PV System Limits based on Service Panel Size

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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #62
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    Re: PV System Limits based on Service Panel Size
    NorthGuy wrote: »
    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?

    in the case of this example it doesn't matter because the rating of all of the sub boxes do not exceed that of the main box. for instance you can't put 3 200a boxes in parallel feeding the main box as it would overload the main box. at the same time if you had a 200a sub box and a 100a sub box you have plenty of room for the main box and can expand by another 100a sub box. unless the breakers are changed from the stock breakers the excess on the bus will be 20% higher than that breaker rating and allow that 20% to be utilized as backfed power up to that rated current. so as in this example with a 200a sub box with a stock 200a main breaker the bus has the ability to handle 240a. if we were to reduce the main breaker to say 125a then that difference of 240a-125a=115a that can be backfed with solar on that sub box.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #63
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    Re: PV System Limits based on Service Panel Size
    ggunn wrote: »
    Here ya go...

    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? :D

    in series? nope these sub boxes are in parallel.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #64
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    Re: PV System Limits based on Service Panel Size
    niel wrote: »
    ... you can't put 3 200a boxes in parallel feeding the main box as it would overload the main box.
    Sure you can if the total of the ratings of all the backfed breakers directly connected to inverters in all the subs added to the rating of the MDP main breaker does not exceed 120% of the rating of the main busbar. The size of the main breaker in a subpanel has nothing to do with the 120% rule and neither does the breaker in the main feeding the sub. The "feed" is the sum of the ratings of the backfed breakers directly connected to the inverters all the way back to the service, i.e., if you have a single 20A backfed breaker in a 200A subpanel, the sub's contribution to the main busbar is still only 20A. If you have three 200A subs each with a 20A backfed breaker in them, their total contribution is 60A.

    NEC 2011 705.12(D)(7)
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #65
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    Re: PV System Limits based on Service Panel Size
    niel wrote: »
    in series? nope these sub boxes are in parallel.
    Yep, and the sub(s) and the main are in series.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #66
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    Re: PV System Limits based on Service Panel Size

    of course with smaller main breakers they may be able to feed that 400a box. standard stock breakers, no and that was my point on that.

    how do you get series from those boxes as the wiring is paralleled back to the 400a box is it not?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #67
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    Re: PV System Limits based on Service Panel Size
    niel wrote: »
    how do you get series from those boxes as the wiring is paralleled back to the 400a box is it not?

    Sub panel(s) are in series with the main breaker box. He (ggunn) is referring to the derating of the sub negating the need to derate the main as per NEC code section cited.

    Without juggling the numbers it works out to 40 Amps on a 200 Amp box, and two of those would be 80 Amps. Which is not-so-coincidentally the same as the 120% on a 400 Amp box.

    So if you decrease the size of the main breakers in the sub panels you can increase the allocation for the back-feed from GTI. With 175 Amp on one it would be allowed 65 Amps back feed, plus the 40 Amps on the other totals 105 Amps. This would be greater than doing the straightforward calc on the main panel. But that doesn't really enter into the equation unless your inspector is an idiot and reads it as "80 Amp limit" based on the main panel and ignores the one sub panel being reduced to 175 Amps so that they can have higher back-feed.
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #68
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    Re: PV System Limits based on Service Panel Size
    Cariboocoot wrote: »
    Without juggling the numbers it works out to 40 Amps on a 200 Amp box, and two of those would be 80 Amps. Which is not-so-coincidentally the same as the 120% on a 400 Amp box.

    That is a pure coincidence. The sizes of main breakers in the subpanels do not have to sum up to the size of the main breaker in the main panel. This 400A panel might've had 3 or 4 sub-panels with 200A breakers.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #69
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    Re: PV System Limits based on Service Panel Size
    NorthGuy wrote: »
    That is a pure coincidence. The sizes of main breakers in the subpanels do not have to sum up to the size of the main breaker in the main panel. This 400A panel might've had 3 or 4 sub-panels with 200A breakers.

