How do I figure the ampacity need to size wire leaving combiner to DC disco
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Technically, that's a PV output circuit conductor. Right? PV source circuits enter the combiner. A PV output circuit exits the combiner, after the source circuits are aggregated.
The basic process of sizing any dc PV power circuit is the same. As described in 690.8(B)(1) and (2), there's basically a twostep evaluation process. You use the largest minimum conductor size indicated in these two steps.
This article describes the process and includes example calculations:
CodeCompliant Conductor Sizing

Originally posted by SolarPro View PostTechnically, that's a PV output circuit conductor. Right? PV source circuits enter the combiner. A PV output circuit exits the combiner, after the source circuits are aggregated.
The basic process of sizing any dc PV power circuit is the same. As described in 690.8(B)(1) and (2), there's basically a twostep evaluation process. You use the largest minimum conductor size indicated in these two steps.
This article describes the process and includes example calculations:
CodeCompliant Conductor Sizing
Not including voltage drop, it used to be a three step process back in NEC2011. The first step was 1.56*Isc would be used to size the terminations, typically from the 75C column of the NEC. The second step was 1.25*Isc/total derate would be used to size the wire at conditions of use, typically from the 90C column of the NEC. These two steps remain in NEC2014.
And in NEC2011, there was a step that says the OCPD where required shall protect the wire as sized. And this section is absent from NEC2014. I'm not sure whether it is simply implied by Article 240, or if it was intentionally removed to no longer be a requirement. I assume it is implied by Article 240, as the first four chapters apply to all installations. And I've understood this to be a case where 240.4(B) applies, which means the wire ampacity at conditions of use AND the termination ampacity must both be greater than the previous OCPD than the one actually used.
It is an important factor to consider whether or not the OCPD affects wire size, because it can make a significant difference.
1. What if it is a case where the OCPD is not required, but is there anyway, simply because that is how the inverter is built? For instance: two identical combiners feeding an inverter, each with an identical fuse. This is a case where the two circuit exception applies.
2. What if you have an excessively larger OCPD, because that is the smallest device that is built for the inverter? For instance: a 4circuit combiner connected to a 225A ocpd. Probably because the other combiners have 12 circuits, and this one is the "runt of the litter". Inverter manufacturers usually "bin" their subcombiner options, such that you might pick 16 breakers from the 110 to 200A range, or 8 big breakers from the 225A to 400A range.Last edited by Carultch; 103115, 02:24 PM.
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