PV System Limits based on Service Panel Size
Comments
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Re: PV System Limits based on Service Panel Size
Replace one 200 amp with a175 amp breaker and update the mistake on your drawing?
Usually, a permit to replace a breaker is not needed...
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: PV System Limits based on Service Panel Size
Changing the breakers feeding the subpanels won't change anything with regards to the 120% rule on the main.Well, the electrician, the solar and I got together this morning and figured out that panel -- the 125a with a 90A will meet the load and allow all the backfeed needed for the desired system.
BUT, another limit pops up -- there is a 400A main panel, and apparently the 2011 NEC doesn't allow more than 80A (120% again) backfeed total, and since I have 40A on my main building, only 40A allowed on the other building, even though I'd like to do 55A. Apparently this isn't amenable to down-rating some of the feeds off the panel, either.
(There's two 200A branches off of it, and one could easily be reduced to 150 or 175).
Any ideas on getting around that? -
Re: PV System Limits based on Service Panel SizeChanging the breakers feeding the subpanels won't change anything with regards to the 120% rule on the main.
Not sure I see that -- 2011 705.12 (D) (2) -
That makes it sound very much as if it is adding up the breaker sizes; So a 200A + 175A seems like there's 375 from the utility on the bus, plus my 95 backfed = 470A; 400* 1.2 = 480.Bus or Conductor Rating. The sum of the ampere rating of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120 percent of the rating of the busbar or conductor.
What section is using 120% and not saying OCPDs at the same time?
Now:
.......400A
meter
> 200A -> underground -> 200A panel << 40A backfeed
..........|
..........|-> 200A -> buss |-> 125A -> underground -> 125A panel -> 90A -> buss << 55 wanted backfeed 2x20, 1x15
...........................|-> 20A
...........................|-> 20A
...........................|-> 20A
Proposed:
....... 400A
meter
> 200A -> underground -> 200A panel << 40A backfeed
..........|
..........|-> 175A -> buss -> 125A -> underground -> 125A panel -> 90A-> buss << 55 wanted backfeed.
...........................|-> 20A
...........................|-> 20A
...........................|-> 20A
thx,
-dB -
Re: PV System Limits based on Service Panel Size
The 400 Amp service can take 1.2X or 480 Amps. Possibly more if the bus bars inside it are rated higher than 400 Amps.
What is connected to this service other than the two 200 Amp sub panels?
If nothing, then the service can take 400 Amps from either direction.
The 120% rule is in place to prevent the connecting bus bars from being fed the full current of the main breaker and the full current of the GTI to a point of exceeding the bars' maximum current rating.
As has been explored in another thread, the GTI feed should be based on the inverter's output, not the breaker rating. But this doesn't always happen. -
Re: PV System Limits based on Service Panel Size-> 175A -> buss -> 125A -> underground -> 125A panel -> 90A-> buss << 55 wanted backfeed.
You can use 125A breaker instead of 175A. It is followed by 125A breaker anyway. Will save you few bucks. -
Re: PV System Limits based on Service Panel SizeCariboocoot wrote: »The 400 Amp service can take 1.2X or 480 Amps. Possibly more if the bus bars inside it are rated higher than 400 Amps.
What is connected to this service other than the two 200 Amp sub panels?
|................. Main Panel ..........|
| sealed |........ accessible ..........|
+
+
+
|........|..............................|
|........|..............................|
| .......|..400A .......................|
| meter -|
> 200A
|
> underground -> 200A panel << 40A backfeed
| .......|....|.........................|
| .......|....|-> 175A -> buss -> 125A -|
> underground -> 125A panel -> 90A-> buss << 55 wanted backfeed.
| .......|...................|--> 20A --|---> local load
| .......|...................|--> 20A --|---> local load
| .......|...................|--> 20A --|---> local load
|........|..............................|
|........|..............................|
+
+
+
The 400A feed from the meter goes to two 200A breakers, one of which feeds the local breaker buss. That could be changed to a 175A.
Here's links to a few pictures:
(1) Main panel exterior
(2) Main panel interior
(3) Main 200A breaker, and wires to 2nd 200A breaker;
(4) 200A breaker feeding the local buss.
