Properly sizing a PV inverter breaker

This post was originally published in 2015. The tips and techniques explained may be outdated.

by Ronnie Raxter (guest post), , 28 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.

28 Comments
  1. hansolar
    hansolar says:

    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
  2. Jerry
    Jerry says:

    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
  3. marc
    marc says:

    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
  4. Christian lorenzo
    Christian lorenzo says:

    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
  5. Mohamed
    Mohamed says:

    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
  6. Minor Rojas
    Minor Rojas says:

    Hola Mike

    I have a question about the calculation of the main conductor of a group of inverters and their OCPD.

    According to what I have read here, for each inverter, I should use the datasheet to select the maximum current and this way not limit the capacity of each investor for future kwdc. this current will be multiplied by a factor of 1.25, but what will happend with the main conductor that’s feed this group of inverter? Do i add this currents and multiply them by 1.25 again?

    Thanks
    Minor

    Reply
  7. Amr S. Tawfik
    Amr S. Tawfik says:

    Sirs,

    What’s the safe distance between the AC breaker and the inverter output .. in other words if I’ve located the breaker in a AC panel for several inverters, what protects the AC cable between the inverter output till the breaker ?

    Reply
  8. Ron
    Ron says:

    The original post states that 220.5(B) can be used to round down if the fraction is less than 0.5, can this still be used in conjunction with 690.8 and 690.9 for inverter output circuit current or do the requirements of 690 now override 220.5(B)? My particular case is 10 micro inverters with max continuous AC output of 1.21 amps, so total of 12.1 amps, 125% give a breaker size of 15.125 amps. Can that use a 15 amp breaker or does it require a 20 amp breaker under 690?

    Reply
  9. Josh
    Josh says:

    Question – is it true that the max input for a PV inverter used in a residential 200A service panel is 40A? If so what does a person do if they want to have two inverters that each can supply more than a total of 40A? Do you have to tie in upstream of the breaker panel?

    Reply
  10. Michael tasker
    Michael tasker says:

    Hello am installing two no 50 kw 3 phase inverters
    Need help on sizing the main mcb in plant room panel
    Have supplied each inverter with a 16 ml 3 phase swa cable
    Could I put a 63 amp mcb in the main panel board even though each phase would or might put out 72 amp per phase for each inverter

    Thanks mike

    Reply

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