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Thread: Existing PV system over sized and potentially dangerous?

  1. #1
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    Existing PV system over sized and potentially dangerous?

    I have a theoretical question:

    I am an electrician, and was performing some work on a customer's house, when I noticed a 70A PV backfeed breaker inside the Cooper B-line 300A Meter-main load center combo. I asked the homeowner about the PV system, and he informed me that he has a 60 module PV system comprised of Enphase M215 microinverters. The spec sheet for the M215 shows a nominal output current of 0.9A, thus making the total output current (0.9A)(60)=54.0A. Sizing the breaker to 125% gives (54A)(1.25)=67.5A, which explains the 70A breaker, but exceeds the allowable feedback of the 300A enclosure by a theoretical 7.5A if considering the actual output current of the microinverters, or 10A if considering the OCP size. The main enclosure is a 300 amp rated enclosure with a 300 amp main breaker, so my understanding is that the maximum allowable feedback breaker would be 60A, with an actual maximum output current on the microinverters of 48.0 amps - to comply with the breaker being 125% of the continuous load. The PV breaker is also located at the opposite end of the buss as the main breaker. To me, Kirchoff's law seems to say that the sum of the current flow through any point of the buss bar cannot possibly exceed the rating of any load breaker, which are all either 2P100 or 2P50 breakers. Am I missing something? Do I need to tell the homeowner that he is at risk of having an electrical meltdown in the enclosure, or does Kirchoff's Law of Current Flow show that, theoretically the system is fine, just violates the 120% rule? I don't want to open my mouth to the homeowner without making sure I know what I'm talking about. Any insight would be appreciated.

  2. #2
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    Quote Originally Posted by MHF View Post
    I have a theoretical question:

    Sizing the breaker to 125% gives (54A)(1.25)=67.5A, which explains the 70A breaker, but exceeds the allowable feedback of the 300A enclosure by a theoretical 7.5A if considering the actual output current of the microinverters, or 10A if considering the OCP size.

    I realize I type this part wrong. What I meant to say is that given the actual output current of the microinverters, the PV feedback is withing the allowable 60 amp range (54.0A total current output), but when taking 125% of the output current, the value 67.5A exceeds the allowable 60A feedback by 7.5A.

  3. #3
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    You can take as given that based on actual current flow at any point on the bus will be below the bus rating at anything up to 200% rather than just 120%. But the code does not let you go that far because no testing has been done of that configuration, and so they are verrry conservative.
    So your example is probably safe but definitely a code violation.
    Keep in mind that the latest codes start the calculation with the rated inverter output rather than the selected or required breaker size.

    Sent from my XT1585 using Tapatalk
    Last edited by GoldDigger; 09-16-18 at 02:59 AM.

  4. #4
    Join Date
    May 2011
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    4,577
    To agree with Goldigger...

    I would not tell the customer that he's at risk of an electrical meltdown because that would be an exaggeration. 6A over an arbitrary and conservative code requirement doesn't call for such language.

    However I would tell the customer that the installation isn't to code, and that it could interfere with passing inspections on any other work and that it's not your responsibility to pay for that fix since it was existing.

    One way to fix might be to change the 300A breaker to a 275. If you can find the right one.

  5. #5
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    Thank you for your responses. I'm not sure yet how this system has been approved, if at all. But, I also agree that given the theory of current flow, it's not a real issue. I've personally always been confused about where the 120% rule came from - other than it being a conservative, arbitrary value as mentioned. I've also heard rumors that the 2017 revision is under review to be increased to 150% of the bus rating, but that is just through conversation in forums like these. I'm trying to find the cold, hard documentation that explains the origination of the 120% rule, and exactly why it is that value, but I just can't find it. Thank you again for your input - it is greatly appreciated.

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