    No, there's nothing coincidental about 2 * 200 = 400.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #70
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    Re: PV System Limits based on Service Panel Size
    ggunn wrote: »
    Sure you can if the total of the ratings of all the backfed breakers directly connected to inverters in all the subs added to the rating of the MDP main breaker does not exceed 120% of the rating of the main busbar. The size of the main breaker in a subpanel has nothing to do with the 120% rule and neither does the breaker in the main feeding the sub. The "feed" is the sum of the ratings of the backfed breakers directly connected to the inverters all the way back to the service, i.e., if you have a single 20A backfed breaker in a 200A subpanel, the sub's contribution to the main busbar is still only 20A. If you have three 200A subs each with a 20A backfed breaker in them, their total contribution is 60A.

    NEC 2011 705.12(D)(7)

    ok, in trying to clarify this i agree with your case example of 3 200a sub boxes backfeeding 20a each, but if you have 3 200a sub boxes and they are backfed their max of 40a each this will introduce a backfeeding to the 400a main box of 120a. this exceeds the max backfed current to that box's bus by 40a. you will be limited by the bus and main breaker differential in this case again as the backfed currents do add. this is no good.

    the bus and main breaker differential on that 400a box will dictate what the max backfed current from all of the sub boxes in total would be. if as you said you had many sub boxes and kept their max backfed total under the max differential of the main box you are safe. that 400a box with a 400a breaker can be backfed 80a and your 60a example falls within that. going beyond the max backfeeding current of the main box voids the whole thing even if the smaller boxes meet the standard individually. this makes my previous thoughts on this with the 175a breaker as being good in tandum with another 200a box with a 200a breaker being fed their max backfed current invalid for each of those boxes. the max the backfed current could be on the box with a 175a breaker would still be 40a if the other 200a box is backfeeding 40a as this totals 80a which is the max for the 400a box to be backfed.

    i don't know what i was thinking before on this as i was wrong on the box with the 175a main breaker being able to pass more than 40a back to the 400a main box if the other box is also backfeeding 40a. that main box can only take an additional 80a to its bus by that 120% rule.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #71
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    Re: PV System Limits based on Service Panel Size
    niel wrote: »
    ok, in trying to clarify this i agree with your case example of 3 200a sub boxes backfeeding 20a each, but if you have 3 200a sub boxes and they are backfed their max of 40a each this will introduce a backfeeding to the 400a main box of 120a. this exceeds the max backfed current to that box's bus by 40a. you will be limited by the bus and main breaker differential in this case again as the backfed currents do add. this is no good.

    Maximum back-feeding of the 400 Amp main panel is 400 Amps: as per the NEC section ggunn quoted there is no derating on it because the derating is done at the sub panels. This is not true if the 400 Amp main also has loads directly connected to it.

    The main panel either flows power from the utility to the subs (in which case its maximum is limited to 400 Amps by the breaker) or from the subs to the utility (in which case its maximum is limited by the output of the GTI's). There is not an instance where you have power flowing in from "both ends" on its bus bars and out to loads connected to same, so the bus bars can never be overloaded.
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #72
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    Re: PV System Limits based on Service Panel Size
    Cariboocoot wrote: »
    Maximum back-feeding of the 400 Amp main panel is 400 Amps: as per the NEC section ggunn quoted there is no derating on it because the derating is done at the sub panels. This is not true if the 400 Amp main also has loads directly connected to it.

    The main panel either flows power from the utility to the subs (in which case its maximum is limited to 400 Amps by the breaker) or from the subs to the utility (in which case its maximum is limited by the output of the GTI's). There is not an instance where you have power flowing in from "both ends" on its bus bars and out to loads connected to same, so the bus bars can never be overloaded.

    In this case, there are loads directly connected to the main panel (3 20A breakers). It shouldn't really matter what would happen if there were no loads.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #73
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    Re: PV System Limits based on Service Panel Size

    ok, try this as a thought. you are looking at the 400a box as almost transparent due to the sub boxes taking on the role they do, but this isn't 100% accurate imo. if the main 400a box had breakers equal to the main breakers of the sub boxes it takes on a different picture as a box with not only backfed power, but also loads governed by the net of each individual sub box. the fact that the circuits are further subdivided is meaningless in view of the main box. that means all load currents + all backfed currents shall not exceed the 480a bus rating on the main box.

