What is the main reason of upgrading panel from 100A to 200A???

BatikikikBatikikik Solar Expert Posts: 141 ✭✭✭✭✭
Within these 2years in solar field in the end I did nor understand whats the reason to upgrading panel from 100 to 200 A
Lets say you have 22 panels each 230watts ... total 5060 KW
And you have a 100 A Main ..
And the choice is between sunny boy SB4000US and SB5000US
Be NEC code the breaker has not to be more than 20% of the main ..
It means max breaker size you can put is 20A .
Sunny design shows that installing 22 panels each 230 with 2 string each pf 11 is danger so you have to install 5000us ...
But for 5000US you need to have 30A breaker
Code says that you cannot but 30A breaker on 100A main ....
But I have installed a lot of 30A , 40A breakers on 100 A
Now how can explain me how it can be like that ???
And whats the best option to install 5KW system without upgrading MAIN
Thank you
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Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,870 admin
    Re: What is the main reason of upgrading panel from 100A to 200A???

    The reason is that a solar grid tied system is adding power to the main box... Now, instead of having 100 amp breaker coming in to a bus bar rated for 100 amps. You now have 100amps+40amps from your GT system and the possibility of 140 Amps of source current.

    Remembering that heat is equal to I^2 * R... the ratio of the two are:
    • (140 amps)^2 / (100 amps)^2 = 1.96
    So, you have almost 2x the heating potential as for what the box/bus bars were designed to support.

    They have you install the solar breakers at the bottom of the bus bar (if the main feed is at the top)--So, in theory, you can never exceed the current in any single piece of bus bar (current feeds from top or bottom or shared)--so you are not exceeding the current capability of the bus bar.

    However, you could, in theory, draw more power during the day time from that box (140 amps in sun, vs 100 amp at night)--so you could over heat the box during the day (A/C + Shop Tools for example).

    At least, that is how I would understand the reasoning behind the 20% limit (at least for residential? Commercial did not allow the 20% exemption in the past--you had to derate the incoming breaker to allow solar to be added--as I recall--I am not a code expert):
    • 120^2 / 100^2 = 1.44x limit
    That 1.44x limit looks suspeciously like the sqrt of 2 (1.414)... There are some engineering rules of thumb that use things like powers of sqrt(2), 2x, 10x, etc... I suspect the 20% is just a number large enough for people to use, but not too large to overheat an average service (lots of hand waving).

    Basically, one of the engineering rules of thumb is that things that are less than 2x something else are almost the same. And things that are 10x something else--you can ignore the smaller item.

    There have been times in the past where 2x load factors (non-power factor corrected computers in a data center) overheated the wiring--So, not allowing 2x but allowing sqrt(2) "overloading" potential is probably about as far as it is safe to press the issue.

    In the end, the NEC is intended to provide a "safe" scheme for designing and implementing building wiring regardless of what the end user does... It does not allow for some things to be "safe" at night vs "unsafe" during the day based on how the user chooses to plug in/turn on their loads.

    -Bill "my two cents worth of guessing" B.

    PS: Your only option to install a solar system without a main panel upgrade (as I understand) is a supply side tap... Basically, you tap into the utility feed (before or after the meter) and run a new set of cables to a second mains panel.

    However, that is not something often done. Many inspectors will have a real problem with allowing this (others here have talked about their issues trying to get taps engineered and approved).
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: What is the main reason of upgrading panel from 100A to 200A???
    Batikikik wrote: »
    Lets say you have 22 panels each 230watts ... total 5060 KW

    5060w - not kw.
    But I have installed a lot of 30A , 40A breakers on 100 A
    Now how can explain me how it can be like that ???

    The NEC allows - for residential only, not commercial - that breakers totaling 125% of the bus bar rating can be installed for LOAD, and breakers totaling 120% of the bus bar rating can be installed for FEED.

    The reason this is allowed on residential and not commercial is "intermittant use". The theory is that in a residential building, it is unlikely that all loads will be turned at the same time.

    On a commercial building, it is more likely that all loads will be turned on at the same time.

    And whats the best option to install 5KW system without upgrading MAIN
    Thank you

    1. Change main service FEED breaker to 80a and put 40a for solar FEED.
    2. Line side tap (rare).
  • SlimDieselSlimDiesel Registered Users Posts: 17
    Re: What is the main reason of upgrading panel from 100A to 200A???