I believe using a 175, we can guarantee the load on the input buss that has the two main breakers never sees more than 375 feed plus 90 backfeed = 465 (OK), and that the local breaker buss never sees more than 175 + the 90 backfeed, right?
thanks,
-dB -
Re: PV System Limits based on Service Panel Size
you have to go by the bus ratings on the 400a panel and that is usually 20% more than the panel's rating.
1.2 x 400a = 480a.
that sets the overall max for everything. now you can use up to 200a + 175a = 375a going by the subbox's circuit breakers and solar can be fed up to the difference of the max rating of the sub boxes to the bus rating of the main box for 480a - 375a = 105a. problem is it would need that much applied to the main box as the smaller sub boxes can't tolerate all 105a unless properly fed for their max bus ratings. that means the 200a box with 200a breaker can be fed up to 40a and the 200a box with the 375a breaker can be fed up to 65a. -
Re: PV System Limits based on Service Panel SizeNot sure I see that -- 2011 705.12 (D) (2) -
That makes it sound very much as if it is adding up the breaker sizes; So a 200A + 175A seems like there's 375 from the utility on the bus, plus my 95 backfed = 470A; 400* 1.2 = 480.
What section is using 120% and not saying OCPDs at the same time?
Now:
.......400A
meter
> 200A -> underground -> 200A panel << 40A backfeed
..........|
..........|-> 200A -> buss |-> 125A -> underground -> 125A panel -> 90A -> buss << 55 wanted backfeed 2x20, 1x15
...........................|-> 20A
...........................|-> 20A
...........................|-> 20A
Proposed:
....... 400A
meter
> 200A -> underground -> 200A panel << 40A backfeed
..........|
..........|-> 175A -> buss -> 125A -> underground -> 125A panel -> 90A-> buss << 55 wanted backfeed.
...........................|-> 20A
...........................|-> 20A
...........................|-> 20A
thx,
-dB
Elsewhere in the code (I don't have it at my fingertips) it says that the rating of the first OCPD that is backfed in series from the inverter is the number that is used throughout. For example, if you have a 100A subpanel fed by a 100A breaker in the MDP and that subpanel contains a 20A backfed breaker, 20A is used for the 120% rule calculation in the MDP as well as the sub, even though the MDP has a 100A backfed breaker in it. In short, from the standpoint of the 120% rule, the breaker that is feeding the busbar in the MDP is the 20A breaker, not the 100A breaker. The 100A breaker is irrelevant.
Another example: If you had 6 inverters whose maximum output current were 16.4A each, you'd need a 25A breaker for each of them in the sub (1.25 X 16.4A = 20.5A). In the main you could have a 125A breaker (6 X 16.4 X 1.25 = 123A), even though the sum of the backfed breakers in the sub is 150A. Granted, that's a contrived example and I don't know that it would ever show up in the real world, but that's the math. It would be weird because I'm pretty sure that by the letter of the code you'd need to use 150A and not 125A for the 120% rule in the MDP.
If I am missing or misinterpreting something in the code, I am sure that someone will correct me.
-
Re: PV System Limits based on Service Panel Size
ggunn; that was what I was getting at. Unless it's been much too hot here lately (and it has) if the only thing connected to the 400 Amp service is two 200 Amp sub-panels then there is no derating of the 400 Amp service at all: it can take 400 Amps "in either direction". The derating is done at the sub panels where the GTI's connect.
Of course 38C is 100F so maybe my brain fried today.
-
Re: PV System Limits based on Service Panel SizeCariboocoot wrote: »ggunn; that was what I was getting at. Unless it's been much too hot here lately (and it has) if the only thing connected to the 400 Amp service is two 200 Amp sub-panels then there is no derating of the 400 Amp service at all: it can take 400 Amps "in either direction". The derating is done at the sub panels where the GTI's connect.
If there was a big main 400A breaker on that main panel, then (400A + 40A + 55A) would be more than 480A and this wouldn't be permitted. But since there's a kludgy connection without a main breaker the rules allow it?
Cariboocoot wrote: »Of course 38C is 100F so maybe my brain fried today.
