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Thread: Panel and Main Circuit Breaker Sizing for Solar (PV) System

  1. #1

    Question Panel and Main Circuit Breaker Sizing for Solar (PV) System

    I'm trying to size a MCB panel for a Photovoltaic grid-tied commercial job.

    I want to make sure that I am sizing it properly.

    I have a total of 14,868 DC watts coming into a PV inverter 96% efficient. It is a 120/208 3-phase system. Which calculation is correct.

    (14,868 VA x 0.96 (efficiency) / (208V x 1.73) = 40 A x 1.25 OCP = 50A approx.

    or

    (14,868 VA x 0.96 (efficiency) / (208V) = 69 A x 1.25 OCP = 86.25 A approx.

    Your help is much appreciated!

  2. #2
    Join Date
    Feb 2003
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    Illinois
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    Does the inverter have a single phase or a 3 phase output?


    correction: It said "5 phase" when I made the orginal post
    Last edited by don_resqcapt19; 10-25-10 at 11:57 AM. Reason: correction
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  3. #3
    Join Date
    Jul 2006
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    San Francisco, CA, USA
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    3,604
    If you are referring to the breaker coming from the inverter, I believe it would need to be based on the maximum current rating of the inverter as shown on the inverter nameplate, not the measured power coming into the inverter.

  4. #4
    The output on the inverter is a L1, L2 (2 hots coming out of each inverter). We have a balanced input into the Electric Panel.

    (3) inverters total. Inverter #1 A,B phase, Inverter #2 C, A phase, and Inverter #3 B,C phase.

    The only thing that I am asking is which one of the calculations is correct? The one calculation that divides by just (208 V) or the calculation that divides by (208V x 1.73)???

  5. #5
    Join Date
    Jul 2009
    Location
    Mesa Arizona
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    PV inverter OCPD

    Most inverter installation instructions will specify a maximum value for the inverter OCPD (Over Current Protection Device = fuse or breaker). This is based on the maximum PV array that can be connected to the inverter and it is possible that your array may be increased in the future. Wire size is based on the OCPD rating. You may have difficulty with the AHJ if you select the OCPD based on PV array rating.

    Once you determine the OCPD rating, then you need to check that the main panel is suitable. The sum of the PV OCPD and the main OCPD can not exceed 120% of the buss rating.

  6. #6
    Join Date
    Sep 2008
    Location
    Los Angeles
    Posts
    6

    RE: Panel and Main Circuit Breaker Sizing for Solar (PV) System

    Efficiency and number of modules will not gone affect your CB size-zing.
    In your case we use 15 kw inverter 3 phase 120/208.
    15 kw /( 208 V x1.73) = 42 A x 1.25= 52 A so CB 60 A .
    Check catalog of manufacturer of the inverter you will gone have" Maximum Cont. Current (AC)" . and than multiple by 1.25 .

  7. #7
    Florinm16 thank you, I know that I have to size to the inverters max output plus the OCP factor, the manufacture has all the breaker sizes in the instruction manual. I just wanted to confirm that since it is 3-phase, I divide by the (208 x 1.73) instead of just 208V in the denominator by itself... That was the only thing I was unsure of and now we will be able to save some money and make more money since we can go with a lower wire size and smaller panel. Beautiful thing, we have another job but this one has four inverters, and it is impossible to have a balanced load with four inverters. Does the same calculation hold true? I'm guessing so, but just want to see some other opinions?

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