DC string fusing puzzle

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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Smart $ said:
...Is that conductor ["D"] ampacity required to be sized N*Isc*125%?
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Well, yes, that is Imax for those conductors, but of course their ampacity must be appropriately derated for 90 degree conditions of use and 75 degree continuous use, and both ampacities must be equal to or greater than Imax.
 
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jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Smart $ said:
Combined, not recombined. For example, consider a fault between string OCPD's and the combiner bus, that short horizontal line in the diagram below. I know it is shown as fusing and a combiner combo, but the string fuses are not required to be connected directly to a combiner bus, right? Is that conductor ampacity required to be sized N*Isc*125%? And even if it is, there may not be a need for additional fusing, but a short circuit in that area will never trip a string OCPD no matter how many strings are combined.... [/IMG]
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Practically speaking, I think the fuses are required to be connected more or less directly to the combiner bus. That's because nowadays PV combiners are required to be listed for PV applications, and I don't think you'll find a listed combiner that isn't constructed that way.
 
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