The "120% rule" in 2014 is 705.12(D)(2)(3)(b). Ugh. Anyway, the inverter current is "125 percent of the inverter(s) output circuit current", aka the minimum OCPD and conductor ampacity value per 705.60(B) and 690.8(B)(1).
This is actually a bit of wiggle room: in 2011 the 120% rule was based on the inverter output circuit OCPD size, which is often larger than 125% of the inverter output circuit current. Cheers, Brian Mehalic NABCEP Certified Solar PV Installation Professional(tm) R031508-59 IREC ISPQ Certified Affiliated Instructor/PV US-0132 PV Curriculum Developer and Instructor Solar Energy International http://www.solarenergy.org On Thu, Mar 27, 2014 at 2:34 PM, Troy Harvey <[email protected]>wrote: > I am wondering about the busbar 120% rule, and if there is any wiggle room > in the 2014 NEC. > > Fundamentally I don't understand the 120% rule. If my solar breaker is > installed properly at the bottom of the busbar, and the grid-tie breaker is > installed at the top, and the busbar itself is rated for 120% of the panel > rating, I don't see any means by which a solar breaker of a size > substantially larger than 120% could cause a problem. There can be no > place on the busbar under any situation (that I can think of) that would > exceed 120% because the supply current is coming from opposite ends of the > bus bar - even in the worst case load situation. So even if I had a huge PV > system (100A), backfeeding the bottom of a 200A panel, I don't see a > situation where there is more than 200A over any one section of busbar. Am > I wrong, or is the NEC just too prescriptive for its own good? > > Also would you say that the 120% is based on the inverter max output or > backfeed breaker size? > > > thanks, > > Troy Harvey > --------------------- > Principal Engineer > Heliocentric > 801-453-9434 > [email protected] > > > > > _______________________________________________ > List sponsored by Home Power magazine > > List Address: [email protected] > > Change email address & 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 > > >
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