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 youre a master at whipping up drawings!
OK, I get the concept, and its 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
theyre 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 wouldnt change
anything in that (or any) scenario. So why would you have to increase
conductor size?
Thanks, Allan
----------
From: re-wrenches-boun...@lists.re-wrenches.org
[mailto:re-wrenches-boun...@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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