Wlliam,

If there were to be a short circuit in the feeder or the main service panel (
the one with no extra breaker spaces ), the solar's maximum 50 amp contribution
would be about the last thing to be concerned about. A short is going to result
in perhaps 1000s of amps coming from the grid into the short. The service 100
amp breaker will trip immediately and clear the fault. The inverter will lose
the grid and drop offline. End of story.

Dick

--- You wrote:
Allan:

At the point of the short circuit, the amperage will exceed the rating of a 
100 amp feeder.  If the main panel had no extra breaker spaces, it could 
still be subject to a short circuit if, for example, a wrench is dropped 
into it.

I should add that I am not an electrical engineer nor am I trying to 
justify the codes that apply.  I am just trying to explain how I see  the 
codes are applied.  It is necessary to understand this to avoid incorrect 
system design and fruitless discussions with building officials.

William



At 07:22 AM 8/2/2009, you wrote:
>William,
>Wow $F6 you$E2re a master at whipping up drawings!
>
>OK, I get the concept, and it$E2s just as I described. The scenarios in 
>drawings one and two are OK as long as both busbars are rated 150A 
>(commercial) or 125A (residential.
>
>You then show that the reason for the upsized feeders is to handle a 
>short. But wait! Even if the short could produce 140A, as you have 
>indicated, the conductors will still not carry more than the 100A that 
>they$E2re rated to carry. One would carry 100A from one panel and 40A from 
>the other, Nowhere would that conductor carry 140A, and the mate to it 
>would carry at most 60A. Increasing conductor size wouldn$E2t change 
>anything in that (or any) scenario. So why would you have to increase 
>conductor size?
>
>Thanks, Allan
>
>----------
>From: re-wrenches-bounces@lists.re-wrenches.org 
>[mailto:re-wrenches-bounces@lists.re-wrenches.org] On Behalf Of William Miller
>Sent: Saturday, August 01, 2009 11:13 PM
>To: RE-wrenches
>Subject: Re: [RE-wrenches] down-sizing main breaker
>
>Allan:
>
>I whipped up three drawings that indicate three overload scenarios 
>possible with incorrectly sized distribution equipment.  They are on our 
>web site at:  http://millersolar.com/case_studies/case_studies.html  Click 
>on Point of connection.  Click on any drawing to see a larger version.
>
>I hope this helps with understanding the concept.
>
>William Miller
__________________
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