Service Panel Upgrade Required?

solemio
solemio Registered Users Posts: 16
Our solar contractor has informed us that the planned installation of a 9.900kw DC, 8.484 kw AC system will require an electrical service panel upgrade. They recommend instead to reduce the # of Sunpower panels by 4 to reduce the system to 7.7 kw AC which will be several thousand dollars less and will not require the electric service upgrade which would cost two to three thousand more. Any thoughts?
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Comments

  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Service Panel Upgrade Required?

    Avoid the service panel upgrade. Reduce the Array size to stay within the limits.

    Well, it mostly depends on how easy it is to tear the old panel out, and put a larger panel in the spot. Some wires may not reach, and need 14" spliced on. You may get a new class of service, with a higher base rate. And it's more permits and fees.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • solemio
    solemio Registered Users Posts: 16
    Re: Service Panel Upgrade Required?

    It sounds like a no brainer...
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Service Panel Upgrade Required?

    Check if it does affect your base utility rate (I don't think a larger panel would in my area--bills are based on energy used for residential accounts, from what I have seen/read for California--commercial accounts do have "reservation fees" -- but that is based on actual power consumed--or possibly 15 minute peak generated would count too???).

    Also, it depends on how difficult the job becomes... Is your electrical service such that you can just put a new box in front of your existing service (aesthetically and to code)? Of course, our 1930's home need a new panel anyway for the new circuits that remodeling to current code would require (even if my overall power use was going down because of conservation measures I took too).

    We did this on our home (we had to move the old meter box anyway--was down next to the gas meter only a few inches above the ground--in theory, could have gotten flooded).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • solemio
    solemio Registered Users Posts: 16
    Re: Service Panel Upgrade Required?

    What size electric service panel is required for an 8.4 kw-ac solar system. We have a 200 amp service panel now, installed in 1978, when the house was built.
  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Service Panel Upgrade Required?
    solemio wrote: »
    What size electric service panel is required for an 8.4 kw-ac solar system. We have a 200 amp service panel now, installed in 1978, when the house was built.

    Doesn't work like that.

    The code allows up to 120% of a panel's rating to be connected on the supply side.

    That means that if you have a 200a rated panel, then you can connect up to 240a of supply, which has to go through breakers.

    Thus, if you have a 200a supply breaker feeding into the buss bars from the utility, then you can connect no more than another 40a breaker feeding the buss bars from the solar.
  • solemio
    solemio Registered Users Posts: 16
    Re: Service Panel Upgrade Required?
    dwh wrote: »
    Doesn't work like that.

    The code allows up to 120% of a panel's rating to be connected on the supply side.

    That means that if you have a 200a rated panel, then you can connect up to 240a of supply, which has to go through breakers.

    Thus, if you have a 200a supply breaker feeding into the buss bars from the utility, then you can connect no more than another 40a breaker feeding the buss bars from the solar.

    Then what determines how many panels we can have on the roof? They have reduced the system from 44 sunpower 225's to 40 panels reducing the output from 9.9 kw dc to 9 kw dc.
  • Ralph Day
    Ralph Day Solar Expert Posts: 1,019 ✭✭✭✭
    Re: Service Panel Upgrade Required?

    Sounds like they have the roof of a Wal-Mart available!

    Ralph
  • solar4me
    solar4me Registered Users Posts: 9
    Re: Service Panel Upgrade Required?
    dwh wrote: »
    Doesn't work like that.

    The code allows up to 120% of a panel's rating to be connected on the supply side.

    That means that if you have a 200a rated panel, then you can connect up to 240a of supply, which has to go through breakers.

    Thus, if you have a 200a supply breaker feeding into the buss bars from the utility, then you can connect no more than another 40a breaker feeding the buss bars from the solar.

    Thus a maximum of 40A x 240v = 9.6kW from the solar, right?
  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Service Panel Upgrade Required?
    solemio wrote: »
    Then what determines how many panels we can have on the roof? They have reduced the system from 44 sunpower 225's to 40 panels reducing the output from 9.9 kw dc to 9 kw dc.

    What determines how many panels you can have on the roof is A) how much room you have and B) how much can you afford.

    You could have two arrays. One connected to the main service panel, and one which runs your hot tub and is not connected to the main service.


    The size of the main service entrance panel determines how much you can connect to the main panel. Again, code allows 120% of the buss bar rating. So, with a panel that has a 200a rating, you can connect up to 240a supply to it. Since you almost certainly already have a 200a breaker in there that is from the utility supply, you can only add up to 40a more from solar...