    And if you've installed 30 and 40A breakers in 100A panels, you've incurred a lot of liability for yourself, are one dangerous SOB and stay away from my house! Keep it up and you're going to give fly-by-nighters a bad name.
  • sub3marathonmansub3marathonman Solar Expert Posts: 300 ✭✭✭
    Re: What is the main reason of upgrading panel from 100A to 200A???

    I thought I read somewhere, but I could be wrong, that some of the "100 amp" main panels are in fact rated as "125 amp," but have just had the 100 amp main breaker installed.

    I also thought it was possible to buy just an upgraded bus bar and install into some main panels. Of course, it has to be an approved installation from the manufacturer.
  • sub3marathonmansub3marathonman Solar Expert Posts: 300 ✭✭✭
    Re: What is the main reason of upgrading panel from 100A to 200A???
    BB. wrote: »
    The reason is that a solar grid tied system is adding power to the main box... Now, instead of having 100 amp breaker coming in to a bus bar rated for 100 amps. You now have 100amps+40amps from your GT system and the possibility of 140 Amps of source current.

    OK, I can understand that. But then dwh said:
    dwh wrote: »
    The NEC allows - for residential only, not commercial - that breakers totaling 125% of the bus bar rating can be installed for LOAD, and breakers totaling 120% of the bus bar rating can be installed for FEED.

    The reason this is allowed on residential and not commercial is "intermittant use". The theory is that in a residential building, it is unlikely that all loads will be turned at the same time.

    On a commercial building, it is more likely that all loads will be turned on at the same time.

    So, wouldn't the absolute maximum use of the bus-bar be 125% of the rating, and only at the time IF every single load is turned on, no matter how much of the backfeed was coming from the PV? Without PV, I assume the main breaker would trip once it got over 100%? Now with PV, you can supply the extra 25% IF somehow that was needed. But if nothing is using electricity, isn't there 0% use of the bus-bar? And now if the PV is supplying the 40 amp (40% of the rating), and there is a need in the house for 40 amps from whatever breakers, isn't the PV supplying it all at that point? As a simplified question, doesn't the PV just offset any pull from the grid, and thus pose no problem even if it had a 100 amp backfeed breaker?

    Now, I will say that I have read the NEC rules, and followed them (with the help of the licensed electrician I hired of course). However, this is a time when I really didn't understand the reason for it.
  • BB.BB. Super Moderators, Administrators Posts: 29,870 admin
    Re: What is the main reason of upgrading panel from 100A to 200A???

    I believe you are ignoring several effects.

    1. The sources of heat in the box is just not the bus bar, but must also include the wiring leaving the breakers too. If you had 100 amps of utility power and 100 amps then somebody could over load the box (too large/too many breakers).

    2. Bus bars are designed and tested for xxx amount of current/power dissipation. Power increases with the square of the current. So heat does not increase linearly with the increase of current.

    3. Things are designed to be idiot resistant. Note the different requirements between residential and commercial installations. I could (probably) design the same thing into a comuter and get it approved by UL because it is designed and tested for a specific application.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • newenergynewenergy Solar Expert Posts: 291 ✭✭✭✭✭✭
    Re: What is the main reason of upgrading panel from 100A to 200A???

    I don't want to suggest that anyone violate the NEC and that there's no purpose to the 120% rule, but...

    As sub3marathonman was suggesting, if you never have a demand for more than 100 amps (never tripped your main) and you add 40 amps of solar, you will still never exceed 100 amps on your busbar.

    Also, if you put the 40 amp solar breaker on the opposite side of the busbar from the main (one where the main is at one end) you will never have more than 100 amps at any given point on the busbar. Any current supplied to the busbar has to be drawn by the loads in between the two feeders.
  • dreesdrees Solar Expert Posts: 481 ✭✭✭
    Re: What is the main reason of upgrading panel from 100A to 200A???
    SlimDiesel wrote: »
    And if you've installed 30 and 40A breakers in 100A panels, you've incurred a lot of liability for yourself, are one dangerous SOB and stay away from my house! Keep it up and you're going to give fly-by-nighters a bad name.
    There's nothing wrong with installing a 30-40-50A+ breaker in a 100A panel.

    My whole neighborhood has 100A panels and at least one 50A (stove) and one 30A (dryer) breakers/circuits installed. And a decent number of these same people also have additional 30A+ breakers for AC, hot tubs, etc.

    My old condo was exactly the same.

    Never heard of anyone having problems blowing main breakers as a result...
  • BB.BB. Super Moderators, Administrators Posts: 29,870 admin
    Re: What is the main reason of upgrading panel from 100A to 200A???