    (yes, this is helping me view it and what you are trying to say too.)

    this does put a limit to what you say as if you have 2 sub boxes backfeeding at their max 240a each for 480a then introducing a 3rd box would present a problem as would any load currents as the bus is already fully occupied. that means it can't accommodate either a load or more backfed current on its bus. each bus on the sub boxes will represent either a net load or a net backfeed to the 400a main box bus and must be handled as such.

    i am going to think this over further for a bit as today is not one of my better thinking days.:confused:
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #74
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    Re: PV System Limits based on Service Panel Size
    NorthGuy wrote: »
    In this case, there are loads directly connected to the main panel (3 20A breakers). It shouldn't really matter what would happen if there were no loads.

    I specifically asked once if there were any loads connected to the main box other than the two 200 Amp sub panels. I do not recall getting an answer, much less "three 20 Amp breakers".

    As I mentioned before, if there are loads on that 400 Amp panel in addition to the two sub panels then they have to be considered as well.

    The question was, I believe, whether or not he could lower the value of the main breakers in one or both sub panels to increase the GTI allowance for them. The answer to that is "yes" as has been demonstrated.

    But if you have 60 Amps of load on the main panel the maximum back feed from the sub panels should be limited to service rating - load rating or 340 Amps as the intent is to avoid putting more current on the bus bars than they are designed to handle. The NEC does not address the issue from this perspective as far as I know.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #75
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    Re: PV System Limits based on Service Panel Size

    actually it would be the 480a bus rating of the main panel - the 60a loads for 420a able to be backfed at that point. if there is nothing that would limit the amount of backfeed current from exceeding that 420a in the presence of a 60a load then i would deem it as unsafe. the ratings of the 2 main breakers of the sub boxes in the original 2 sub box example would accommodate that limitation. even stock 200a main breakers would meet that.
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #76
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    Re: PV System Limits based on Service Panel Size
    Cariboocoot wrote: »
    I specifically asked once if there were any loads connected to the main box other than the two 200 Amp sub panels. I do not recall getting an answer, much less "three 20 Amp breakers".

    May be they're not. I just looked at the diagram in post #34, which shows 3 20A breakers. The picture of the panel referred to in post #37 definitely show more than two breakers, but it's hard to tell what they are.
    Cariboocoot wrote: »
    But if you have 60 Amps of load on the main panel the maximum back feed from the sub panels should be limited to service rating - load rating or 340 Amps as the intent is to avoid putting more current on the bus bars than they are designed to handle. The NEC does not address the issue from this perspective as far as I know.

    Rules need to be followed rather than common sense. You probably need to dig dip into definition of junction boxes and load centers. Perhaps, even having a place where breakers for more loads can be installed is enough to classify the panel as a load center. Because you can install loads later.

    Also, as Niel said, the breakers coming to sub panels can also be considered loads - e.g. one breaker feeding current and other breaker taking it.

    I think the situation requires a deeper knowledge of the code, and inspector might not go into such depths. Chances are that he may allow it even though the strict following of code rules may not permit it. So, it's better to ask him instead of trying to fingure that out independently.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #77
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    Re: PV System Limits based on Service Panel Size

    Well any time you change the wiring configuration from what was originally "stamped approved" you change the rules. So adding load breakers to the 400 Amp main later would not be an approved design, obviously.

    Since power can not flow in two directions at once, the only time this matter arises is when you have a conductor (i.e. bus bars in a breaker box or wire in a wall) that can be fed by two or more power sources (as in utility and GTI) and is connected to loads. In that case the loads have the potential for drawing the combined power of the multiple sources, which may exceed the current rating of the conductor.

    The NEC expects a straight forward "one box" design, with the nod to two in series as per the section ggunn quoted (wherein the derating applies only to that box which the GTI connects to as that is where the overload potential exists). With two sub panels both back feeding one main panel and that having loads on as well then it isn't as straightforward about where the derating needs to be applied. They probably didn't think such a situation would ever arise.