    Unless you either: A) upgrade the panel to a higher rated panel, or B) reduce the size of the breaker controlling the feed from the utility.

    I.e., if you took out the 200a main breaker that is there now, and replaced it with a 180a, then you could put up to a 60a from the solar instead of the 40a you are limited to now.
  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Service Panel Upgrade Required?
    solar4me wrote: »
    Thus a maximum of 40A x 240v = 9.6kW from the solar, right?

    Unfortunately no.

    The breakers have to be rated at 125% of the amperage that the inverter is putting out. 9600w of solar (if it all goes through the inverter with 100% efficiency) would be 40a, true...but 125% of 40a would be 50a. So an inverter putting out a full 40a would need to be connected to a 50a breaker, which would be 250a on a 200a panel, which is too much.

    So, with a 40a breaker supplying to an existing 200a panel, the max coming from the inverter is 7,680w.

    Which does not mean that's the max size of the array...

    There are efficiency losses from wiring and inverter. There are some tricks - for instance it's possible to feed from a larger array into a separate sub-panel and move some of the loads from the main panel to the sub-panel. But, no more than a 40a breaker can be used to feed from that sub-panel to the main panel (for a 200a main panel) unless you downgrade the main service disconnect from the utility.


    Also - I should have noted at the beginning: The 120% exception is ONLY for dwelling units (residential). It is NOT allowed on commercial buildings.


    Here's a good article by John (The Code Man) Wiles:

    http://www.homepower.com/view/?file=HP111_pg94_CodeCorner
  • tallgirl
    tallgirl Solar Expert Posts: 413 ✭✭
    Re: Service Panel Upgrade Required?
    solemio wrote: »
    Our solar contractor has informed us that the planned installation of a 9.900kw DC, 8.484 kw AC system will require an electrical service panel upgrade. They recommend instead to reduce the # of Sunpower panels by 4 to reduce the system to 7.7 kw AC which will be several thousand dollars less and will not require the electric service upgrade which would cost two to three thousand more. Any thoughts?

    Ignore everyone elses advice -- a larger system will have a much LONGER period of usefulness.

    Pay for the panel upgrade (you need it) and go with the larger array (you'll need it when you get your electric car in a few years ...)
  • tallgirl
    tallgirl Solar Expert Posts: 413 ✭✭
    Re: Service Panel Upgrade Required?
    dwh wrote: »
    Doesn't work like that.

    The code allows up to 120% of a panel's rating to be connected on the supply side.

    That means that if you have a 200a rated panel, then you can connect up to 240a of supply, which has to go through breakers.

    Thus, if you have a 200a supply breaker feeding into the buss bars from the utility, then you can connect no more than another 40a breaker feeding the buss bars from the solar.

    First, it's 125%, not 120%. You're confusing the 80% derating for continuous loads. 1 / 0.8 = 1.25.

    Second, it depends on the total ampacity ON the buss bars. I'm getting ready to go find dinner, but if you'd like to argue about it after dinner, I'd be happy to.

    The back feed breaker is connected to the buss on the load side of the service entry breaker. If there were no other breakers on the load side, the largest back feed breaker would be 200A.

    The goal is to protect the buss bar, which is a conductor, from over current. If you had 400A of breakers to subpanels, and a 200A service breaker, anything more than 50A from a back feed breaker would allow the buss bar to be overloaded.
  • sub3marathonman
    sub3marathonman Solar Expert Posts: 300 ✭✭✭
    Re: Service Panel Upgrade Required?
    tallgirl wrote: »
    First, it's 125%, not 120%. You're confusing the 80% derating for continuous loads. 1 / 0.8 = 1.25.

    Second, it depends on the total ampacity ON the buss bars. I'm getting ready to go find dinner, but if you'd like to argue about it after dinner, I'd be happy to.

    The back feed breaker is connected to the buss on the load side of the service entry breaker. If there were no other breakers on the load side, the largest back feed breaker would be 200A.

    The goal is to protect the buss bar, which is a conductor, from over current. If you had 400A of breakers to subpanels, and a 200A service breaker, anything more than 50A from a back feed breaker would allow the buss bar to be overloaded.

