I don't think this is expressed in the code, but in my opinion, the 120% applies to the building distribution equipment, not to parts of the solar system. In the case where a panel is being used to combine multiple inverter outputs, the panel is part of the solar system only. The 120% rule was an accommodation to allow solar to feed a building distribution panel and is not applicable to solar system components. It would be good if the code could indicate this more clearly.
On Wed, Jun 27, 2012 at 8:29 AM, Jason Szumlanski <[email protected]>wrote: > Kirk, > > That's basically what I said. Unfortunately, your opinion holds no weight > with my local AHJ's. I've argued the point till blue in the face. Although, > I have never had the instance where all available slots were filled in the > combiner panel - I might be able to argue that case successfully. > > Jason Szumlanski > Fafco Solar > > > > > On Tue, Jun 26, 2012 at 4:53 PM, Kirk Herander <[email protected]> wrote: > >> Jason,**** >> >> ** ** >> >> In your email below you state:**** >> >> ** ** >> >> “You DO need to observe the 120% rule for the combining subpanel, >> regardless of whether there are loads present, at least in jurisdictions >> where I have worked. I've heard that some inspectors will allow you to >> ignore it if it is labeled as a PV combiner with "add no loads" notation. >> ”**** >> >> ** ** >> >> NEC 705.12 (D) states that the distribution equipment (in this case the >> combiner panel, fed by multiple inverters and a utility source) must be >> “capable of supplying multiple branch circuits or feeders or both” for >> (D)(1) through (7) to apply. If you fully populated a combiner panel with >> inverter breakers, leaving no slots for load breakers, it is not capable of >> supplying branch circuits or feeders, and IMO the 120% rule does not apply >> to the combiner buss or the conductors back to its point of utility >> interconnect. I have argued this point as well as label combiners “load >> circuits prohibited” (with or without available slots) and received AHJ >> approval.**** >> >> ** ** >> >> You could also just lock shut a combiner that had spare slots as a >> deterrent to adding load breakers.**** >> >> ** ** >> >> ** ** >> >> Kirk Herander**** >> >> VT Solar, LLC**** >> >> dba Vermont Solar Engineering**** >> >> NABCEPTM Certified installer Charter Member**** >> >> NYSERDA-eligible Installer**** >> >> VT RE Incentive Program Partner**** >> >> 802.863.1202**** >> >> ** ** >> >> *From:* [email protected] [mailto: >> [email protected]] *On Behalf Of *Jason >> Szumlanski >> *Sent:* Tuesday, June 26, 2012 8:28 AM >> *To:* RE-wrenches >> *Subject:* Re: [RE-wrenches] Enphase grid tie question**** >> >> ** ** >> >> I'll email you off-list a 1-line diagram from a system with 164 >> microinverters broken down into 8 strings in a 208V system. This particular >> system used two subpanels to accumulate PV, but that was only because we >> had to backfeed two existing subpanels due to the size of existing 480/208V >> transformers. You will have to look at the utility service and all existing >> equipment.**** >> >> ** ** >> >> Regarding the breakers in the subpanel, you will only need a maximum of a >> 20A breaker for each string. The max inverters per string is 25 and the >> calculation for OCPD is:**** >> >> ** ** >> >> 215W / 208V x 25 inverters / 1.732 x 1.25 = 18.65A**** >> >> ** ** >> >> “You DO need to observe the 120% rule for the combining subpanel, >> regardless of whether there are loads present, at least in jurisdictions >> where I have worked. I've heard that some inspectors will allow you to >> ignore it if it is labeled as a PV combiner with "add no loads" notation. >> ”**** >> >> ** ** >> >> Use a MLO panel with a fusible disconnect between the subpanel and the >> interconnection point. If you use a 225A panel, you can feed it with 270A. >> With eight 20A backfed PV circuits, you would need to protect the line side >> of the panel with a 100A fusible disconnect. That probably isn't going to >> work. You may be best off from a cost perspective using two 225A subpanels >> and two 60A fusible disconnects. Anything larger than a 60A 3P disconnect >> and the price skyrockets. It all depends on your circuit calculations and >> the existing equipment. Of course, you would need two spaces for your >> interconnection point.**** >> >> ** ** >> >> Jason Szumlanski**** >> >> Fafco Solar**** >> >> ** ** >> >> _______________________________________________ >> List sponsored by Home Power magazine >> >> List Address: [email protected] >> >> Options & settings: >> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org >> >> List-Archive: >> http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org >> >> List rules & etiquette: >> www.re-wrenches.org/etiquette.htm >> >> Check out participant bios: >> www.members.re-wrenches.org >> >> >> > > _______________________________________________ > List sponsored by Home Power magazine > > List Address: [email protected] > > Options & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > List-Archive: > http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out participant bios: > www.members.re-wrenches.org > > > -- Chris Mason President, Comet Systems Ltd www.cometenergysystems.com Cell: 264.235.5670 Skype: netconcepts
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