    Nevertheless, it did. :roll:
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #78
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    Re: PV System Limits based on Service Panel Size

    we would be assuming the wiring to be correct and all other aspects also good.

    technically you are right that it needs reinspected, but we are following lines of what would be proper for approval are we not? technically backfeeding 2 200a sub boxes with 240a each (if you had 240a breakers) to a 400a main box would be ok. adding anything else in or out to the main panel with this much being backfed will not be safe or fly. if limited by the stock 200a breakers of which there would be 2 would limit the backfeed to 400a from the sub boxes and then this could allow the connection of up to 80a of loads to the main box due to the bus rating of 480a. if done with good wiring and practices this should pass inspection.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #79
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    Re: PV System Limits based on Service Panel Size

    Niel; We've been talking about this so much I've forgotten what the original question was! :p

    I think it was about being able to reduce one of the sub panels' main breakers to increase the size of the GT allowance on that. So trading off load capacity in a sub panel for solar capacity?

    It's nap time, right?
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  • WayneTPVS
    WayneTPVS Registered Users Posts: 9 ✭
    #80
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    Re: PV System Limits based on Service Panel Size
    niel wrote: »
    technically you are right that it needs reinspected, but we are following lines of what would be proper for approval are we not? technically backfeeding 2 200a sub boxes with 240a each (if you had 240a breakers) to a 400a main box would be ok. adding anything else in or out to the main panel with this much being backfed will not be safe or fly. if limited by the stock 200a breakers of which there would be 2 would limit the backfeed to 400a from the sub boxes and then this could allow the connection of up to 80a of loads to the main box due to the bus rating of 480a. if done with good wiring and practices this should pass inspection.

    WOW, this thread has been one hell of a ride, and I'm not sure where LOAD's came into play on the 120 rule, but lets say in this instance they dont matter, as each is protected with its own OCPD.
    The NEC is clear "The SUM of all Source circuits shall not exceed 120% of the busbar or conductor rating".. this includes all panels mains or subs in series to the main.. and is introduced because if you count the majority of home panels the LOAD OCPD far out weight the Source current OCPD and it would be easy to overload a main panel supplied by 2 seperate sources if not regulated.

    So back the question at hand: we have a 400A service panel ( allowed 480a source currents) 400a potential from utility, 40a potential from Bldg 1 and owner wants 55a from bldg 2, not going to fly 495> 480.... So what do we do? what I have'nt seen here is how is the 400a from the utility OCPD'ed.
    If it's a LO connection then I would introduce a 375A breaker between the utility and 400a panel, as this would provide the sum currents of 470a onto the 480 allowed main panel... if the OCPD is controlled by utility, then you have some sweet talking to do.

    PS, I agree with Carabocoot, in that current can only flow in one direction, and that the source current has no where to flow to overload the bus.. but code and inspectors dont see it that way.. they will always look a worst possible senario of "what could happen" in insure their butts from being liable..

    Thanks for the ride guys:cool:
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #81
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    Re: PV System Limits based on Service Panel Size

    The loads come in to play when they can draw from two power sources across one conductor (the service panel bus bars). Each of those sources can provide their limit (either determined by breaker rating from the source or the absolute maximum output of the inverter). Combined load circuits can therefor draw more across the bus bars from the two sources than the bus bars were designed to handle. Hence the practice of reducing the main breaker from utility in order to increase the amount that can be fed from the GTI; trading one source capacity against the other but keeping the total available current the same.

    That is why if there are no loads connected to the 400 Amp service there is no derating of it, as the power either flows from the utility through that or to the utility through that. Utility current maximum is limited by the 400 Amp breaker, and the sub panel back-feed current is limited by the capacity of the GTI's.

    As Niel said, you have to consider one sub panel feeding current and the other taking it, but as spec'd that would still be limited by the sub panel main breaker which, since it is 1/2 the capacity (or less) of the 400 Amp service, should not be a problem.

