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<font size=3>Allan:<br><br>
I whipped up three drawings that indicate three overload scenarios
possible with incorrectly sized distribution equipment. They are on
our web site at:
<a href="http://millersolar.com/case_studies/case_studies.html" eudora="autourl">http://millersolar.com/case_studies/case_studies.html</a>
Click on Point of connection. Click on any drawing to see a larger
version.<br><br>
I hope this helps with understanding the concept.<br><br>
William Miller<br><br>
<br><br>
At 06:19 PM 8/1/2009, you wrote:<br>
</font><blockquote type=cite class=cite cite><font face="Times New Roman, Times" size=3>William,<br>
I have been following this back-and-forth thread and need you to please
explain one thing:<br>
<br>
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: “</font>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.
<ol>
<li><font face="Times New Roman, Times" size=3>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).</font>
<li><font face="Times New Roman, Times" size=3>100A + 40A tapped into
subpanel ahead of 100A main breaker:</font>
<ol>
<li><font face="Times New Roman, Times" size=3>maximum current passing
through 100A main breaker = 100A. Adding 40A of PV only reduces amount
through conductors from main AC panel to 60A.</font>
<li><font face="Times New Roman, Times" size=3>no AC load in daytime =
40A of current back to main AC load center</font>
<li><font face="Times New Roman, Times" size=3>some subpanel loads in
daytime reduces 2.b. above</font>
</ol>
<li><font face="Times New Roman, Times" size=3>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.</font>
</ol><font face="Times New Roman, Times" size=3> <br>
What am I missing here, please?<br>
<br>
Allan Sindelar<br>
</font><font face="Times New Roman, Times" size=2><a href="mailto:Allan@positiveenergysolar.com">Allan@positiveenergysolar.com</a><br>
NABCEP Certified Photovoltaic Installer<br>
EE98J Journeyman Electrician<br>
</font><font face="Times New Roman, Times" size=3>Positive Energy,
Inc.<br>
3201 Calle Marie<br>
Santa Fe</font>, New Mexico 87507<br>
<font face="Times New Roman, Times" size=3>505 424-1112<br>
</font><font face="arial" size=2><a href="http://www.positiveenergysolar.com">www.positiveenergysolar.com</a><br>
<hr>
<div align="center"></font></div>
<font face="tahoma" size=2><b>From:</b>
re-wrenches-bounces@lists.re-wrenches.org [<a href="mailto:re-wrenches-bounces@lists.re-wrenches.org" eudora="autourl">mailto:re-wrenches-bounces@lists.re-wrenches.org</a>] <b>On Behalf Of </b>William Miller<br>
<b>Sent:</b> Saturday, August 01, 2009 6:11 PM<br>
<b>To:</b> RE-wrenches<br>
<b>Subject:</b> Re: [RE-wrenches] down-sizing main breaker<br>
</font><font face="Times New Roman, Times" size=3> <br>
Dick:<br><br>
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.<br><br>
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.<br><br>
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.<br><br>
William<br>
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