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Thread: load side connection calculation

  1. #1

    Default load side connection calculation

    I'm installing my first system with 2 arrays. I need help understanding the calculations for the back feed breakers.

    I have a 200 amp service

    I'm installing 2 arrays. I planned to run out from each inverter to their own AC disconnect then to individual meters for the PA Sunshine rebate program and then into the service panel with a backfeed breaker for each array. Forgive me, but I seem to be having brain freeze at the point of interconnection to the service panel. Below are the specs. All assistance is much appreciated.

    Solar System Specs:
    Array #1
    3 strings of 5 - Sanyo HIP-215NKHA5

    Fronius IG4000
    The nominal output is 16.7A (According to Section 690.8, the output circuit must be sized at 125% of the rated output) Then the safety factor would make it 20.875A.

    There is an Outback combiner box mounted on the roof. EMT runs from the combiner through attic to DC disconnect in garage.


    8 AWG wiring from the combiner to the DC disconnect (account for heat running through attic to diminish voltage drop)

    EMT runs from the DC Disconnect to the inverter located in the basement. (Utility supplied lockbox for key to basement)
    10 AWG wire runs from the DC Disconnect to the Fronius inverter.

    Array #2
    3 strings of 6 with the same setup as described above.

    From what I understand my backfeed breakers would need to be 25A for each array. When the code refers to OCPDs supplying power would that be the backfeed breaker? So then I would have 50A supplying power.

    "The sum of the ratings of the breakers may not exceed 120% of the rating of the bus bar or the ampacity of the conductor." John Wiles

    In a 200 amp service is the bus bar rating 200amps? (I believe it is the terminology that throws me not the concepts/theory) If yes then 120% of 200A is 240A. Then my two 25A backfeed breakers are safe to install, right?

    My partner spoke to an electrical inspector. The inspector suggested that he has seen installers with two arrays go into a sub panel with one AC disconnect for panel and then only 1 meter. My partner is not trained in solar PV, but he does have a very good knowledge base, but I don't trust that he accurately described the system we are installing. (Since this is my first post I have to figure out how to upload a photo.)

    I'm coming to all of you because my electrician seems to be in never never land when I try to discuss this with him about code.

    I took my SEI class last year and there is much to remember and understand, let alone try and decipher the NEC 2008 code book.

    If I have missed supplying you information please yell and I will get it to the discussion.

    Thanks,
    Kim
    Attached Images Attached Images  
    Last edited by HTG PVinPA; April 25th, 2010 at 23:18 PDT. Reason: Forgot to sign my name

  2. #2
    Join Date
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    Default Re: load side connection calculation

    120% of the busbar rating for a 200A service is 240A. 240A would then be the maximum sum total of allowable amps of all breakers that feed power into the main busbar. You cant use two 25A breakers with 200A service and a 200A main breaker. That would total 250A, which is 10A more than allowable. per the NEC.

    You will either need to do a line side tap outside
    OR
    Install a 150A or 175A main breaker in your main panel
    (240A - 150A = 90A of allowable backfeed)
    (240A - 175A = 65A of allowable backfeed)
    Then, install your two 25A backfeed breakers at the BOTTOM of the busbar.


    Note: You'll likely find that when you go looking for main breakers on all the usual places, that 175A are more expensive and generally harder to come by than 150A main breakers. A new old stock 150A cuttler-hammer cost me $60.
    Xantrex XW6048, XW-MPPT-60 SCC, Midnite E-Panel, 36 SunXtender PVX-890T @ 801ah, Briggs Elite 8kw, Midnite PV Combiner, 12 Sharp 224s / 2.6kw , Bogart Tri-Metric Meter, 2 EKM Meters (On AC1 and Load)

  3. #3

    Default Re: load side connection calculation

    Great! Now I understand what they mean. Since I'm feeding the service panel with 50A from the PV and the utility is feeding it with the 200A from the street then we overload the panel.

    Could someone like you write the NEC code?

  4. #4
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    Default Re: load side connection calculation

    You should be able to combine the 2 PV feeds into one, and run a single meter. connection to the panel. The breaker rating would then be the next size up for the 2 amp ratings of the inverters max AC output.

    Now as far as de-rating you panel main, you need to consider the max draw that would possibly take place. If your load side can exceed the main de-rated breaker, you will be looking at a service entrance upgrade. At least that is what happened here and was required by the local code guys. We tried to do a line side tap, however the code guys shunned that kind of solution and said the only time they allowed that was a large commercial install.
    My TED 5000 system
    Sticking it to the power company one watt at a time!
    60 Ningbo Electric 175 watt panels and 12 Canadian Solar 180 watt panels with 2 PVP 5200 Inverters

  5. #5
    Join Date
    Jun 2003
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    pittsburgh, pa
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    10,282

    Default Re: load side connection calculation

    sd,
    technically when feeding an electric panel with both solar and with the utility ac there will always be more power available within the buss of the panel than the main breaker could handle. the problem is that the total power should not exceed that of the buss rating. one way to insure the buss rating is to lower the main breaker rating, that i assume you would not want to derate your solar input to the panel, which is not always conducive with the home requirements and hence a panel upgrade is then warranted.
    NIEL

  6. #6
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    Default Re: load side connection calculation

    Quote Originally Posted by solar_dave View Post
    You should be able to combine the 2 PV feeds into one, and run a single meter. connection to the panel. The breaker rating would then be the next size up for the 2 amp ratings of the inverters max AC output.
    Not going to help in this case as he has two inverters rated at 16.7A.

