Properly sizing a PV inverter breaker

From Ronnie Raxter (guest post) on 02/17/2015 in Category Technology with 14 Comments
PV Inveter Breaker - SMA Inverted

Believe it or not, code references for determining the calculation to adequately size a PV inverter breaker are longer than the calculation itself. Don’t be intimidated into making a costly mistake when designing a customer’s solar system.

The calculation is simply the maximum output current of the inverter multiplied by a 125 percent safety factor, then rounded up to the nearest breaker size.

Two standard PV breaker examples:

A maximum output current of 16A multiplied by a 125 percent safety factor equals 20A. This happens to be a standard breaker size.

A maximum output current of 22A multiplied by a 125 percent safety factor equals 27.5A. The next standard breaker size is 30A.

An odd PV breaker example:

A maximum output current of 24.1A multiplied by a 125 percent safety factor equals 30.12A. If you rounded up, the next standard ampere rating is 35A. However, if you have a decimal fraction that is smaller than 0.5 the fraction can be dropped. Therefore you would only have 30A, which happens to be a standard breaker size.

NEC Code References

NEC® 2014 690.8 (A)(3) Inverter Output Circuit Current.

NEC® 2014 690.9 (B) Overcurrent Device Ratings.

NEC® 2014 240.6 (A) Standard Ampere Rating.

NEC® 2014 220.5 (B) Fractions of an Ampere.

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Ronnie Raxter (guest author)

Design Application Engineer

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14 Comments

  • avatar

    hansolar

    03/27/2015 at 6:17 pm

    Hi
    what starts to get confusing is figuring the main PV breaker size for multiple single phase inverters when its a three phase service – it seems there are times when 2 inverters will have a higher amperage output than three inverters which are balanced and breakers landed on all three phase equally.

    example would be (2) 9kw inverters (18,000w/208vX1.25) = 108A – 3p125A main PV breaker
    compared to (3) 9kw inverters (27000w/208v/1.73X1.25) = 94A – 3p100A main PV breaker
    am I figuring this right? If so, if in the 3 inverter setup and one inverter goes down or is taken out of service what then?
    thanks for the input

    Reply »
    • avatar

      Justin Dyke

      03/30/2015 at 3:01 pm

      Hi Hans,

      Great question. I chatted with Ronnie to make sure we could answer completely. For more feedback on the examples you pose, please email socialmedia@sma-america.com so we can get deeper into the details.

      Single phase inverters onto a three phase system is a little more complicated because it is not the kW output, divided by the voltage, divided by the square root of three, and then multiplied by the safety factor.

      Each of these inverters are single-phase inverters and they need to be calculated as individual inverters onto each phase, as tedious as this seems. We find excel quite helpful especially if you expect to perform these calculations regularly.

      Two inverter example: (2) 9kW inverters are only going to be connected to two out of the three phases. Our SB9000TL-US inverter at 208V has a maximum output current of 44A and you will need individual 60A 2-pole breakers per each of the inverters (44A x 1.25 = 55A). This is where the math becomes too much for this comment field and to expand, email would work best.

      Hope to hear from you,
      Justin

      Reply »
  • avatar

    Jerry

    11/04/2015 at 6:44 am

    Ronnie,

    Like you did above, I’ve always multiplied the inverter max continuous current by 1.25 in order to properly size the inverter output circuit breaker, but I can’t find the requirement to do so. 690.9(B) applies to only PV source and output circuits. 690.8(A)(3) says that the max current shall be the inverter continuous output current rating. Have I been doing this wrong? Where is the requirement for the safety factor here? Thanks!

    Reply »
    • avatar

      Ronnie Raxter

      11/04/2015 at 10:26 am

      Hi Jerry,

      You are doing the calculation correctly. We have in the past referenced 690.8(A)(1) as why we’ve added the 1.25 safety factor to the value from 690.8(A)(3) which is not entirely accurate because it does state Photovoltaic Source Circuit Current. However, in NEC 2014 they added 690.9(B) which is titled Overcurrent Device Ratings and it states “Overcurrent device ratings shall be not less than 125 percent of the maximum currents calculated in 690.8(A)” which now clearly encompasses the inverter continuous output current rating.

      Please let me know if you have any further questions.
      Regards,
      Ronnie

      Reply »
      • avatar

        John

        09/25/2016 at 9:33 am

        Yes understood, but where in the code does it state that the OCPD cannot be LARGER than 125% of inverter output as long as the conductor, perhaps oversized) is protected?
        Is there a guideline to limit OCPD size to protect the inverter?
        Thx

      • avatar

        Justin Dyke

        10/20/2016 at 2:21 pm

        Hi John. Responding on behalf of Ronnie, who says that The NEC code doesn’t state that the OCPD cannot be larger than 125% of inverter output, rather it states that 125% more than the inverter output is the minimum size allowed for an OCPD. The maximum size of an OCPD is defined by the manufacturer, because it is up to the manufacturer to determine how much current their device is rated to withstand without damage. The size that you would then utilize would be any OCPD size that is above the min and below the max. Which particular size is the most optimal depends on your specific application.

        Code references listed below.

        NEC 690.8 (A)(3) Inverter output circuit current
        States that the calculation for the maximum circuit current (i.e. the max the inverter is capable of producing) shall be the inverter continuous output current rating.
        NEC 690.9 (B) Overcurrent device ratings
        States that the OCPD cannot be less than 125% of the maximum currents calculated in NEC 690.8(A)

        Hope this helps!

