the service size now. It will not be able to go above the 20% limit this way and you'd eliminate the labor of moving all those existing circuits over to a new panel. There would be some other details to figure out (especially if you choose an inverter with two load panels), and the details of your current system may make such a change easy or very hard. And whether 125A will handle all your current and future loads will still need to be addressed. But something along these lines could be done with minimal wiring changes (maybe), yet allow for the immediate and future expansion.
A possible solution might be to keep the existing main panel but make it the load panel for this AC coupled inverter. Put in a new panel with very few slots (or maybe none, just pass-thru). Put a 125A main breaker in it but choose one with a 200A buss. Not necessary to increase the service size now. It will not be able to go above the 20% limit this way and you'd eliminate the labor of moving all those existing circuits over to a new panel. There would be some other details to figure out (especially if you choose an inverter with two load panels), and the details of your current system may make such a change easy or very hard. And whether 125A will handle all your current and future loads will still need to be addressed. But something along these lines could be done with minimal wiring changes (maybe), yet allow for the immediate and future expansion.
DIYrich
Solar Wizard
Get the Gridboss when it comes out.
Put the gridboss between the meter and main panel.
Put enphase into one smart port.
add flexboss 21 for more pv.
Put main panel on the 200a load port or the 125a smartload port (save the load port for a future 200 amp expansion panel).
Put ev charger on a smartload port so it disconnects when grid goes down (but you can manually turn back on to charge).
You can limit export to the 34a limit of the enphase capability, or some lower amount to avoid utility hassles.
Note: in order to use your solar panels when the grid is down, you have to put something between the main panel and the grid. Might as well be the gridboss. I thought about the 18kpv, but it only has the one smart load port, and you have 2 needs: enphase, and ev charger. While both can go on the same port, it is not optimal when the grid goes down. You want to shed the ev charger to avoid overloading the inverter, and you don't want to loose the enphase in the process.
My first thought on the gridboss is that it is a solution looking for a problem. As people ask questions, a lot of problems have been asked where a gridboss solution is much easier to implement.
Put the gridboss between the meter and main panel.
Put enphase into one smart port.
add flexboss 21 for more pv.
Put main panel on the 200a load port or the 125a smartload port (save the load port for a future 200 amp expansion panel).
Put ev charger on a smartload port so it disconnects when grid goes down (but you can manually turn back on to charge).
You can limit export to the 34a limit of the enphase capability, or some lower amount to avoid utility hassles.
Note: in order to use your solar panels when the grid is down, you have to put something between the main panel and the grid. Might as well be the gridboss. I thought about the 18kpv, but it only has the one smart load port, and you have 2 needs: enphase, and ev charger. While both can go on the same port, it is not optimal when the grid goes down. You want to shed the ev charger to avoid overloading the inverter, and you don't want to loose the enphase in the process.
My first thought on the gridboss is that it is a solution looking for a problem. As people ask questions, a lot of problems have been asked where a gridboss solution is much easier to implement.
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zanydroid
Solar Wizard
As I said you can switch to sum of breakers without changing the main.
To use that you relocate all load breakers into a new 225A busbar sub. The sub is then fed from the main from a single 100A breaker. The sub can accommodate 100A of backfeed via either 100% or 120% rule
(100% , 120%, and sum of breakers are three of the allowed calculation methods in 705.12)
1201
Net zero
The grid tie system will not reduce the amount of pv you can add to your hybrid.
Do you frequently lose power? How much additional pv do you want to add?
It might be easier to ac retrofit instead of ac couple
I think there might be confusion here, most likely me. The off grid system will be 6-7K PV and 30kwh battery backup. My goal is for the off grid system to supplement / elevate my grid tied system, since I am already drawing more than I return to the grid and plan to add an EV soon. I can see different ways to do this, but confused on how I can do this and be able to utilize the grid tied panels should the grid shut down.
1201
Net zero
Do you frequently lose power?
