[RE-wrenches] down-sizing main breaker

William Miller william at millersolar.com
Sat Aug 1 22:12:38 PDT 2009


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



At 06:19 PM 8/1/2009, you wrote:
>William,
>I have been following this back-and-forth thread and need you to please 
>explain one thing:
>
>Your 140A point makes perfect sense (from the point of 690.64 in requiring 
>that the busbar to which both utility and PV feeds are connected. No 
>question there. But you write: "It is my understanding that if one is 
>going to install a load side tape, the back feed calculations need to work 
>for every link of the distribution system upstream to the meter." It seems 
>to me that there's still no way to exceed main disconnect current rating.
>    * 100A utility (using 100A subpanel main) + 40A PV tied into subpanel 
> busbar = 140A, which is OK if the subpanel busbar is rated at least 125A 
> (residential) or 150A (commercial).
>    * 100A + 40A tapped into subpanel ahead of 100A main breaker:
>        * maximum current passing through 100A main breaker = 100A. Adding 
> 40A of PV only reduces amount through conductors from main AC panel to 60A.
>        * no AC load in daytime = 40A of current back to main AC load center
>        * some subpanel loads in daytime reduces 2.b. above
>    * At the main panel, maximum current flow through breaker feeding 
> subpanel is 100A, as any PV reduces load at breaker. However, main panel 
> must itself meet 690.64, as it's still possible to feed 40A of PV 
> (daytime with no subpanel load) into main panel. So same busbar 
> requirements apply in the main panel as in the subpanel. But that's all. 
> The subpanel feeders don't need to be upsized.
>
>What am I missing here, please?
>
>Allan Sindelar
><mailto:Allan at positiveenergysolar.com>Allan at positiveenergysolar.com
>NABCEP Certified Photovoltaic Installer
>EE98J Journeyman Electrician
>Positive Energy, Inc.
>3201 Calle Marie
>Santa Fe, New Mexico 87507
>505 424-1112
><http://www.positiveenergysolar.com>www.positiveenergysolar.com
>
>----------
>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 6:11 PM
>To: RE-wrenches
>Subject: Re: [RE-wrenches] down-sizing main breaker
>
>Dick:
>
>Your logic is correct but incomplete.  Consider it like this:  The buss 
>bars in the service panel can handle 100 amps (they are rated for that 
>amount).  The main breaker feeds 100 amps into that buss, maximizing it's 
>capabilities.  If you feed an additional 40 amps into that buss bar, you 
>have the potential of exceeding the ampacity of that buss assembly.  If no 
>other loads are fed from that buss bar, there is no over current 
>scenarios.  Say, however, someone puts more breakers on that buss bar and 
>draws 140 amps from them.  100 amps comes in from the utility, 40 amps 
>comes in from the PV and the buss is overloaded.
>
>This is the scenario for which 690.64(B) was written.  This will occur 
>only if the overload amperage is not drawn from a space on the buss bar 
>between the utility feeder and the PV feeder.  If, however, the PV feeder 
>breaker and the utility feeder are both on one end of the buss and the 
>load is on the other end, then the loads are additive on the buss assembly.
>
>Some want the code passage re-written to say that if the feeder is on one 
>end of the bus assembly and the PV is on the other, there is no 
>possibility of an overdraw on the buss bar and the installation is 
>legal.  This has not happened yet.
>
>William
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