    No it isn't. The requirement is that on a residential home the sum of circuit breakers feeding current to a conductor or busbar cannot exceed 120% of the rating of the busbar [NEC 690.64(B)(2) Exception]. A 200 amp panel busbar can thus have at most (200 + 0.2 * 200) 240 amp supplied to it, which usually is 200 amp main breaker plus a 40 amp backfed breaker from the solar PV. Also, I read that rule to state that even if there is just a 15 amp circuit connected to the panel that doesn't matter, you still must supply only a maximum of 200 amps + the 20% overage allowed for residential homes of 40 amps. So if you reduced the main breaker to 180 amps then you could use a 60 amp backfed breaker (which others here have already said too).
  • tallgirl
    tallgirl Solar Expert Posts: 413 ✭✭
    Re: Service Panel Upgrade Required?
    No it isn't. The requirement is that on a residential home the sum of circuit breakers feeding current to a conductor or busbar cannot exceed 120% of the rating of the busbar [NEC 690.64(B)(2) Exception]. A 200 amp panel busbar can thus have at most (200 + 0.2 * 200) 240 amp supplied to it, which usually is 200 amp main breaker plus a 40 amp backfed breaker from the solar PV. Also, I read that rule to state that even if there is just a 15 amp circuit connected to the panel that doesn't matter, you still must supply only a maximum of 200 amps + the 20% overage allowed for residential homes of 40 amps. So if you reduced the main breaker to 180 amps then you could use a 60 amp backfed breaker (which others here have already said too).

    I sit corrected (drinking my coffee) on the 125%.

    And that's a dumb code section. I'll have to find out if it's too late to complain for the '11 revision.

    I can take a plain conductor, not call it a "buss bar", put two 200A breaker in series and use that to hook up a 200A output inverter. But as soon as I call it a "200A buss bar" I can only carry 120A on it?

    I think (B)(7) is confusing and contradictory to how you're reading it. The '08 ROP was much clearer and explained how to get around the limitation you're reading into (B)(2).
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Service Panel Upgrade Required?
    tallgirl wrote: »
    Ignore everyone elses advice -- a larger system will have a much LONGER period of usefulness.

    Two pretty bold statements.

    Both are arguable.

    > Ignore everyone elses advice
    We don't have enough info from the OP as to up sizing the panel is the way to go, or not. Since the OP asked, I'm assuming a non-electrical person, who does not know, and to this point, has not learned much about electricity. The power harvest difference, is small. Perhaps in a few years, there may be more motivation to upgrade and up size the system, and do the panel work then. Electrical contractors may be even more hungry for work in a year.


    > LONGER period of usefulness ? Larger ≠ Longer
    Why would a 12KW system last any longer than a 8KW system? Same panels = 20+ years Different or more inverters, still a 10 -15 year item. Mounting rails last the same.
    Do you mean more useful or more power ? that would be true
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • GreenerPower
    GreenerPower Solar Expert Posts: 264 ✭✭✭✭✭✭
    Re: Service Panel Upgrade Required?

    Here is how I think the installer calculations are:

    * They use 2 x 120V 4800W inverters for 240V -> Max 9.6 KW AC (also 9600W/240V = 40A feedback)
    * 200A panel - > 40A max feedback (I have to concur with dwh, see Wiles article he cited on post #11) -> 40A breaker is used
    * Breaker is 125% wire rating - > 40 A / 1.25 = 32A max current from inverters
    * 32A -> 32A*240V = 7.7 kW AC

    My 2 cents:
    * It is still code compliant with 9.6 KW AC if 40A feedback breaker and 50A rated wire are used. The code states that the max breaker could use is 125% the wire rating but using a smaller breaker is perfectly fine. A 40A breaker would properly protect a 50A-rating wire (and proper for 200A panel backfeed). A 50A rating wire is proper for 40A continuous AC current.

    So, the OP could stay with 200A service panel and the 9.6kW AC. Just use 40A feedback breaker and 50A rated wire from inverter to breaker. I would split the panels, most installed this year and the rest early next year to maximize rebates/tax credits.