No. Just something about seeing those panels up there becoming useless should the grid shut down bugs me. Maybe it’s the apocalypse prepper in me
1201
Net zero
No. Just something about seeing those panels up there becoming useless should the grid shut down bugs me. Maybe it’s the apocalypse prepper in me
if you ac couple you will be able to fully utilize the power in an outage. just set the hybrid inverter to zero export so it doesnt send any of the new pv to the grid
Per "My main panel is already maxed out and I will be trying to add an EV to the mix as well! Upgrading to a 200amp service isn’t an option either, so I think I’m stuck with an independent off grid system that can use grid power if necessary, but not backfeed. Also something that will help lower the draw from the grid."
You may be able to avert an expensive upgrade of the 125A panel by using something like Lumin. One of their main uses has been TOUD and offloading the heavy hitters in PV setups. As I understand it today, the 702.4 (A) (2) 2023 NEC code complicates matters IF I really understand all I've read iin the last 12 months considering adding inverters + batteries to my residence.
"Section 702.4(A)(2) of the National Electrical Code applies to optional standby systems that use automatic load connection. Energy storage systems must either be capable of supplying the full load or use an Energy Management System (EMS) to manage the connected load. For example, if a homeowner backs up their house with solar and energy storage or with a generator on an automatic transfer switch, those systems would be considered optional standby systems and fall under article 702."
-> https://www.luminsmart.com/blog/what-installers-should-know-about-nfpa-70-section-7024
-> https://www.luminsmart.com/platform/smart-electrical-panel
(I have no financial interest in Lumin).
You may be able to avert an expensive upgrade of the 125A panel by using something like Lumin. One of their main uses has been TOUD and offloading the heavy hitters in PV setups. As I understand it today, the 702.4 (A) (2) 2023 NEC code complicates matters IF I really understand all I've read iin the last 12 months considering adding inverters + batteries to my residence.
"Section 702.4(A)(2) of the National Electrical Code applies to optional standby systems that use automatic load connection. Energy storage systems must either be capable of supplying the full load or use an Energy Management System (EMS) to manage the connected load. For example, if a homeowner backs up their house with solar and energy storage or with a generator on an automatic transfer switch, those systems would be considered optional standby systems and fall under article 702."
-> https://www.luminsmart.com/blog/what-installers-should-know-about-nfpa-70-section-7024
-> https://www.luminsmart.com/platform/smart-electrical-panel
(I have no financial interest in Lumin).
Hedges
I See Electromagnetic Fields!
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If inverter landed at end of breaker panel busbar where fed by main breaker, any amps would add to current from main breaker, and could exceed busbar rating given sufficient loads landing on breaker further down busbar.
If inverter landed at far end of busbar (per 120% rule), current in each of L1, L2 busbars flows one direction from main breaker, other direction from PV breaker for new zero export inverter. No point on L1, L2 busbars carries more current than the larger of the two breakers.
However, if you load say L1 with single phase 120V loads, up to main breaker amperage (e.g. 125A) plus PV breaker amperage (e.g 50A) can feed 120V loads, and will dump their 175A combined current into N busbar.
Depending on how N wires are distributed, that could be 175A in one branch of the busbar, significantly overloading it. Or, could be distributed half to the left and half to the right, only 87A in each branch.
But either way, the 175A N current in the busbar will flow out N wire of utility drop to transformer. That will overload and overheat it.
You must use some scheme that limits N current to what wires and busbars are rated for. The 120% rule means it is possible to overload 20%, but that is undesirable.
If your single phase 120V loads on each busbar are no greater than busbar rating, then you are OK.
I had always been told (perhaps incorrectly), that it is not a point analysis of the buss, but the total amps potentially flowing thru it that trigger the 20% rule. Most folks have more amps worth of breakers in their panel than the main breaker can supply. This is rarely a problem cause all circuits are not running at or near their max at the same time (when was the last time your main breaker popped open in normal use).
In theory though, with two feeds into the panel buss (main breaker and inverter breaker) they could exceed 120% of the buss's current rating. Likely? No, no more so than the normal loads popping the main breaker. But in theory one could put more current into the panel buss than it is designed for, and there would be no breaker protection to stop that. The 20% rule keeps the buss "safe", just as the various breakers keep their wires "safe".
In real life is this a concern? Probably not in almost all cases, but it is a potential fire danger...
Hedges
I See Electromagnetic Fields!
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If you turn on too many loads and trip main breaker, no harm no foul.
