[RE-wrenches] down-sizing main breaker

[William Miller]

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 – you’re a master at whipping up drawings!
>
>OK, I get the concept, and it’s 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’re 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’t change 
>anything in that (or any) scenario. So why would you have to increase 
>conductor size?
>
>Thanks, Allan
>
>----------
>From: re-wrenches-bounces at lists.re-wrenches.org 
>[mailto:re-wrenches-bounces at 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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