    It is the difference between a 50 amp load and a 50 amp grid tied power source. The original statement was vague as to if the large breaker was a source or a sink.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • solar_davesolar_dave Solar Expert Posts: 2,347 ✭✭✭✭
    Re: What is the main reason of upgrading panel from 100A to 200A???

    If you have homeowners insurance (and hopefully told your agent about your solar install so it is covered), if there ever is a claim and the panel is not up to NEC or been approved by both your local code dept. and the utility, they surely would be foolish to pay off.
  • sub3marathonmansub3marathonman Solar Expert Posts: 300 ✭✭✭
    Re: What is the main reason of upgrading panel from 100A to 200A???
    BB. wrote: »
    I believe you are ignoring several effects.

    1. The sources of heat in the box is just not the bus bar, but must also include the wiring leaving the breakers too. If you had 100 amps of utility power and 100 amps then somebody could over load the box (too large/too many breakers).

    -Bill

    But, according to the NEC, you can only do 125% of the bus bar rating for Loads, so that shouldn't apply.

    I still can't understand the theory behind only allowing the 120% of the bus bar for "Feeds." Does that mean the bus bar really only can withstand 120% of the rated capacity? And is that only be for a certain amount of time? Why is it allowed to be "overloaded" at all? Also, if 120% is the true maximum for safety, why not drop the 125% rating for Loads back to 120%, and allow 200% for the Feeds? Am I correct that you can't "feed" unless you've got the "load?" And if the PV is "feeding" the panel, up to the 100% bus bar rating, won't it be pushing electricity back onto the grid, with no electricity coming in from the main breaker and the bus bar remaining in the safe zone? And if a "load" came it would just diminish the amount going back onto the grid, down to the total PV "feed."

    Also, I can understand limiting the "Feed" from power generated on-site and backfed into the panel because the excess power continues past the panel and back onto the grid.

    Now, I'm not saying violate the NEC, I'm just saying I would like to understand why the NEC decided to make this specific rule.
  • dreesdrees Solar Expert Posts: 481 ✭✭✭
    Re: What is the main reason of upgrading panel from 100A to 200A???
    But, according to the NEC, you can only do 125% of the bus bar rating for Loads, so that shouldn't apply.
    I thought the minimum service calculations for NEC were a lot more complicated than that...
  • BB.BB. Super Moderators, Administrators Posts: 29,870 admin
    Re: What is the main reason of upgrading panel from 100A to 200A???
    But, according to the NEC, you can only do 125% of the bus bar rating for Loads, so that shouldn't apply.
    Only for residential under the theory that homes rarely have everything running at once--Commercial only allows 100% rating.

    Also, remember that a 100 amp box is only supposed to have 80% hardwired loads (heater, A/C, electric stove, Lights, etc.)... At 100 amps (with 100 amp main breaker), the main breaker may trip over time.
    I still can't understand the theory behind only allowing the 120% of the bus bar for "Feeds." Does that mean the bus bar really only can withstand 120% of the rated capacity? And is that only be for a certain amount of time? Why is it allowed to be "overloaded" at all? Also, if 120% is the true maximum for safety, why not drop the 125% rating for Loads back to 120%, and allow 200% for the Feeds? Am I correct that you can't "feed" unless you've got the "load?"
    At least for my understanding, You have the utility feed at the top, and the solar panel GT feed at the bottom of the box... Nowhere, in that box, will there be a 120% current load (with GT running or not). If you had the Main and GT feed breakers both at the top of the bus--then you would have 120% going through the bus bars.

    But, remember that the bus bars are not the only source of heat in the box. The circuit breakers (typically thermal breakers) and the branch circuit wiring all add heat energy to the box too...

    So, it is not fair to say that the Bus Bar is 100 amp rated--it is the entire system, bus bars, breakers, wiring cavity, box itself, that is rated for 100 amp (or whatever) total current flow/load.
    And if the PV is "feeding" the panel, up to the 100% bus bar rating, won't it be pushing electricity back onto the grid, with no electricity coming in from the main breaker and the bus bar remaining in the safe zone? And if a "load" came it would just diminish the amount going back onto the grid, down to the total PV "feed."
    Besides my "hand waving" above--look at the possible fault conditions... Remember that residential/commercial boxes are only rated at 125% / 100% for installed breakers. You look at that 40 amp GT breaker as a source--That 40 amp breaker can also be a load in some conditions (shorts, failure in GT inverter, etc.)...