  • avatar

    marc

    08/18/2017 at 6:28 am

    hi,

    quick question, I installed a 2kw pv but the grid tie inverter is 5kw . the question is, will my ac breaker be base on the inverter size or on the pv size? common sense tells me that the inverter can not give 5kw ac output since my pv is just 2 kw. I know that ocpd is a protection, but will it still protect the system if I use for example a 30amp ac breaker knowing that my pv source is just 2kw? our voltage here is 230 volts 60hz

    Reply »
    • avatar

      Mike

      08/25/2017 at 9:38 am

      Hi Marc, Great question. You are right, the breaker is protecting the AC side of the PV system – the inverter output side. In particular, the AC wires from the inverter terminal to the panel (breaker terminals).

      With only a 2kW (DC STC rating) array, you can be assured that you will not get 5kW AC out of your inverter (if you are not concentrating sunlight onto your array). However, you could potentially get more than 2kW – as long as your array is experiencing greater than 1000W/m^2 irradiance and lower than 77 deg F module temperature. So, electrically you could choose a breaker sized for maximum inverter output power all the way down to the size calculated from 2kW AC output (at your panel voltage), PROVIDED that the AC wires installed can also safely handle the current that OCPD is rated to limit to.

      Going down in breaker size may not prohibited by code, but can lead to nuisance tripping if it is sized too small. Not sizing the breaker to the full capacity of the inverter could be possible. I say could be, as most inspectors I have asked would like to see the breaker (and conductor) sizing based on the inverter max capacity. This is to prevent any safety or operational concern if the system owner decides to “upgrade” the PV side of the system at a later date using the same inverter (and AC wiring). So as long as your AC conductors can safely handle the current that the breaker is designed to limit to, the breaker can be sized to the full output power of the inverter, and this can prevent any unintended consequences in the future.

      Be safe out there!

      Reply »
      • avatar

        Peter

        09/28/2017 at 9:39 am

        Hi Mike,

        Thanks for your reply. Just to clarify, do you mean both methods listed below for calculating minimum backfeed breaker size are acceptable:

        1) DC array power output at STC / system voltage * 1.25
        2) Inveter “AC maximum output current” from spec sheet * 1.25

        Another question: In a system where DC array is oversized in relation to inverter rated AC output, and the AC current value is higher from 1) than 2), do we still use 2) to calculate backfeed breaker size?

        Thank you!

      • avatar

        Mike

        10/03/2017 at 9:11 am

        Looking at what code directs for inverter AC breaker sizing, in 2014 NEC 690.9(B) you will find:
        “Overcurrent device ratings shall be not less than 125 percent of the maximum currents calculated in 690.8(A).”

        And in 690.8(A)(3): “Inverter Output Circuit Current. The maximum current shall be the inverter continuous output current rating”

        So you would be basing the OCPD size (and AC conductor size too!) on inverter rating from spec sheet/manual. That will prevent any safety issues in the future if the PV array is upsized from its current 2kW. This is why inspectors would like to see this even with a 2kW array – if smaller conductors or OCPD was used, what would ensure that those would be upsized if the array was upgraded? Nothing but installer diligence! Conversely, the inverter will only ever output its maximum current, even with an oversized array, so that is what an inspector wants to see and why it is the code directive.

        Continuing this line of thought will answer your additional question. If the inverter is working at maximum output power, the oversized DC array will be operated at a point where it is producing enough DC current to run the inverter at max power, even if the array were capable of producing more. The inverter is always in control (as long as its electrical limits have been respected!). So sizing the AC conductors and OCPD to maximum inverter output is always the best solution.

  • avatar

    Christian lorenzo

    05/23/2018 at 7:21 am

    Sir im using a 3 20kw sma grid tie inverter.what should be the breaker for each inverter and the main breaker and wire size for main line and inverter to breaker.thank you

    Reply »
    • avatar

      Alejandro

      05/24/2018 at 10:10 am

      Hello Christian! The rated current on the AC side for the STP TL-US 20K is 24 A, which means that if you are tied to a 277/480 WYE service you could typically use a minimum 30A breaker, however the maximum permissible protection is 50A. As far as the conductors landing on the inverter output terminals, they need to be 6AWG – 8AWG (10sqmm – 6sqmm cross section). The size of the Main Breaker depends on the rating of your main panel and the amount of loads on the panel. SMA recommends contacting a qualified local electrician to ensure that all applicable code and safety requirements are being satisfied. Best Regards.

      Reply »
  • avatar

    Mohamed

    08/15/2018 at 5:46 am

    Hello,

    I am designing a PV system using SMA SunnyTripower core 1 50000 kW. It is stated in the inverter data sheet that the maximum output current is 72.5 A. Is this value is the current of all 3 phases or the current per phase. How should i size my AC wires and Circuit breaker in the main panel if the voltage is 220/380 and the distance is 30 meters from inverter to main panel. Thank you.

    Reply »
    • avatar

      Mike

      08/17/2018 at 8:53 am

      Mohamed – The current is for each phase. The size of the wire and circuit breaker must be determined in accordance with local codes and regulations, and in consideration of the environmental conditions the equipment will experience. SMA recommends having a licensed solar contractor or electrician determine the correct conductor and breaker size. Best Regards. Mike

      Reply »

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