[RE-wrenches] sizing a sub-panel used to combine multiple inverteroutputs

[Mark Frye]

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: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Al Frishman
Sent: Tuesday, March 29, 2011 7:46 PM
To: re-wrenches at lists.re-wrenches.org
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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