    Edit to add: This might cause nuisance trip of the breaker on those cold mornings or "edge of cloud effect" that the inverter could momentarily push the feedback current above 40A.
    GP
  • sub3marathonman
    sub3marathonman Solar Expert Posts: 300 ✭✭✭
    Re: Service Panel Upgrade Required?
    Here is how I think the installer calculations are:

    * They use 2 x 120V 4800W inverters for 240V -> Max 9.6 KW AC (also 9600W/240V = 40A feedback)
    * 200A panel - > 40A max feedback (I have to concur with dwh, see Wiles article he cited on post #11) -> 40A breaker is used
    * Breaker is 125% wire rating - > 40 A / 1.25 = 32A max current from inverters
    * 32A -> 32A*240V = 7.7 kW AC

    My 2 cents:
    * It is still code compliant with 9.6 KW AC if 40A feedback breaker and 50A rated wire are used. The code states that the max breaker could use is 125% the wire rating but using a smaller breaker is perfectly fine. A 40A breaker would properly protect a 50A-rating wire. A 50A rating wire is proper for 40A continuous AC current.

    So, the OP would stay with 200A service panel and the 9.6kW AC. Just use 40A feedback breaker and 50A rated wire from inverter to breaker. I would split the panels, most installed this year and the rest early next year to maximize rebates/tax credits.

    Edit to add: This might cause nuisance trip of the breaker on those cold mornings or "edge of cloud effect" that the inverter could momentarily push the feedback current above 40A.
    GP

    OK, I was following pretty much everything up until the 2 cents. I thought that then if you upsized the wire for example to 50 amp rated and used a 40 amp breaker that would not be acceptable because, quoting from the John Wiles article Making the Utility Connection, www.nmsu.edu/~tdi/pdf-resources/CC111.pdf , "These limits include the normal 80 percent maximum continuous operating-current limitations on the circuit breakers." I thought that if the inverter was thus changed to supplying 40 amp continuous, which it can do, that the breaker would then need to be upsized to a 50 amp breaker. It was originally sized to supply 32 amps continuous with a 40 amp breaker. You could upsize the wire, but you'd still have to stay at 32 amps continuous. Unless somehow there was a 40 amp continuous rated for AC breaker.
  • Windsun
    Windsun Solar Expert Posts: 1,164 ✭✭
    Re: Service Panel Upgrade Required?
    tallgirl wrote: »
    I sit corrected (drinking my coffee) on the 125%.

    ...But as soon as I call it a "200A buss bar" I can only carry 120A on it?

    BTW, it is BUS, not BUSS :p

    After 100 years I still find myself using both.. :blush:
  • GreenerPower
    GreenerPower Solar Expert Posts: 264 ✭✭✭✭✭✭
    Re: Service Panel Upgrade Required?

    Darn, you are correct marathonman :blush:. Actually it's NEC 210.20 (edit: corrected now that I have the book in hand) to size AC breakers, 690-8 is similar but for PV DC side.
    GP
    edit :
  • sub3marathonman
    sub3marathonman Solar Expert Posts: 300 ✭✭✭
    Re: Service Panel Upgrade Required?
    Darn, you are correct marathonman :blush:. Actually it's NEC 384-16 to size AC breakers, 690-8 is similar but for PV DC side.
    GP

    WOW!!! You mean all those nights reading the NEC until 3 a.m. paid off???
  • GreenerPower
    GreenerPower Solar Expert Posts: 264 ✭✭✭✭✭✭
    Re: Service Panel Upgrade Required?

    I guess the viable route for the OP is to replace and reduce the main breaker down from 200A to 180A as dwh suggested and use 50A wire, 50A breaker instead of upgrading the service panel or cutting down the PV panels.
    GP
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Service Panel Upgrade Required?

    Well--I know who to blame for 95+ years of the Bus/Buss confusion:
    1914 — Bussmann founded by brothers Al, Frank, Joe, Harry and Lee Bussmann in St. Louis, Missouri.

    As in Bussmann fuses...

    Regarding effective maximum current... Please feel free to ignore if it confuses you (the previous posts keep confusing me too :roll: )...

    Limit on maximum power/current from a solar array/inverter system...
    1. The array may have a 35% increase in maximum output (or more?) during cold/edge of cloud/snow reflection events.
    2. A MPPT based controller or GT inverter will have internal electronic limits that prevent their output ratings from being exceeded (for example, my GT 3.0 inverter will output a maximum of 3,000 watts because of hardware/software limits--no matter how large the input array is. The maximum fuse rating that the GT inverter may be connected to is listed in the installation manual (and may, or may not, be related to its maximum power/current output).
    3. People here are using 240 VAC nominal voltage... The GT inverter is a constant power device and its maximum current output (fuse/wiring requirements) is based on its minimum operational voltage (GT manual pdf) of 212 (to 263) VAC.
    4. For inverters being operated under there maximum rating (smaller PV Arrays), the maximum power limit will be based on which ever is the least--the actual output of the array or the actual limit of the inverter's output.
    Put all of the above together:

    32 amps * 212 volts = 6,784 Watts output maximum

    Assume that inverter is 95% efficient, max array size would be:

    6,784 watts / 0.95 = 7,141 watts maximum array on a "nominal day"...