    The code is designed to protect the wiring/boxes/etc. from dangerous conditions in single (and subsequent) fault failures. It is not, however a requirement that the equipment protect against multiple simultaneous faults (this is not a life-critical/flight/etc. design).
    Also, I can understand limiting the "Feed" from power generated on-site and backfed into the panel because the excess power continues past the panel and back onto the grid.

    The problem with having both sinks and sources in the same wiring design is that the current fault analisys gets complicated... For example, if you have two GT systems on either side of the home, and they run into separate (or even the same) box--you now have to look at fault current supplied by main, and GT 1, into (for example) a failure in GT 2... Or all two of your GT systems, plus any on th same transformer as you, supplying current into the shared grid.

    Typically, power transformers for home distribution systems (as I understand, certainly no expert) are limited to 10,000 Amps maximum (they saturate their magnetic fields and will not transfer any more current). So you will the typical home circuit breaker has a 10,000 Amp Interrupt rating. You place a couple large solar arrays on a single transformer drop, and you run the risk of creating an unsafe condition where there is too much available current on the transformer drop and could weld a breaker shut in a neighbor's home.

    In real life--probably nothing bad is going to happen. The "band" from don't care to concern is probably 10% to 50% of 10,000 amps... Or 1,000 to 5,000 amps of Solar GT power (~240kW to 1.2 MWatt of solar panels).

    The NEC code is fairly conservative and it is difficult to provide absolute "safety" with a single document.
    Now, I'm not saying violate the NEC, I'm just saying I would like to understand why the NEC decided to make this specific rule.

    From what I have seen when trying to design and approve larger computer systems--There is not enough publicly available information from internal design and Listing Approval documents as to the actual factors of safety in any particular design.

    I had designed some equipment that, I thought, met the letter of the Listing (I don't remember the details now)--but UL had to go back into their private vendor files to confirm what I was doing was OK within the larger scope of the approval process (and they would not show the information to me).

    So, for building wiring--You are pretty much left to following what is typed in the NEC (and other code books) and, hopefully, your local inspector will agree with your interpretations...

    We had one nearby city that, one day, just decided that the minimum size of all 120/240 VAC wiring in a home was to be 12 AWG. Period. It did not matter that 14 AWG was in the code as good for 15 amps--That city required all wiring to be a minimum of 12 AWG--Made wiring homes more expensive and difficult to fit the wiring for 2 and 3 way switches for lighting and such (actually, according to the tables, 14 AWG is good to 20 amps--In the NEC, they just say that use 15 amp maximum for 14 AWG copper, and 20 amp Maximum for 12 awg copper--I could never figure out where that requirement came from).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: What is the main reason of upgrading panel from 100A to 200A???

    Bus bar = piece of metal. Designed to carry, for example, 125 Amps, "in one end and out the other".
    Now back feed it with additional power from an inverter.
    You have 100 Amps potential from the grid, plus whatever potential the inverter can supply; 20 Amps, say. This means that anywhere along the bus bar you could draw off 120 Amps from a piece of metal rated up to 120 Amps. Doesn't mean it will happen or that it's likely too. As we all know, nobody turns on all the circuits to maximum load all at once (difference between residential and commercial rules). If you do, the main breaker pops. But with two "main breakers" feeding one bus bar there is the potential to over-load the bar without tripping either breaker.

    Get it? :D
  • newenergynewenergy Solar Expert Posts: 291 ✭✭✭✭✭✭
    Re: What is the main reason of upgrading panel from 100A to 200A???

    If you put the solar breaker on the opposite end of the busbar from the main, then you can size the solar feed at 100% of the busbar capacity and the main at 100% of the busbar capacity and still have no chance of overloading the busbar. This would, of course, be a violation of the NEC. But, many service panels have the main in the middle, and many installers will not put the solar breaker all the way opposite from the main. In these cases, it is possible to overload the busbar.

    Still, if the service is properly sized for the loads, you aren't going to be overloading the busbar and hence not tripping the main before you get solar. Also, in residential, it's really rare that anyone would be near their max load for a sustained period.

    So, the question is, given the fact that there are some unlikely ways for people to overload the busbar because of adding solar and that people want to install solar and it will almost always be safe, how do you write rules that reasonably allow for it and reasonably protect people and equipment even when best practices are not followed?