You know, that's about a correct operating temperature for the brain
It's terribly hot here too. 35C (96F). But they say huge storms with huge hail and 100 km/h winds are coming tonight, and it'll get much cooler afterwards. -
Re: PV System Limits based on Service Panel Size
maybe i should say straight up what i meant to say. the bottom line going to the 400a main is a backfeeding of 80a. going to the 200a box with 200a breaker it is 40a there that can be backfed. the other 200a box with 175a breaker can be backfed 65a.
it must feed to the smaller boxes up to those ratings and cannot be transposed in any other manor when going to those sub boxes. for instance you can't feed the 200a box with 45a while feeding the other one with 60a as the first box has an overloaded bus at this point.
if feeding the sub boxes with solar to its max bus ratings then you can't feed the main box at the same time with more solar. the sub boxes will allow for more solar because the main box breaker was lowered to 175a instead of 200a which allows an extra 25a to the bus so it does matter about the main breakers. -
Re: PV System Limits based on Service Panel Sizemaybe i should say straight up what i meant to say. the bottom line going to the 400a main is a backfeeding of 80a. going to the 200a box with 200a breaker it is 40a there. the other 200a box with 175a breaker can be fed 65a.
it must feed to the smaller boxes up to those ratings and cannot be transposed in any other manor when going to those sub boxes. for instance you can't feed the 200a box with 45a while feeding the other one with 60a as the first box has an overloaded bus at this point.
Exactly. Get the derating right at the sub panels and the main panel should have no issue. -
Re: PV System Limits based on Service Panel Sizemaybe i should say straight up what i meant to say. the bottom line going to the 400a main is a backfeeding of 80a. going to the 200a box with 200a breaker it is 40a there that can be backfed. the other 200a box with 175a breaker can be backfed 65a.
Looking at the main panel. At max it can be fed:
400A from utility
40A solar through a 200A breaker
55A solar through a 175A breaker
When I sum that up, it gives 400+40+55 = 495A > 480A, a little bit too much
Is that a correct way to apply rules?
If you count breaker sizes instead, it only gets worse:
400A from utility
200A solar through a 200A breaker
175A solar through a 175A breaker
400A + 200A + 175A = 775A - way too much -
Re: PV System Limits based on Service Panel Size
you are confusing yourself. the total on any bus is 1.2x the stock main breaker amp rating so the difference between that bus rating and the main breaker is what can be backfed on the box. reducing a main breaker size increases the amount of amps available to be backfed. being the sub boxes are in parallel then each can go to the max differential backfeed currents calculated by me because it will not overload the max bus current rating on the 400a box.
it is not 55a on the other box as it has a 175a main breaker. the max bus current on that box is 240a. the difference between the main breaker and the bus rating is the amount you can backfeed or 240a-175a=65a. it would be a similar situation on the 400a box if the main breaker were to be reduced to say 375a and then it would allow for 480a-375a=105a. that got a 25a reduction just as one of the sub boxes did and it works out to the same in doing so. -
Re: PV System Limits based on Service Panel Size
What I said was that the ratings of the main breakers in the subs do not matter to the 120% rule calculations in the MDP. The ratings of the breakers connected directly to the inverters, wherever they are, are what are used as if they were actually in the MDP.maybe i should say straight up what i meant to say. the bottom line going to the 400a main is a backfeeding of 80a. going to the 200a box with 200a breaker it is 40a there that can be backfed. the other 200a box with 175a breaker can be backfed 65a.
it must feed to the smaller boxes up to those ratings and cannot be transposed in any other manor when going to those sub boxes. for instance you can't feed the 200a box with 45a while feeding the other one with 60a as the first box has an overloaded bus at this point.
if feeding the sub boxes with solar to its max bus ratings then you can't feed the main box at the same time with more solar. the sub boxes will allow for more solar because the main box breaker was lowered to 175a instead of 200a which allows an extra 25a to the bus so it does matter about the main breakers. -
Re: PV System Limits based on Service Panel Sizeyou are confusing yourself. the total on any bus is 1.2x the stock main breaker amp rating so the difference between that bus rating and the main breaker is what can be backfed on the box. reducing a main breaker size increases the amount of amps available to be backfed. being the sub boxes are in parallel then each can go to the max differential backfeed currents calculated by me because it will not overload the max bus current rating on the 400a box.
it is not 55a on the other box as it has a 175a main breaker. the max bus current on that box is 240a. the difference between the main breaker and the bus rating is the amount you can backfeed or 240a-175a=65a. it would be a similar situation on the 400a box if the main breaker were to be reduced to say 375a and then it would allow for 480a-375a=105a. that got a 25a reduction just as one of the sub boxes did and it works out to the same in doing so.