    If one of the sub panels could back-feed 200 Amps and the other could draw 250 Amps the 400 Amp service would be in trouble as it could have 450 Amps of current on its bus bars. Likewise if it had loads attached they could draw from utility and inverters just the same.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #82
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    Re: PV System Limits based on Service Panel Size

    actually the main bus is not in trouble with 450a as that bus is rated for 480a. now if the 400a main box is only capable of being tied to the 2 sub boxes and there are no other positions for breakers available (or some other means of preventing other connections in there) then i would say the pass through either way is safe as there's no possibility of loads added to the 400a main box. that would keep it in compliance. at that point maybe the ahj would be satisfied with only a sign stating no other circuits added or it may fly as is, who knows? the 55a and 40a would be ok to do if the sub box main breakers are reduced as in 200a and 175a. that gives the max input as 40a + 65a respectively for a total of 105a capability of which the 95a proposed qualifies for easily.

    now that's just me and i'm not an expert with nec and neither are most ahjs. this is only my opinion on what would be safe. now i do have to ask if there are breaker positions involved in the internal connections for the 400a main box? if so then there would be duplication of the breakers at the main inputs to the sub boxes. the ahj would make the final decision on what is good. no ahj, then call your utility.

    it felt good to exorcise my mind on this for the record and i hope this all helped.
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  • WayneTPVS
    WayneTPVS Registered Users Posts: 9 ✭
    #83
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    Re: PV System Limits based on Service Panel Size
    Cariboocoot wrote: »
    If one of the sub panels could back-feed 200 Amps and the other could draw 250 Amps the 400 Amp service would be in trouble as it could have 450 Amps of current on its bus bars. Likewise if it had loads attached they could draw from utility and inverters just the same.

    Coot I have to agree to disagree, where are we getting a 200A subpanel backfeeding 250a onto the main 400a panel when it only had 40a available backfeed( bldg1)?!
    You have 3 source circuits 400a from the utility, 40a bldg 1, and proposed 55a bldg 2.. 495a can exist on the the bus at a given time, but they need somewhere to go. ( you even said it yourself, current only flows in one direction) even with the 3 additional 20a circuits they will only flow 20a on any given circuit.
    The only way there will be 495a on the main is a direct short, wear your PPE, cuz it will get hot.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #84
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    Re: PV System Limits based on Service Panel Size

    I said if it could, not that it does.

    In the original example both sub panels were 200 Amps so there is no problem. The power from the GTI's is the max they can produce, as per spec'd 120% rule. As such the 400 Amp main can't get in trouble even if one sub panel had its main breaker reduced to accommodate 65 Amps from GTI as the feed from either or both solar sources is far below the current limit of the main panel.

    The theoretical and the real keep getting their wires crossed in this thread. Metaphorically at least. :D
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  • jaggedben
    jaggedben Solar Expert Posts: 230 ✭✭
    #85
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    Re: PV System Limits based on Service Panel Size

    I think that unfortunately this thread has now got a lot of misinformation in it. I will try to sort it out.
    dbrower wrote: »
    Back to my proposed situation.

    The main 400A buss would have a 200 and a 175 on it.

    There is a very important pair of questions that need absolute clarification before we go further:
    Is there a single 400A main breaker connected directly to the meter, or not?
    and
    How many breakers are connected directly to the meter (i.e. with no other breakers between it)?
    Please answer this question definitively. It sounds like you are saying there is no 400A breaker, and that you have two service disconnecting breakers connected directly to the meter, one at 200A and one at 175. However I have to say that for me your diagrams and pictures do not make this absolutely clear. The pictures are especially confusing, as it looks to me like you have a single 200A main breaker in that panel.

    Can we get some better pictures of the main panel? It's a really weird arrangement you are describing.

    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.

    This arrangement, as described, is fine. You simply have two separate loadside connections, both of which are compliant. It should perhaps be said that it isn't entirely clear in the code how much total backfeed is allowed on a service with multiple loadside connections, but it's logical that it should be the rating of the service, which you are well under.

    The local buss, rated 400A, can't have more than 175 + 125 backfeed = 300A.

    This is way off track. If the bus is rated 400A then the utility+backfeed can be 480A. Moreover, the backfeed you stipulated is only 55A. You do not count the 125A breaker feeding the sub, just the actual solar backfeed.
    The main buss can't have more than 375 + 90, being OCPD's out + their backfeeds, I don't think.

    It depends whether this 'main bus' is connected to the load side of a main breaker or not. If it is connected directly to the meter, it can have 400A of backfeed. If it is connected to the load side of a main breaker, it can only have 80A.
    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?