    ----

    Ok, if you had branch circuit breakers that were larger than the main, you could overload the busbar even with the solar at the opposite end. That might be another thing that makes the rule makers a little extra on the conservative side - even though it's probably impossible for UL listed service panels and breakers. A branch circuit breaker could also fail to trip.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: What is the main reason of upgrading panel from 100A to 200A???
    newenergy wrote: »
    If you put the solar breaker on the opposite end of the busbar from the main, then you can size the solar feed at 100% of the busbar capacity and the main at 100% of the busbar capacity and still have no chance of overloading the busbar.

    No, absolutely not. The physical location of the connection is irrelevant. Adding grid-tie feed is the functional equivalent of upping the size of the main breaker, no matter at what point it is tied in.

    Let's look at an actual 100 Amp service:
    (1) 50 Amp 240 Volt "stove" feed
    (2) 30 Amp 240 Volt "appliance" feeds (hot water & dryer)
    (14) 15 Amp 120 Volt "household" feeds
    Total: 215 Amps @ 240 Volts

    Obviously it is possible to overload the main breaker without overloading even one branch circuit. The thing is, you want the main "feed" breaker(s) to be what "goes" before the bus bars.

    A lot of NEC is, in my opinion, over-kill and "product pushing." This part isn't; it's logical and common sense.
  • newenergynewenergy Solar Expert Posts: 291 ✭✭✭✭✭✭
    Re: What is the main reason of upgrading panel from 100A to 200A???
    No, absolutely not. The physical location of the connection is irrelevant.

    The physical location is quite relevant and that's why in 2008 690.47(B)(7) allows the 120% rule on commercial installations for the PV breaker IF the breaker is located at the opposite end of the busbar from the main breaker. (I think it is also mentioned in the 2005 and is required by some inspectors and mentioned by John Wiles, etc.)

    The reason the location matters is that if they are on opposite ends ALL of the loads are in between them. Say you have a 100 amp main at the top and a 20 amp solar feeder at the bottom. Say the total load fed by that panel is 120 amps. Since you really can't have >100amp breakers for branch circuits, let's just say you have two 60 amp breakers. You have 100 amps on the busbar just before you get to the upper 60 amp breaker. You have 40 amps on the busbar in between the breakers. You have 20 amps on the busbar below the bottom 60amp breaker.

    If you had the main, then the solar, then the two 60 amp breakers, you'd have all 120 amps just below the solar breaker.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: What is the main reason of upgrading panel from 100A to 200A???

    Okay, you try it at your house first. :roll:

    Me, I would not be so certain about the even distribution of current load.
  • BB.BB. Super Moderators, Administrators Posts: 29,870 admin
    Re: What is the main reason of upgrading panel from 100A to 200A???

    I believe the current distribution is actually that exact (circuit breakers at each end of the bus)...

    The GT inverter is a current mode (or constant power mode) device... It outputs current based on the amount of power and present voltage (high impedance source).

    The "battery" is the utility feed... It maintains voltage by supplying or accepting current (low impedance source).

    If both were voltage sources--then you could have high current through the bus bar from once source to the other--depending on which one was deciding it was 1.0 volts higher than the other.

    The circuit breakers will draw their current from the center of the bus bar at XX amps. The GT inverter will supply YY amps to the bus, and the Utility will either sink or source ZZ amps as needed (basically the difference between the GT inverter and the current going to the loads).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BatikikikBatikikik Solar Expert Posts: 141 ✭✭✭✭✭
    Re: What is the main reason of upgrading panel from 100A to 200A???

    When I said I`ve install a lot of 30a 40a breakers in 100a main , I did not mean Solar power source breaker , I mean breaker for house circuits , whatever ,
    AS U GUYS NOTICED there is a difference in 30A power source and 30A supply breaker .
    When you read NEC book you see some rules that in real experience nobody follow that rule even inspectors and electrical engineers .
    But in this case I WANT to know exactly , I know that you can change main from 100a to 80a and I understand what is the reason of that .It works I know.

    But I want to know , in what solar installation cases you have to upgrade your main 100A to 200A ????(without any modifications in panel(changing basbar or main breaker))

    I think this way and share with your opinions please.

    On 100A main maximum size of the inverter you can connect is SB4000US .Because SB4000us`s output Amp is 16,6 = > 16,6*1.25=20,75A . And breaker size will be 20 A or 25 A ...So in this case yo dont need to upgrade panel .
    But if you have larger array and you have to install inverter more that SB4000US so you have to upgrade your panel MANDATORY .
    I called SMA America Tech support and asked this question , and they were agree with me .