    I know that the NEC has methods to calculate the maximum array output margin of safety (I don't have code handy--I am not going to look it up--since I have been out of that side of the business for too long to give accurate answers without a lot of work)--but assume it is 1.25*1.25=1.5625 required derating factor (I think that this is the correct number--but I am not sure).

    So, now the limit of the inverter output will be either:
    • 7,141 watts / 1.5625 = 4,570 Watts of solar PV panels on a 40 amp "240 VAC nominal" branch circuit.
    • Or the maximum rated limit of the inverter (unless it has a programmable current limit that meets NEC requirements) would require an inverter rated at 6,784 watts (or less) maximum.
    Now--will that be saying that other configurations here listed won't work. No. Am I saying that a breaker may nuisance trigger... Maybe.

    Can you assume that your home voltage will not reach 212 VAC on a 220/240 VAC circuit--don't know... My GT system reports around 240-254 VAC when running (mostly >245 VAC when running near maximum output).

    From what I have seen years earlier, the 4,570 watt maximum was a "hard limit" as defined by code interpretations by "John (The Code Man) Wiles"... I have argued that the Inverter Power Limit (as tested and approved by UL/CSA/NRTL's/etc.) are hard power limits and can also be used as operational current limits (i.e., solar array can be way oversized, and it will not affect the safety of the AC side of the modern MPPT Charge Controller / GT Inverter).

    Can you define your power to be 240 VAC and above--when the system will operate down to 212 VAC? That I am not sure about.

    However, when I was designing and building larger computer systems--I always used the minimum line voltage to calculate my maximum current (constant power switching power supplies)--because I could not afford nuisance trips on Telephone gear (voice mail systems in my case).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Service Panel Upgrade Required?
    BB. wrote: »
    Well--I know who to blame for 95+ years of the Bus/Buss confusion:

    As in Bussmann fuses...

    Aye, I believe you are right. I always used buss and I had this idea in my mind that it was something named after a person. Which seems to be completely wrong.

    [Nods to Windsun, "Good catch."]


    Limit on maximum power/current from a solar array/inverter system...

    3. People here are using 240 VAC nominal voltage... The GT inverter is a constant power device and its maximum current output (fuse/wiring requirements) is based on its minimum operational voltage (GT manual pdf) of 212 (to 263) VAC.

    Can you define your power to be 240 VAC and above--when the system will operate down to 212 VAC? That I am not sure about.

    -Bill


    I was just doing a quick and dirty roundoff for illustrative purposes (which is, as you know, my preferred method since if there is detailed math to be done, I'd rather hand it off to the engineers), to answer a quicky question (a question, I point out, from someone other than the OP) which I interpreted as, "Can you add up to 9600w to a 200a panel?"

    And the answer of course is...it depends. :cool:


    To illustrate my point, I stole some numbers from the Wiles article that I referred to (where he used 240v as the basis):

    "Where the main circuit breakers and panels have the same rating, the exception to 690.64(B)(2) allows 20 amps of backfed PV circuit breakers to be added to a 100-amp panel and 40 amps to be added to a 200-amp panel. Although these numbers translate to a 3,840-watt (AC inverter output) PV system on a 100-amp panel and a 7,680-watt PV system on a 200-amp panel, some people want to install bigger PV systems, which requires creative thinking. These limits include the normal 80 percent maximum continuous operating-current limitations on the circuit breakers."


    Fortunately, I could rattle off an answer to his question and still avoid the detailed math, because the only one here worse than me at math is Icarus (and I'm not even sure about that...I might be worse despite his claims to the title :D).


    And to TG: I am usually confused, but it's pretty easy for me to remember that you are allowed to over-breaker the supply by 120% of the panel's bus bar rating, and over-breaker the load by 125% of the panel's bus bar rating...

