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Thread: 120% RULE adding PV to a loadcenter

  1. #1
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    120% RULE adding PV to a loadcenter

    One of the PV Salesmen i install for sold an 8kv kw system to a customer with a 150 amp service. My first thought was to include a service panel upgrade to 200amp with my estimate. While discussing this, the customer mentioned that most manufactures use the same size buss bars in 150 and 200 amp panels.
    i have been to Eatons site and Square D's site and can find no data or cut sheets on the actual physical size of the buss bar. Can i use standard buss ratings of 1000amps per square inch, or just 120% and thats it?

  2. #2
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    You can use 120% of whatever the manufacturer says the panel is rated at.
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  3. #3
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    PV BREAKER

    Back to the basics i guess...
    30 M215 micro inverters max output 215wts being installed on a home with a 150amp main breaker panel.
    150 amps + 20% = 30amps additional service may be added to this buss bar.
    30 x 215 = 6450wts 6450/240v= 26.875 amps additional service.
    Now sizing the PV breaker that i connect to the bus bar.... 26.875amps x 125% = 33.59amps round up to 35amp breaker.

    does this meet my requirements?

  4. #4
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    No. The maximum permitted total of all of the breakers that can supply power to the 150 amp rated bus is 180 amps.
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  5. #5
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    M215 inverters on 240V must be used only with the proper Enphase harness, and the harness can not be connected to a breaker exceeding 20A. The limit is 17 M215 inverters per harness. If 13 inverters are used on a harness, a 15-amp CB can be used (0.9A x 13 /.8=14.6A), so the limit is 26 inverters for 30A of breakers. The 120% rule will allow only a 30A CB on a 150A panel with a 150A main breaker.

    You have 5 choices:
    1. Replace the 150A panel with a 200A panel (with a 150A main CB if the service conductors are not rated for 200A).
    2. Downsize to 26 inverters;
    3. Do a load analysis and see if the 150A main CB can be downsized to 125A;
    4. Investigate a custom replacement buss bar for your panel, Eaton makes these for many applications (but they are expensive);
    5. Line side connection made between the meter and the main breaker.

  6. #6
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    Article 705.12

    I think this 120% rule is coming from Code Article 705.12(D)(2). Which is very easy to overlook because of Article 690 of the NEC . It says the sum of the ampere ratings of the overcurrent devices in circuits supplying power to the busbar or conductor shall not exceed 120% the rating of the busbar or conductor.

    According to my interpretation. If you have 150A utility MCB and a 30A inverter breaker you add those together which is 180A , multiply by 120% which is 216 amps, so you need even larger than 200 Amps. Unless you can do what Billz suggested and reduce your MCB size.

    in addition many manufacturers use larger busbars in their panel boards because it is cheaper to make 1 bus bar size and put a different nameplate on it, than it is to make slightly different size bus bars for every panel size. The easiest way to see is to call your SquareD rep, and ask if your particular panel board falls in this category. If it does, then ask if they can send someone out and change the nameplate for you. I have done this before but only a 600A panel i needed to be a 800A panel.

  7. #7
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    Quote Originally Posted by crazedengineer View Post
    ...According to my interpretation. If you have 150A utility MCB and a 30A inverter breaker you add those together which is 180A , multiply by 120% which is 216 amps, so you need even larger than 200 Amps. Unless you can do what Billz suggested and reduce your MCB size. ...
    No. The rule permits the total rating of all of the breakers that can feed power into the panel bus to exceed the bus rating by 20%. If the panel bus is rated at 150 amps you are permitted to add a 30 amp breaker that is fed from interconnected power production equipment.
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  8. #8
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    Quote Originally Posted by don_resqcapt19 View Post
    No. The rule permits the total rating of all of the breakers that can feed power into the panel bus to exceed the bus rating by 20%. If the panel bus is rated at 150 amps you are permitted to add a 30 amp breaker that is fed from interconnected power production equipment.
    Opps, Don you are right. I read this one backwards.. So I will back pedal this one.

    Since your Panel board is 200 amps. You are fine if the sum of your supply breakers is less that 240A (200Amps x 120%)

    So 150amps + 30 Amps is 180Amps which is less than 240amps.

    My apologies for the original response.

  9. #9
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    I'd like to jump in here. I have dealt with the 120% rule on multiple occasions and I have a new scenario.

    Consider a 250A/3-Phase MLO distribution panel as our service panel. The panel utilizes the (6) handle rule as it is fitted with (2) 150A/3P to feed branch panels. There is also a 40A/3P for TVSS and a 75A/3P back-feed PV interconnection breaker.

    The 120% rule allows 1.2 x 250 = 300A. But what does that mean? Because there is no main breaker, are we allowed up to a 300A/3P PV breaker?

    Anyone encountered this before?

  10. #10
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    Quote Originally Posted by cdcengineer View Post
    I'd like to jump in here. I have dealt with the 120% rule on multiple occasions and I have a new scenario.

    Consider a 250A/3-Phase MLO distribution panel as our service panel. The panel utilizes the (6) handle rule as it is fitted with (2) 150A/3P to feed branch panels. There is also a 40A/3P for TVSS and a 75A/3P back-feed PV interconnection breaker.

    The 120% rule allows 1.2 x 250 = 300A. But what does that mean? Because there is no main breaker, are we allowed up to a 300A/3P PV breaker?

    Anyone encountered this before?
    I would see that as a supply side connection and would permit the PV breaker to equal the rating of the service.
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

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