    So what is the "rule" regarding changing main panel ???
    When do you need and when u dont ? :)_))))))))
    Thank you
  • BB.BB. Super Moderators, Administrators Posts: 29,870 admin
    Re: What is the main reason of upgrading panel from 100A to 200A???
    Batikikik wrote: »
    When you read NEC book you see some rules that in real experience nobody follow that rule even inspectors and electrical engineers.

    I sure did... But again, if "I did not follow" the NEC rules--it was because I had to document and test using an outside agency (like UL or AT&T, etc.) to ensure that what I did was "safe"... And remember, safe is a relative term. I could light off the inside of the computer (which as happened due to defective components)--but the smoke/fire that resulted was contained inside the cabinet (steel/aluminum box, certain sized holes, metal shelves designed to prevent flame spread, catch burning debris, etc.).
    But I want to know , in what solar installation cases you have to upgrade your main 100A to 200A ????(without any modifications in panel(changing bus bar or main breaker))
    Given the 120% of box capacity rule, and the option of reducing the incoming mains breakers to allow larger solar breaker--I am not sure what your question is asking.

    The options of using a smaller mains breaker is not always possible (limited current rating options for breakers), changing out bus bars to higher rating in same box (only a couple vendors ever seemed to have the documentation that would allow this), or supply side tap (difficult to get hardware to do this, most utilities and inspectors seem to disallow this option--at least what I have read here)...

    If your installation does not fall into one of the above options--then you are left with ether a smaller rated solar array/inverter or changing out to a larger mains panel (and possibly a larger utility drop).

    What other options/variances are you seeing as "legal"?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BatikikikBatikikik Solar Expert Posts: 141 ✭✭✭✭✭
    Re: What is the main reason of upgrading panel from 100A to 200A???

    Straight question ....
    what is the maximum solar array size and maximum inverter size that you can connect to 100a Main and 125 A main .
    thanks
  • BB.BB. Super Moderators, Administrators Posts: 29,870 admin
    Re: What is the main reason of upgrading panel from 100A to 200A???

    If I understand the code and comments here correctly:
    • 100 Amp Main (box rating with 100 A main breaker) * 20% = 20 amp circuit (120 VAC or split phase 240 VAC)...
    • 125 Amp Main * 20% = 30 amp branch circuit
    Now, practically speaking, assuming 240 VAC, 80% circuit derating, 0.77 typical array derating:
    • 20 amps * 240 VAC * 0.80 derating * 1/0.77 typical derating = 4,987 watts STC rated panels
    • 30 amps * 240 VAC * 0.80 derating * 1/0.77 typical derating = 7,481 watts STC rated panels
    Now, in practice, the above assumes that the output rating of the inverters is limited to:
    • 20a*0.8=16 amps
    • 30a*0.8=24 amps
    If the inverter is not rated at 16 or 24 amps--then (guessing--I have not studied the code), there is a 0.8 derating for circuit and another 0.8 derating for solar panel conditions--So, if you had a large inverter > 16 amps, then you would have to limit the solar panels to worst case conditions (including low line voltage of 212 VAC for a 238 vac nominal circuit)[edit: removed one of the 0.8 deratings below--forgot the 20 to 16 amp derating already accounted for one]:
    • 16 amps * [removed] * 0.8 * 212 VAC low voltage = 2,713.6 watt solar array
    • 24 amps * [removed] * 0.8 * 212 VAC low voltage = 4,070.4 watt solar array
    The above shows what I, as a design engineer, was always limited to when I was designing power systems for large computers. The worst case conditions were always really bad (from a design point of view)... I have all this power available, but when low line, safety factors (and sometimes dual safety factors), efficiencies are included--It gets very hard to fit everything into the power requirements.

    So, in summary, if you have an inverter that is output current limited to the circuit limit (16/24 amps in the above example)--there is really no Array Wattage Limit because the inverter provides the "Listed" limit regardless of the solar array input power available. Just the practical derating (0.75, 0.77, 0.80, 0.85 or whatever numbers you like) of the solar array so it is cost effective.