May be I'm not reading the diagram correctly. As I read it, there's no main breaker, but rather the bus is connected to the utility directly. It has five branch breakers (200A with 40A backfeed, 175A with 55A backfeed, and 3 20A without backfeed). Is that so? -
Re: PV System Limits based on Service Panel Size
My interpretation is that there is a main service panel which has a 400 Amp main breaker and two 200 Amp breakers going to two 200 Amp sub panels. There are no other connections to the 400 Amp main. As such, it doesn't really enter into the matter of sizing back-feed; it's all about getting the sub panels right.
So if the sub panels have 240 Amp bus bars on 200a sub panels (edited for clarification-niel) and you down-size their main breakers you can increase the back-feed allowance.
The whole point of the 120% rule is to prevent too much current flowing on the bus bars as provided by two power sources and a variable amount of loads. If all the power from the GTI is flowing to the grid, then there is no problem (as in the main panel here). If power is sourced 200 Amps from grid and 40 Amps from GTI going to 240 Amps of load, then there's 240 Amps on the bus bars. This is where trouble starts. -
Re: PV System Limits based on Service Panel SizeCariboocoot wrote: »As such, it doesn't really enter into the matter of sizing back-feed; it's all about getting the sub panels right.
So, if you have a main panel and subpanels, these rules only apply to subpanels and do not apply to the main panel at all. Therefore, if the subpanels satisfy the rule, it doesn't really matter what is in the main panel. Correct? -
Re: PV System Limits based on Service Panel SizeSo, if you have a main panel and subpanels, these rules only apply to subpanels and do not apply to the main panel at all. Therefore, if the subpanels satisfy the rule, it doesn't really matter what is in the main panel. Correct?
Yes, providing the sub panels are the only thing connected to the main (no other loads) and it is designed to handle both. It essentially becomes just "wiring to the grid", not really a panel. 400 Amps can flow through it in either direction, but there will never be a case where current is flowing through it from both directions because there's no place else for it to go. -
Re: PV System Limits based on Service Panel Size
Not really. The sum of the ratings of all the backfed breakers connected to inverters in all the subpanels plus the rating of the main breaker in the MDP must also be less than 120% of the busbar rating of the MDP. This is assuming that there are or can be loads everywhere. If the whole assembly is strictly for combining PV inverter outputs and there are no loads anywhere, the situation can get murky in the real world.So, if you have a main panel and subpanels, these rules only apply to subpanels and do not apply to the main panel at all. Therefore, if the subpanels satisfy the rule, it doesn't really matter what is in the main panel. Correct? -
Re: PV System Limits based on Service Panel SizeCariboocoot wrote: »Yes, providing the sub panels are the only thing connected to the main (no other loads) and it is designed to handle both. It essentially becomes just "wiring to the grid", not really a panel. 400 Amps can flow through it in either direction, but there will never be a case where current is flowing through it from both directions because there's no place else for it to go.Not really. The sum of the ratings of all the backfed breakers connected to inverters in all the subpanels plus the rating of the main breaker in the MDP must also be less than 120% of the busbar rating of the MDP. This is assuming that there are or can be loads everywhere. If the whole assembly is strictly for combining PV inverter outputs and there are no loads anywhere, the situation can get murky in the real world.
I think you're saying the same thing, which is that if there are other loads then the main panel should be treated with 120% rule. Since OP has three 20A breakers, which are loads, the 120% rule should apply to his main panel. Is that what you're saying? -
Re: PV System Limits based on Service Panel Size
It depends on where the breakers are in relation to the bus bars.