Following 120% rule e.g. 200A bus, 200A main breaker one end, 40A main breaker other end, the L1 & L2 busses will never see more than 200A at any point, even if you feed up to 240A of loads.
I read discussion prior to adoption of NEC rule considered someone might improperly relocate PV breaker adjacent to main breaker in the future. 120% of current squared is 144% of design temperature rise, not the end of the world.
I had previously thought it ought to be 200% rule, OK to feed 200A from far end of 200A bus.
But I have since figured out that PG&E split-phase transformer is acting as an auto-transformer, supplying neutral current for any single-phase 120V loads you supply by backfeeding 240V. So the problem is, neutral busbar and neutral utility drop carries sum of main breaker and PV breaker current. Only a problem if you have that much imbalance in 120V loads, but I'm certainly capable of stuffing the panel with a degenerate case of 400A 120V load and burning up their unprotected neutral wire.
I am not aware that NEC or PG&E has figured out what I have. Never seen it addressed outside this forum, where only with great effort have I been able to convince electrical professionals that center tap of auto-transformer carries sum of L1, L2 (not difference.) At least some auto-transformers on the market do provide suitable protection. I don't think PG&E's Pole Pigs do.
Following 120% rule e.g. 200A bus, 200A main breaker one end, 40A main breaker other end, the L1 & L2 busses will never see more than 200A at any point, even if you feed up to 240A of loads.
I read discussion prior to adoption of NEC rule considered someone might improperly relocate PV breaker adjacent to main breaker in the future. 120% of current squared is 144% of design temperature rise, not the end of the world.
I had previously thought it ought to be 200% rule, OK to feed 200A from far end of 200A bus.
But I have since figured out that PG&E split-phase transformer is acting as an auto-transformer, supplying neutral current for any single-phase 120V loads you supply by backfeeding 240V. So the problem is, neutral busbar and neutral utility drop carries sum of main breaker and PV breaker current. Only a problem if you have that much imbalance in 120V loads, but I'm certainly capable of stuffing the panel with a degenerate case of 400A 120V load and burning up their unprotected neutral wire.
I am not aware that NEC or PG&E has figured out what I have. Never seen it addressed outside this forum, where only with great effort have I been able to convince electrical professionals that center tap of auto-transformer carries sum of L1, L2 (not difference.) At least some auto-transformers on the market do provide suitable protection. I don't think PG&E's Pole Pigs do.
@Hedges I don't disagree with your analysis. It makes sense, and indeed no point on the buss will be carrying more than the max current rating of either breaker.
I am speaking only to what/how AHJ inspectors interpret the rule. I think you are probably right that from the POV of protecting the panel buss the 20% rule is not necessary. However that is not how the PoCo and building inspectors see it, from my experience.
One could construct a case or two where there might be a fire risk in the main panel, but it is exceedingly rare. For example, let's say you have only a double (240v) breaker in the panel as load. There is a high draw but not complete short in that circuit. And it's 240v breaker has a fault, and does not trip. It would be possible to draw just under the trip current for the main and inverter feed breakers for an extended time, eventually overheating the buss, and so on. Not a likely scenario by any means, but in theory possible. Granted the wiring to that circuit would likely overheat and burst into flames before the panel did in this scenario. But if it did not unfold that way I would not want to be arguing with my insurance co that it was the faulty breaker not my overfeeding the panel that caused the house to burn down.
I am speaking only to what/how AHJ inspectors interpret the rule. I think you are probably right that from the POV of protecting the panel buss the 20% rule is not necessary. However that is not how the PoCo and building inspectors see it, from my experience.
One could construct a case or two where there might be a fire risk in the main panel, but it is exceedingly rare. For example, let's say you have only a double (240v) breaker in the panel as load. There is a high draw but not complete short in that circuit. And it's 240v breaker has a fault, and does not trip. It would be possible to draw just under the trip current for the main and inverter feed breakers for an extended time, eventually overheating the buss, and so on. Not a likely scenario by any means, but in theory possible. Granted the wiring to that circuit would likely overheat and burst into flames before the panel did in this scenario. But if it did not unfold that way I would not want to be arguing with my insurance co that it was the faulty breaker not my overfeeding the panel that caused the house to burn down.
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