    If you have a large inverter that does not manage the output current to the circuit max continuous current limit--Then you are severly limited in the array design output limits. You have to take multiple safety factors into account to comply with NEC ratings because you are dealing with multiple variables that will rarely (if ever) occur on the typical system... For example, my "engineered" limits assume:
    • Clean array with heavy gauge wiring (low losses)
    • Very cold/clear day, edge of could effects (boosts light levels/panel efficiencies)
    • Low line voltage in the face of high output current from a constant power output inverter (P=V*I--Line Voltage falls, inverter current rises)
    • On this same cold day with maximum current, the wiring box and conduit are exposed to maximum ambient temperatures for worst case ambient+I^2*R heating limits.
    Would all of that occur in my temperate climate just south of San Francisco? Probably not. Can I get rid of one of the 0.8 safety factors and assume that my system runs at 240+ VAC (every time I have looked, line is > 240 VAC)--Perhaps I can hand wave my inspector to sign it off.

    If there was a fire and my insurance was looking for reasons not to pay--Would they use my non-compliance with NEC as a reason to avoid payment (and I avoid liability for neighbor's property damage from my fire, and would I avoid criminal charges for injuries/death from such a system design even if there is no evidence that the overheating in the wiring box was not related to my system design)--I haven't a clue--But I am not willing to be the guinea pig.

    Again, just my 2 cents. Others may disagree with my analysis (especially on the code side)--But this is the first pass of the complexity analisys that I see.

    -Bill

    PS: Fixed above where I used an extra 0.8 derating...
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jcgee88jcgee88 Solar Expert Posts: 154 ✭✭
    Re: What is the main reason of upgrading panel from 100A to 200A???
    Batikikik wrote: »
    ...whats the reason to upgrading panel from 100 to 200 A.

    Others who replied based their answers on load limits,
    heat dissipation, NEC codes, etc.

    For me, the answer to your question is more elementary:
    my old 100A breaker box didn't have enough open slots
    to support my needs.

    Actually, while I did do the upgrade to 200A, I didn't do it
    for solar. I ran out of slots when I tried to add some
    circuits during a basement remodeling project a couple of
    years ago. It was serendipity that allowed me to re-use
    that investment when I added a solar array this year.

    John
  • BB.BB. Super Moderators, Administrators Posts: 29,870 admin
    Re: What is the main reason of upgrading panel from 100A to 200A???

    Note: I had a mistake in a couple formulas in my previous post--Have updated to "correct" (in my humble opinion) deratings.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • newenergynewenergy Solar Expert Posts: 291 ✭✭✭✭✭✭
    Re: What is the main reason of upgrading panel from 100A to 200A???
    Batikikik wrote: »
    On 100A main maximum size of the inverter you can connect is SB4000US .Because SB4000us`s output Amp is 16,6 = > 16,6*1.25=20,75A . And breaker size will be 20 A or 25 A ...So in this case yo dont need to upgrade panel .

    Seems to me that 20,75 > 20 and then you need a breaker bigger than 20, so you do need to make some changes. But, that's not really the main point here.

    Batikikik wrote: »
    But if you have larger array and you have to install inverter more that SB4000US so you have to upgrade your panel MANDATORY .
    I called SMA America Tech support and asked this question , and they were agree with me .

    So what is the "rule" regarding changing main panel ???
    When do you need and when u dont ? :)_))))))))
    Thank you

    IMHO, SMA tech support did not give you all your options. You can downgrade the main breaker in this case as long as the loads in the house allow for it. To determine whether that is the case or not, you need to do a load calculation based on all the appliances, outlets, lighting, etc., not just look at what breakers are in the panel.

    There is no "rule" regarding changing the panel. There are changes that are allowed and changes that aren't. Sometimes upgrading the whole panel to 200 amps will be the best solution, sometimes it won't, but there will almost always be alternatives.
  • BatikikikBatikikik Solar Expert Posts: 141 ✭✭✭✭✭
    Re: What is the main reason of upgrading panel from 100A to 200A???

    BB..
    Usually in my installations derate facor is not 0.77 it is 0.803
    when there is some shading issues in derate drops in that case I use microinverters ,
    But when I use central invereter it means for me derate factor is the maximum 0.803

    80% of the 100A I have ever seen those are rating 125 A...
    It means busbar rating is 125 , but main is 100A .
    And th ^formula^ of upgrading panel is if your solar array is less than 5000watts , you can avoid upgrading main panel(with not more than SB400us)
    And if you have more than 5KW system you have to upgrade the panel ...
    I will not suggest to change main from 100 to 80 A ...I know it works I know it will pass inspection but it is not the best way ....Whatever guys depend circumstances .

    I think I am not wrong , If not we can close his topic:)
    Thank you
  • BB.BB. Super Moderators, Administrators Posts: 29,870 admin
    Re: What is the main reason of upgrading panel from 100A to 200A???