Normal GT system has a simple design of power from mains going to bus bars and power from GTI going to bus bar with loads drawing from bus bars. Pretty easy to understand where power comes from and goes to and how much current is involved at any given point. The 120% rule is there to prevent the current on the bus bars going to the loads from exceeding the current rating of those bars.
When you have two sub panels like this connected to a common main panel it becomes more complex. If there are any loads connected to the main panel, then that has to be treated the same way to prevent the current from the utility plus the current from the GTI(s) from exceeding the bus bar current rating.
If the main panel is just a connection point for the two sub panels, then it is essentially a combiner box with a disconnect. The most either sub panel can handle either in or out is their main breaker rating, so in the example given there can be no current greater than 400 on the main bus bars. Connect a load on there and this changes, as that can pull current from mains and GTI and potentially cause an over-current condition on the bars.
The other big problem is getting inspectors to understand what is going on. This is when they might look at the 400 Amp service and limit the GT connection based on that, rather than evaluating the derating at the sub panels only which is where it should be done. This is double derating. -
Re: PV System Limits based on Service Panel Size
This is fascinating, and I'm not seeing any conclusivity, even tentative yet. For those thinking there's an absolute limit of 80A backfeed for a 400A panel, can someone cite 2011 code section? I quoted the one that seemed relevant to me, and it seemed to be counting the OCPD values in the equation. Where's the one that does not take those into account? I didn't find it.
thx,
-dB -
Re: PV System Limits based on Service Panel Size
I'm not getting out of this without drawing a diagram, am I :P
Anyway, in this post I'm just going to veer off track a bit and point out this is very similar to the problem with the plug-n-play inverters: it all depends on how the wiring is done. That's what ultimately determines how much current can flow on any given conductor and is the reason for all the rules.
With the P-N-P dilemma we have the issue of the loads "downstream" of both GTI and main service potentially drawing more current than the wiring can handle. The OCPD on neither the GTI or main service will trip because neither is being overloaded; only the wiring from the point where the two power sources combine to where the loads draw. So you end up with a length of 14 AWG capable of 20 Amps max having 30 Amps of current potential from the two power sources.
This is the same thing, but with bus bars in breaker boxes instead of wires in walls.
It is also the reason why if the GTI is connected at one end of the bus bars and the main breaker at the other the problem is mitigated (no single point on the bar will have the combined maximum current of both power sources). If the main breaker is reduced in capacity it is mitigated. -
Re: PV System Limits based on Service Panel Size
It does take the OCPD ratings into account, but it's a matter of which ones. The breakers in the MDP feeding and taking power to/from the subpanels are not counted, but the breakers in the subs which are backfed by the inverters are counted. If two 20A backfed breakers are in a 200A sub fed by a 200A breaker in the MDP, the number for figuring the 120% rule in the MDP is 40A, not 200A. It is figured as if the 20A breakers were in the MDP. It's not that complicated.This is fascinating, and I'm not seeing any conclusivity, even tentative yet. For those thinking there's an absolute limit of 80A backfeed for a 400A panel, can someone cite 2011 code section? I quoted the one that seemed relevant to me, and it seemed to be counting the OCPD values in the equation. Where's the one that does not take those into account? I didn't find it.
thx,
-dB -
Re: PV System Limits based on Service Panel Size
Yeah. The common sense and laws of physics say: "Do not run more current through wires (or buses) that they can handle". Sounds very easy to follow.
Now, they try to make it easier with their 120% rule. This rule by itself hugely decreases the size of the backfeed breaker that could be installed based on common sense. The rule is probably put in there by electric companies to limit the size of solar installations. And it does! People really install less solar that they would if not for that rule. You can find examples on this forum.
When you go to more complex situations with multiple panels, this rule leads you even further from common sense and becomes even harder to apply. As a result, the inspector has to find his own interpretation which cannot be argued against. This gives him enormous power to screw installers and owners.