    The relevant code section is 705.12. All of it.
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  • tholius
    tholius Registered Users Posts: 2 ✭
    #86
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    Re: PV System Limits based on Service Panel Size

    Alright, now I am confused. I have a 12,700 nameplate watts system on a 200A panel; actually, the main panel is a 200A and I have a 125A sub panel that has the solar coming into it.

    My real question is, I want to add 24-28kw to my home that has a 200A panel, how would I do this? We're talking around 100A of solar at 240. When I started to do this a few years ago, the installers at first said I needed to change my panel, and then came back and said they could do it with the existing panel.
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  • ARTsolar
    ARTsolar Registered Users Posts: 14 ✭✭
    #87
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    Re: PV System Limits based on Service Panel Size

    tholius,

    Have you looked into a line side connection?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #88
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    Re: PV System Limits based on Service Panel Size

    It also depends on what your Utility has as a drop to your home (or office/business).

    Over ~10 kWatt GT Solar power systems--The utility is going to probably do more than just "rubber stamp" your application.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #89
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    Re: PV System Limits based on Service Panel Size
    tholius wrote: »
    Alright, now I am confused. I have a 12,700 nameplate watts system on a 200A panel; actually, the main panel is a 200A and I have a 125A sub panel that has the solar coming into it.

    My real question is, I want to add 24-28kw to my home that has a 200A panel, how would I do this? We're talking around 100A of solar at 240. When I started to do this a few years ago, the installers at first said I needed to change my panel, and then came back and said they could do it with the existing panel.

    A 28kW system would be ~117A at 240V. And the NEC requires you to multiply that by 1.25. So 146A. The first limit is that the inverter output amperage must not be greater than the service amperage. You are OK on that with a 200A service. And if you do a line side tap (upstream of the main breaker, with overcurrent protection coming only at the combiner subpanel) that would be all that you need.
    But the combiner panel may be subject to the 120% rule also, depending on whether that sunpanel also contains loads. In that case your 150A just will not go into a 125A panel.
    SMA SB 3000, old BP panels.
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #90
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    Re: PV System Limits based on Service Panel Size
    inetdog wrote: »
    A 28kW system would be ~117A at 240V. And the NEC requires you to multiply that by 1.25. So 146A. The first limit is that the inverter output amperage must not be greater than the service amperage. You are OK on that with a 200A service. And if you do a line side tap (upstream of the main breaker, with overcurrent protection coming only at the combiner subpanel) that would be all that you need.
    But the combiner panel may be subject to the 120% rule also, depending on whether that sunpanel also contains loads. In that case your 150A just will not go into a 125A panel.

    You could do it with a second 200 amp panel if that is the only thing there, no loads, might fly with the AHJ. That would require a 400 amp service line and a dual 200 amp service entrance. Sounds like a line side tap is in order but it too may require a line to transformer upgrade.
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  • ronaldmaustin
    ronaldmaustin Registered Users Posts: 2
    #91
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    Re: PV System Limits based on Service Panel Size

    I am completely new here so I apologize in advance if this is the inappropriate place to post this question. I'm installing a SMA SunnyBoy 11000TL-US (240) inverter. The Max Output current is 45.8 amps. The service panel I now have is some jerry-rigged garbage without a cover on it so I have no idea what it is rated at. But I see a 100 Amp main breaker in it, so I assume it's a 100 amp breaker panel that I can only add a 20 amp breaker to for the inverter, which is insufficient at the 45.8 amps max from the inverter.

    Question 1: Will a 125 amp panel with a 100 amp main breaker do in this case? On the one hand, 125 amps x 1.2 = 150 amps ... less the 100 amp main breaker gives me room for a 50 amp breaker by code, which is over the max output of 45.8 amps. BUT, earlier on in this thread, someone multiplied by .8 after doing this simple calculation. If I do that, that then drops the number to 40 amps, which is obviously less than the 45.8 amps.

    Question 2: If no, how about a panel with a rated 150 amp bus?

    Question 3: If no, how about a 200 amp panel upgrade? While this would seem an obvious choice as the price difference is small, people have noted local building inspectors not going for derating the main breaker to, say, 175 amps. And then there are the additional concerns raised here about the drop from the transformer at the pole possibly not being capable of 200 amps.

    Thanks.
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