NEC 690.8(A)(1)(1): module Isc and MLPE max input Isc compatibility issue

[SolarMon]

When designing PV systems smaller than 100kW, should NEC 2017 690.8(A)(1)(1) be followed by multiplying the 1.25 factor to the module Isc to check for MLPE and module level rapid shutdown device compatibility? I got myself confused since I've gotten different responses from various industrial professionals and articles.

1) For example, if my module Isc is 14A and my DC optimizer has a maximum short circuit input current of 15A, should I just calculate the adjusted module Isc based on temperature and irradiance (only slightly larger than 14A, thus still assuming 14A), and conclude that since 14A < 15A, the DC optimizer is compatible with the module? Or instead of doing the calculation, should I follow NEC 2017 690.8(A)(1)(1) and do 14A (module Isc shown on the datasheet with no adjustment applied) x 1.25 = 17.5A, 17.5A > 15A, thus the DC optimizer is not compatible with the module?

2) Does the same rule apply to module level rapid shutdown devices, which mostly have 15A max Isc input?

3) What about compatibility with string inverters without the use of MLPEs or module level rapid shutdown devices?

The points where I got confused are:
1) NEC defines PV source circuit current as module Isc x 1.25 per 690.8(A)(1)(1), and PV source circuit current is defined as the circuits between PV modules and from PV modules to a common connection point of the DC system. This definition of source circuit seems applicable to the case with MLPEs thus the max current should x1.25 based on the code. However, some MLPE manufacturers, such as Enphase and SolarEdge, indicate that the 1.25 factor does not need to be applied to the module Isc when checking for their device compatibility. Adjusted module Isc based on temperature can be directly referenced. Won't this conflict with the code?

2) Some string inverter manufacturers, such as SMA and Fronius, require the application of the 1.25 factor to module Isc when checking for inverter compatibility as written in their published whitepapers. Their max input Isc written on the inverter datasheets actually includes the 1.25 multiplier. So why does MLPE and string inverter differ in this requirement? Or does it depend on the manufacturer? Does this code only apply to a string of modules instead of individual module? And if so, why? Did I miss anything?

Sorry for the super long post since this conflict troubled me for a little while. I really appreciate it if someone can help clarify or advise. Thank you!

 

[jaggedben]

690.8 doesn't say anything about module compatability with inverters or MLPE devices. That stuff is all subject to datasheets and instructions, not code. Not sure how you got confused but 690.8 is only for sizing field installed wiring.

 

[SolarMon]

690.8 doesn't say anything about module compatability with inverters or MLPE devices. That stuff is all subject to datasheets and instructions, not code. Not sure how you got confused but 690.8 is only for sizing field installed wiring.

Thank you for your reply! Correct, 690.8 doesn't mention module compatibility with inverters or MLPEs. The origin of my confusion came from the determination of the maximum source circuit current, which is then used to check for inverter compatibility. It seems some manufacturers include the 1.25 factor for the maximum source circuit current calculation whiles others do not - that's my point of confusion.

690.8(B) defines wire sizing as you mentioned, which also requires the application of the 1.25 factor on top of the max source circuit current, but that's not really the scope of my concern. Hopefully I make sense...

 

[jaggedben]

NEC defined source circuit current is not used to check for inverter compatibility unless the inverter documentation says so. As you've noted, manufacturers are not consistent in their approach. Some may state for you to approach it like the NEC and some may not. So you must always consult their documentation and never generalize an approach to compatibility based on the NEC. Clear?

 

[SolarMon]

NEC defined source circuit current is not used to check for inverter compatibility unless the inverter documentation says so. As you've noted, manufacturers are not consistent in their approach. Some may state for you to approach it like the NEC and some may not. So you must always consult their documentation and never generalize an approach to compatibility based on the NEC. Clear?

Ok thanks, that clears it up.

 

[wwhitney]

So for the NEC, when 690.8(A)(1)(1) refers to the "PV module–rated short circuit currents," does that refer to I_sc at STC per the spec sheet, or the temperature corrected I_sc per the spec sheet temperature coefficient?

I.e. what effects is the 1.25 factor required by 690.8(A)(1)(1) supposed to cover, and in particular is it supposed to cover the difference between I_sc at STC and the actual worst-case I_sc?

Cheers, Wayne

 

[jaggedben]

So for the NEC, when 690.8(A)(1)(1) refers to the "PV module–rated short circuit currents," does that refer to I_sc at STC per the spec sheet, or the temperature corrected I_sc per the spec sheet temperature coefficient?

I.e. what effects is the 1.25 factor required by 690.8(A)(1)(1) supposed to cover, and in particular is it supposed to cover the difference between I_sc at STC and the actual worst-case I_sc?

Cheers, Wayne

You make a valid observation that this is ambiguous in the NEC.

The historical understanding would be STC, because when 690 was written and until a few years ago those would have been the only 'ratings' documented for a typical PV module. Nowadays I'd probably make the argument that 690 generally requires STC to be used, but 690.8(A)(1)(2) and 691.8, where applicable, would allow other datasheet figures to be used if appropriate for the location.

The extra 1.25 factor in 690.8(A)(1)(1) is to account for higher irradiance and/or lower temperatures than STC, i.e. possible conditions that raise the short circuit current. That's my understanding anyway. 690.8(B) adds an additional minimum 1.25 factor for continuous use.