    Provided that A) you do not put in more total breakers than the panel is rated for, and B) that it's a residential system, since both are "exceptions" which only apply to residential.
  • tallgirl
    tallgirl Solar Expert Posts: 413 ✭✭
    Re: Service Panel Upgrade Required?
    I guess the viable route for the OP is to replace and reduce the main breaker down from 200A to 180A as dwh suggested and use 50A wire, 50A breaker instead of upgrading the service panel or cutting down the PV panels.
    GP

    The problem is the buss bars in the panel (No, I will not change. I've been calling them buss bars (okay, being inconsistent at time ...) for almost half as long as the Bussman brothers have been causing confusion), not something else. If the panel was a buss bar kit with a higher rating, that would be one solution. Another is a simple panel swap.

    And speaking of confusion, I memorized the text in the '08 ROP and then didn't un-memorize it when I bought my '08 last year. The comment was rejected prior to finalizing the '08 Code. Prolly need to re-read my '08 and flush out any stale bits ...
  • GreenerPower
    GreenerPower Solar Expert Posts: 264 ✭✭✭✭✭✭
    Re: Service Panel Upgrade Required?
    BB. wrote: »
    ...the previous posts keep confusing me too :roll:
    ...

    [*]A MPPT based controller or GT inverter will have internal electronic limits that prevent their output ratings from being exceeded
    ... to be more confused, now that I have my NEC book in hand. If the inverter is internally limited to its maximum continuous AC backfed current rating (40A for example), I wonder whether NEC 210.20 (A) exception could be used to allow using a 40A breaker (100%) instead of 50A (125%) i.e. analogous to 690.8(B)(1) exeception on the PV side.
    Tallgirl wrote:
    The problem is the buss bars in the panel ... not something else. If the panel was a buss bar kit with a higher rating, that would be one solution. Another is a simple panel swap.
    Julie, using 180A main breaker would make 180+50=230A (40A backfed, 50A breaker), well below the 240A allowed for the existing bus-bar. This is also cited as one of the solution on an article from "John (the code man) Wiles". If the panel is Square-D, changing the main breaker is much easier.
    GP
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Service Panel Upgrade Required?

    GP,

    I am not quite sure what question you are asking--so I will attempt to answer the question with Made Up Numbers inserted to make sure we are talking about the same thing.

    My understanding of the NEC (and really, the safety rules behind them) is that the limit for current and fusing/breaker...

    Basically, you must define and run the circuit at the wiring/breaker at 80% maximum (and conduit, etc.).

    Now, if you are talking about a "32 amp" maximum rated inverter--then you can run it on a 40 amp breaker/wire branch service. (32A * 212 VAC=6,784 watts --or-- 32A*240VAC=7,680 watts maximum inverter output; depending on how you role...)

    So, if you are talking about a GT inverter that tells you it requires a 40 amp service--then I would assume it is limited to a maximum current of 32 amps.

    If the GT inverter tells you it is a maximum output of 40 amp (240v*40a=9,600watt), then you would need a 50+ amp branch service.

    From what I have seen before, J. Wiles was requiring Solar Charge Controllers and GT Inverters that had current/power limiting circuits as part of their design, the branch service required a 1.25*1.25 safety derating vs the solar array Pmp output--and that is what I have always argued was not a correct interpretation of the safety requirements (was requiring a 1.25 wiring safety factor--that is OK; with a 1.25 solar array edge of cloud/cold weather safety factor--which was not required if the GT inverter was setting the branch current/power limit). If my paragraph here makes any sense. :roll:

    If you are have, for example, a 10 kWatt inverter and a 40 amp branch circuit available (for example, because of the 120% bus bar rule)--It would be fair to add the 1.25*1.25 derating of the solar array because it can easily exceed STC rating in some conditions. And the 10kWatt inverter will easily exceed the 32 amps allowed on a 40 amp branch service because it will output anything fed to it--up to 10kWatts.

    When you start putting in the 1.25*1.25 derating factor on (A) 7kW array with a 10kW inverter instead, of for example, using (B) a 6kW inverter and its "hard limits" and a 7kW array--the NEC really requires you to install less solar panels with A than you could get away with using B. Ignoring round-off errors--system B can install 1.25 times more solar arrays (or more) and generate 1.25x (or more) power than system A. (when you are limiting branch circuit ampacity).

