Al, The sub panel can be rated at 80A. With no other loads in the subpanel, 80A is the most that could flow out past the 80A breaker. If you had other loads in the sub-panel, then it would have to be no less than (50+50+50+80)/1.2 or 200A. Note from 2008 690.64 (B) that the sub-panel is NOT "capable of supplying multipule branch circuits......" Still this part of the new text for 690.64 in 2008 is not really that well thought out, since with this understanding we see none of the provisons 1 -7 would apply to that particular sub-panel at all. In the diagram from the article you could ask, "Where is the point of interconnection of the PV system?" Effectively, it is at the 80A breaker, not at the individual 50A breakers (so long as the sub-panel is a dedicated PV backfeed panel). The 80A breaker is the boundry below which it could be said all is PV circuit, and above it, premise wiring.
Mark Frye Berkeley Solar Electric Systems 303 Redbud Way Nevada City, CA 95959 (530) 401-8024 <http://www.berkeleysolar.com/> www.berkeleysolar.com _____ From: [email protected] [mailto:[email protected]] On Behalf Of Al Frishman Sent: Tuesday, March 29, 2011 7:46 PM To: [email protected] Subject: [RE-wrenches] sizing a sub-panel used to combine multiple inverteroutputs Hi Wrenchers, When using a sub-panel for the sole purpose of combining the output of multiple single phase inverters does NEC 690.64 (B) (2) apply? I know that this topic has been discussed but I am still not clear. The "Can we Land" Article pg 11 Diagram 2b shows that using a Subpanel eliminates the problem of exceeding the 120% rule illustrated in diagram 2a but it does not show the rating of the sub-panel. In the example it shows 3 Fronius 7500W single phase inverters landing in a sub-panel and then the 3-phase sub-panel grid connected to a 80A 3-phase breaker in a 400A main panel. What is the minimum Amp rating required for the 3-phase Sub-panel in diagram 2b? Do we only need to size it for the OCP for the combined outputs of the 3 inverters (80A) or do the three 50A two-pole breakers need to be considered? For easy reference here is a link to the article: http://www.spgsolar.com/wp-content/uploads/2010/05/SPGLeBlancYates.pdf All feedback is appreciated. Al Frishman AeonSolar (917) 699-6641 - cell (888) 460-2867 <http://www.aeonsolar.com/> www.aeonsolar.com
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