    16.7A * 2 = 33.4A * 1.25 = 41.75A

    Rounding up he could use a 45A breaker - not that common, though, so he'd likely still end up feeding the main panel with a 50A breaker.

    Only way around it is to downsize the main breaker as previously suggested or tap into the feed in between the main meter and main panel.

  7. #7
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    Default Re: load side connection calculation

    Quote Originally Posted by drees View Post
    Not going to help in this case as he has two inverters rated at 16.7A.

    16.7A * 2 = 33.4A * 1.25 = 41.75A

    Rounding up he could use a 45A breaker - not that common, though, so he'd likely still end up feeding the main panel with a 50A breaker.

    Only way around it is to downsize the main breaker as previously suggested or tap into the feed in between the main meter and main panel.
    i think you missed my point. In my case we planned to downsize the main, however with the loads already in the service were over the required downsize value. Code dudes said unacceptable. So if you add up all the loads and then add in the solar amperage it can't be more than 120% of the buss bar rating. Their fear was a max load in off hours constantly tripping the de-rated main. Yeah I know, I would have had to run both AC, units, the stove and oven, clothes dryer and washing machine, pool pump, microwave, every light, hair dryer, TV in the place all at the same time.
    My TED 5000 system
    Sticking it to the power company one watt at a time!
    60 Ningbo Electric 175 watt panels and 12 Canadian Solar 180 watt panels with 2 PVP 5200 Inverters

  8. #8
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    South of Erie, North of Pittsburgh PA
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    Default Re: load side connection calculation

    For What is worth Pa was still using 2005 code last december when I installed my fronious inverters. Think 2005 nec is easier to meet. I have 2 inverters into a breaker box and 1 240 v output to shutoff switch at my power pole that is for a mobilhome. Slarvic
    16 KC 158G & 3 KD185GX-LPU panels on Fronius IG PLUS 3.0-1 inverter and 14 SHARP NDU3A & 1 KD185GXU panel on FRONIUS IG-3000 inverter. All mounted on pole top racks. Retired and enjoying it!!

  9. #9
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    Default Re: load side connection calculation

    Quote Originally Posted by solar_dave View Post
    i think you missed my point. In my case we planned to downsize the main, however with the loads already in the service were over the required downsize value. Code dudes said unacceptable. So if you add up all the loads and then add in the solar amperage it can't be more than 120% of the buss bar rating. Their fear was a max load in off hours constantly tripping the de-rated main. Yeah I know, I would have had to run both AC, units, the stove and oven, clothes dryer and washing machine, pool pump, microwave, every light, hair dryer, TV in the place all at the same time.
    Eh? It's not as simple as adding up all the loads and seeing if they add up to more than the size of the main breaker or not. Never-mind that the whole point of the NEC allowing 20% busbar rating (as long as you put your backfeed on the opposite end of the main breaker) already accounts for possible bus-bar overloading.

    There's a proper way to calculate your minimum main breaker size based on the size of your house, the number of small appliance and laundry circuits and all your hard-wired appliances.

    This is typically called a "Load service calculation" or similar. It's spelled out exactly in the NEC.

    Up to the first 10,000 VA, the loads get counted at 100%, but after that, most loads (except HVAC) only get counted at 40% of their rated value.

    And all your hard-wired appliances only get rated at their nameplate Volt-Amps value, not the size of the circuit they're plugged in to.

    The size of your PV system doesn't come in to play in this calculation at all.

    For example for my house, I have a 100A main breaker, but probably have close to 200A of circuit breakers plugged in to the main panel (counting only half of the single pole breakers since the main breaker is a 2-pole). But doing a proper load service calc reveals that a 100A main breaker is sufficient (though just barely in my case).

  10. #10
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    Default Re: load side connection calculation

    What can I say, I can only go by what the local code guys would accept. My 200 amp panel with a 50 amp breaker add for the solar would have overload the bus, reduction to 175amp was insufficient, and a 150amp de-rating would have been required. With 2 AC units, electric stove/oven, electric dryer & washer, ........ 150 amp would not have made it according to the code department.

    All of this required a $4300 service entrance upgrade to satisfy them. It is fine that NEC makes those statements but the ultimate stamp of approval comes from the local code department.
    My TED 5000 system
    Sticking it to the power company one watt at a time!
    60 Ningbo Electric 175 watt panels and 12 Canadian Solar 180 watt panels with 2 PVP 5200 Inverters

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