    I hope what I wrote makes sense.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • sub3marathonman
    sub3marathonman Solar Expert Posts: 300 ✭✭✭
    Re: Service Panel Upgrade Required?
    BB. wrote: »

    From what I have seen before, J. Wiles was requiring Solar Charge Controllers and GT Inverters that had current/power limiting circuits as part of their design, the branch service required a 1.25*1.25 safety derating vs the solar array Pmp output--and that is what I have always argued was not a correct interpretation of the safety requirements (was requiring a 1.25 wiring safety factor--that is OK; with a 1.25 solar array edge of cloud/cold weather safety factor--which was not required if the GT inverter was setting the branch current/power limit). If my paragraph here makes any sense. :roll:

    If you are have, for example, a 10 kWatt inverter and a 40 amp branch circuit available (for example, because of the 120% bus bar rule)--It would be fair to add the 1.25*1.25 derating of the solar array because it can easily exceed STC rating in some conditions. And the 10kWatt inverter will easily exceed the 32 amps allowed on a 40 amp branch service because it will output anything fed to it--up to 10kWatts.

    When you start putting in the 1.25*1.25 derating factor on (A) 7kW array with a 10kW inverter instead, of for example, using (B) a 6kW inverter and its "hard limits" and a 7kW array--the NEC really requires you to install less solar panels with A than you could get away with using B. Ignoring round-off errors--system B can install 1.25 times more solar arrays (or more) and generate 1.25x (or more) power than system A. (when you are limiting branch circuit ampacity).

    I hope what I wrote makes sense.

    -Bill

    OK, I do understand some of this. But, should you have said (where I've added bold), "and that is what I have always argued was not a correct interpretation of the safety requirements (was requiring a 1.25 wiring safety factor--that is OK; with a 1.25 solar array edge of cloud/cold weather safety factor--which was really not required if the GT inverter was setting the branch current/power limit), but the NEC has it there anyway. "

    So that leads to the question about the final paragraph, where you say "could get away with." Will the NEC allow a 1.25x larger PV array if your inverter is a 6kw inverter, since the inverter is the limiting factor, or do you still, according to the NEC, have to do the derating factor and essentially still end up in the same place as with a 10kw inverter?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Service Panel Upgrade Required?

    In the end, code compliance is up to the local inspector. Not me (or even the NEC).

    My argument basically boils down to this. I have a 60 watt AC to DC power supply for my laptop PC. And I connect that to a 15 amp 120 VAC circuit... Is the power brick's output based on 12 volts and 60 watts (i.e., 5 amps) or 15 amps and 120 VAC down converted to 12 volts (or 150 amps)?

    And the obvious answer is that that power brick's output is designed/configured based on 5 amps--not 150 amps. Because the power brick is output limited (and there are other safety issues--i.e., a double insulated barrier between AC input and DC output, and the output has been designed and inspected to meet SELV--safety extra low voltage for its output).

    A GT inverter has the same basic parts... A input that is isolated from the output and conversion hardware/software that limits is current/power transfer capabilities--just like the laptop power brick.

    So--I could put a 10kW array on the input of my 3kW GT inverter, and the output will never exceed 3kW by design and construction. If it could, the GT would have never achieved NRTL Listing.

    For example, looking at the GT 5.0 user manual from Xantrex (page 77). They do not list the maximum panel wattage--but the Imp and Isc (max power and short circuit). For the GT 5.0, it is 22 amps/24 amps. And the unit operates from 235 to 550 VDC array voltage...

    24 amp Isc means that the wiring/protection should be designed for 24A*1.25=30amps (I would argue that you can put an array with Isc>24 amps if you use a 30 amp fuse/breaker on the PV input to the GT inverter and meet the input specifications).

    Maximum Array input specifications for GT 5.0:

    22 amps * 235 VDC = 5,170 watts array input
    22 amps * 550 VDC = 12,100 watts array input

    Maximum output current (240 VAC) is 21 amps:

    21 amps * 212 VAC = 4,452 watts
    21 amps * 240 VAC = 5,040 watts
    21 amps * 263 VAC = 5,523 watts

    Maximum output current protection (branch circuit breaker?) is 30 amps.

    21 amps * 1.25 = 26.25 amp minimum branch circuit (round up to 30A).

    A couple of notes--I used Xantrex here because I am familiar with their manual and installation (I have a 3kW GT from them in my home).

    The NEC limitations/recommendations appear to have been written to include J. Wiles version of how he would design something (RF grounding for example, which has no place in the NEC). As best as I can determine from my limited readings--his calculations would only apply to "simple" solar charge controllers (straight On/Off or PWM) where current through the device is limited by the input array--not with internal limitation circuits.

    Also, the above is my personal observations and opinion--others (including inspectors) may disagree. And I have made one or mistakes in my interpretations (many safety issues are proprietary and not released--an inspector or designer sometimes needs to go back to the manufacturer/listing holder/NRTL lister to find "hidden limitations"--I had found that to be a real frustration at times when designing equipment).