I would ask the inspector how it should be done before doing anything. Because that's him who's going to decide what's right. Hopefully he's a good guy. -
Re: PV System Limits based on Service Panel Size
I don't think that's the case; it's more about creating a safety rule that covers all or at least most of the possible scenarios.Now, they try to make it easier with their 120% rule. This rule by itself hugely decreases the size of the backfeed breaker that could be installed based on common sense. The rule is probably put in there by electric companies to limit the size of solar installations. And it does! People really install less solar that they would if not for that rule. You can find examples on this forum.
There is no limit on the number and rating of load breakers you may put in a panel because the probability of all the load breakers themselves being fully loaded at once is low, and even if that happens, when the load current surpasses the rating of the main breaker, the breaker opens and protects the busbars. Once you add a backfed breaker, then the current available to the busbar before the main breaker opens (the backfed breaker cannot be tripped by anything other than a fault on the branch) is increased. The 20% overage allowed is sort of arbitrary, I'll grant you that, but there has to be some limit. The placing of the backfed breaker at the opposite end of the busbars from the main makes sense as well; if it were up right under the main and the load breakers were to draw more than the busbar rating, then the part of the busbar immediately below the backfed breaker would experience a higher current density than it was rated for. When the backfed breaker is placed at the opposite end, that cannot happen.
I won't say that there isn't any part of the code that does not make sense from a safety standpoint, but I do not believe that this is one of them. -
Re: PV System Limits based on Service Panel Size
Back to my proposed situation.
The main 400A buss would have a 200 and a 175 on it.
The 200 goes to a panel with a 40A backfeed.
The 175 goes to a local buss with 60A load breakers, and a 125 to a panel that would have 55A backfeed.
The local buss, rated 400A, can't have more than 175 + 125 backfeed = 300A.
The main buss can't have more than 375 + 90, being OCPD's out + their backfeeds, I don't think.
Again, what is the code section that's not taking the size of the OCPD's on the main buss into account, applying an absolute value of the total backfeed?
We'll clearly be taking this to the County for permit clearance with the drawings, but want to understand how to argue it.
thx,
-dB -
Re: PV System Limits based on Service Panel SizeI don't think that's the case; it's more about creating a safety rule that covers all or at least most of the possible scenarios.
There is no limit on the number and rating of load breakers you may put in a panel because the probability of all the load breakers themselves being fully loaded at once is low, and even if that happens, when the load current surpasses the rating of the main breaker, the breaker opens and protects the busbars. Once you add a backfed breaker, then the current available to the busbar before the main breaker opens (the backfed breaker cannot be tripped by anything other than a fault on the branch) is increased. The 20% overage allowed is sort of arbitrary, I'll grant you that, but there has to be some limit. The placing of the backfed breaker at the opposite end of the busbars from the main makes sense as well; if it were up right under the main and the load breakers were to draw more than the busbar rating, then the part of the busbar immediately below the backfed breaker would experience a higher current density than it was rated for. When the backfed breaker is placed at the opposite end, that cannot happen.
I won't say that there isn't any part of the code that does not make sense from a safety standpoint, but I do not believe that this is one of them.
I'm not saying it is not safe. I'm saying it requires much more than is needed for safity.
1. Think of a bus as a wire. It is rated 100A. On one end you have a utility breaker feeding 100A. On the other end you have a solar backfeeding breaker feeding X A. In the middle you have loads that consume these (100+X)A connected in some order. A question. How big can you make X to make sure that nowhere on the bus there's a current exceeding 100A. Answer - 100A! 120% rule says 20A. This is 5 times smaller than you need to make sure that the 100A bus rating is not exceeded. A little bit overboard.
2. Look at the OP's main panel. Three 20A breakers. You cannot feed anything more than 60A through these breakers. The existing configuration is totally safe. But the rule says that you cannot look at the loads, must only look at supplies. If followed, he needs to install a 600A breaker and bigger panel. Would this increase the safety? I don't think so. -
Re: PV System Limits based on Service Panel Size
Here ya go...Again, what is the code section that's not taking the size of the OCPD's on the main buss into account, applying an absolute value of the total backfeed?
NEC 2011 705.12(D)(7) "In systems with panelboards connected in series, the rating of the first overcurrent device directly connected to the output of a utility-interactive inverter(s) shall be used in the calculations for all busbars and conductors."
Where shall I send my bill for this consultation?
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