    If you are working with a 125 amp panel that only has a 100 amp breaker in it (for whatever reason)--Of course you are allowed to install the maximum breaker allowed (based on the service wiring--which means that if the service/feed wiring may need to be changed out to support the larger breaker rating).

    I guess I am still confused about what you are asking/saying.

    I had a new service installed on my home for various reasons and only went to 125 amp. Should I have gone larger--perhaps. Have I ever come close to its rated capacity in a 3 bedroom home with natural gas appliances and no A/C--No...

    If I ever get an electric vehicle--perhaps I will be kicking myself... But electric power is already so ridiculously expensive (peak summer rates in the $0.40 to $0.60 per kWH range) in Northern California (with the tiered rates)--I have really resisted adding any to my power usage.

    Regarding whatever deratings you wish to use--they do not affect the system electrical design. I don't care if you choose 81% or whatever for derating.

    And the derating should not apply to shading conditions at all--It should represent the ration of output power / input sun around the middle of the day with no shade.

    Obviously, you can fudge the derating as one way to account for AM/PM shading--but it is not what I normally do to "estimate" a good "return on investment" sizing level of Array to Inverter ratio. The lower derating factor also somewhat accounts for dirty panels, slight solar array degridation, etc. over time (6 months to 5 years out). I just like to error on the conservative side when estimating available power. With year over year weather variability of 10-20% or more in many locations--3% among friends is not probably even a measurable difference without lab grade instrumentation.

    It is the output limits of the inverter that guide the branch circuit/panel requirements regardless of the size of the array. Unless you choose a large inverter that can exceed the branch circuit limits--then, I would guess, you need the additional deratings (1.25x safety factor for the solar panels, and low line calculations). Which, dramatically reduce the maximum installed solar array for current limited branch/mains in older homes.

    So--I am still confused about what you are saying is your "disagreement" with how to size the system/main breaker box...

    Are you saying that many 100 amp services really are rated by NEC/UL/NRTL's for 125 Amp breakers and that many folks can avoid the costs of a major physical upgrade from 100 amp to 125/200 amp main service by just swapping out the 100 amp breaker for a 125 amp breaker (plus possibly some pulling a bit of new wire/installing a larger conduit (to account for the 100 amp to 125 amp main breaker change)?

    And that change allows the difference between a ~5kW maximum array vs a ~7.5 kW array without spend thousands of dollars on a service upgrade???? (obviously, there would then be the option of leaving the 100 amp breaker and assuming you now have 20 amp 120% for solar + 25 amps for the 100+25 amp rating of the box or 40 amp branch breaker as no 45 amp breakers are available).

    If that is true (main panel is really rated for 125 amp service)--then I would have no issues with your statement (again--I am certainly no licensed electrician or expert in building codes).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • solar_davesolar_dave Solar Expert Posts: 2,347 ✭✭✭✭
    Re: What is the main reason of upgrading panel from 100A to 200A???

    My best advise is to get a drawing of the design you want to do and take it to the local code department and see if they will buy off on the use of the 100/125 amp panel. Also consult with the Utility involved. Show your loads side breakers in the design as well.

    My installer look high and low for a solution on my install with an exiting 200 amp panel, and the code guys shot him down left and right because of the loads on the 200 amp panel. They even tried a meter side interconnect and the code guys said no way on a residential. The requirement here was to upgrade the service to 400 amps, 400 amp meter head with two 200 amp panels. The required a increased gauge wire pull in the existing conduit from the ground mount transformer, and a full upgrade of the service entrance and breaker panel(s).

    First pic is the old setup, second and third one is the new setup with the added 200 amp sub panel. You will note the main panel has two main breakers, one for the sub and is a 175amp on a 200 amp panel with a 60 amp solar feed in on the sub panel.

    the green sticker is the Code compliance acceptance.
  • newenergynewenergy Solar Expert Posts: 291 ✭✭✭✭✭✭
    Re: What is the main reason of upgrading panel from 100A to 200A???

    BB, I believe the calc for 125 amp busbar and 100 amp main is 125 *1.2 = 150 amps allowed on the busbar - 100 amp main = 50 amps for solar.

    solar_dave, did they do a load calc? It's certainly possible that your load calc comes up to > 175 amps (for downgrading the main), but that is a huge load. I've done load calcs on 6000sf homes with two AC, pool, spa, and more and been around 150 amps. Whatever though, if you are even close, a 400 amp service might be better for you.
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