    I am here to learn.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • GreenerPower
    GreenerPower Solar Expert Posts: 264 ✭✭✭✭✭✭
    Re: Service Panel Upgrade Required?

    OK, I'm gonna beat this dead horse to death so that we don't have to "labor" on this over or after the labor day.
    Marathonman, you threw me off my original thought. Now that I've got my thoughts back and straightened myself, let see if I can clarify these "confusions".

    I think these are commonly mis-interpreted:
    * The amperage calculation for a wire is the "minimum required", not "must use" i.e. can use bigger wire and still meet code and certainly not a fire hazard.
    * The amparage calculation for a breaker is the "maximum allowed" i.e. can use smaller breaker and also still meet code , not a fire hazard. This is commonly interpreted as "must use" which causes confusion.

    For example, on a branch circuit with a 15A breaker on a #14 wire for a light fixture, you can replace the wire with #8 and the breaker with 5A rating one (if one exists). This might cause nuisance trips of the breaker which is inconvenient but is still perfectly to code and not a fire hazard.

    Back to the OP case:
    * The 120% rule on the bus bar calls for "maximum allowed" 40A breaker. Using a normal breaker rated 40A is perfectly to code (even a 30A if you want to), even though it might trip at continuous 32A in a closed panel box (the breaker is tested to operate at 40A continuous in an open air environment, it would guarantee not to trip at 32A continuous i.e. at 80% rating in a closed panel box). Using a breaker rated for 100% at 40A (i.e. designed and tested for in a closed box) would prevent the nuisance trips.
    * 50A rated wire for 40A continuous and the 40A branch breaker are OK i.e. to code (and perfectly not a fire hazard) even if the inverter can provide more than 40A continuous. If the inverter spec says max backfeed current in fault condition is 50A and max breaker used is 50A. It is OK to use 40A breaker, 50A spec'ed is "maximum allowed", not "must use". with 50A wire, 40A breaker, the wire and the busbar are properly protected from over-current conditions even though it could cause nuisance trips which is inconvenient but not fire hazard (the breaker would likely trip at more than 32A continuous, not burned) and perfectly to code.

    So, for the OP, his choices are:
    * Use 50A wire, normal 40A breaker and risk often nuisance trips
    * Use "100% rated" 40A breaker and reduce the chance of nuisance trips. If the inverters backfed current is spec'ed to be maximum at 40A, then the chance of nuisance trip is close to nil. But I'm afraid that with 40A breaker, the inspectors would use J.Wiles articles and dictate 32A backfeed as limit on the PV/inverter.
    * Use 50A breaker and replace the main 200A breaker down to 180A breaker (for inspectors that use "the code man" articles as their bible).
    * Use a higher rated service panel

    Now to the "exception" cases:
    * The 125% calculation for PV amperage is for cold days, edge-of-cloud and higher than 1 kW/meter-square "norm" insolation. This is considered the maximum current the PV could put out. Wiring and breaker sizing are based on this calculated current i.e. another 125% resulted in 156% of the PV rated current. But keep in mind that wire sizing is "minimum required" and the breaker size is "maximum allowed", not "must use". As above, if "100% rated" breaker is used, only 125% of PV rated current could be used for that breaker (normal breaker could trip at 80% rated continous current in a closed box). Due to the nature of PV, if "156%" or larger wire is used, breaker is not required (for a single PV series string).

    Make sense ?
    GP
  • tallgirl
    tallgirl Solar Expert Posts: 413 ✭✭
    Re: Service Panel Upgrade Required?
    ... to be more confused, now that I have my NEC book in hand. If the inverter is internally limited to its maximum continuous AC backfed current rating (40A for example), I wonder whether NEC 210.20 (A) exception could be used to allow using a 40A breaker (100%) instead of 50A (125%) i.e. analogous to 690.8(B)(1) exeception on the PV side.

    Only if it is rated at 40A continuous. 40A x 1.25 = 50A ...
    Julie, using 180A main breaker would make 180+50=230A (40A backfed, 50A breaker), well below the 240A allowed for the existing bus-bar. This is also cited as one of the solution on an article from "John (the code man) Wiles". If the panel is Square-D, changing the main breaker is much easier.
    GP

    Right, but it would reduce the available current when the sun isn't shining.

    In my professional opinion, a panel upgrade is a better long term solution.