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<p class=MsoNormal><font size=3 color=black face="Times New Roman"><span
style='font-size:12.0pt;color:black'>William,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 color=black face="Times New Roman"><span
style='font-size:12.0pt;color:black'>Wow – you’re a master at
whipping up drawings!<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 color=black face="Times New Roman"><span
style='font-size:12.0pt;color:black'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 color=black face="Times New Roman"><span
style='font-size:12.0pt;color:black'>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. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 color=black face="Times New Roman"><span
style='font-size:12.0pt;color:black'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 color=black face="Times New Roman"><span
style='font-size:12.0pt;color:black'>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?<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 color=black face="Times New Roman"><span
style='font-size:12.0pt;color:black'><o:p> </o:p></span></font></p>
<div>
<p class=MsoNormal><font size=3 color=black face="Times New Roman"><span
style='font-size:12.0pt;color:black'>Thanks, </span>Allan </font><o:p></o:p></p>
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<p class=MsoNormal><b><font size=2 face=Tahoma><span style='font-size:10.0pt;
font-family:Tahoma;font-weight:bold'>From:</span></font></b><font size=2
face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma'> re-wrenches-bounces@lists.re-wrenches.org
[mailto:re-wrenches-bounces@lists.re-wrenches.org] <b><span style='font-weight:
bold'>On Behalf Of </span></b>William Miller<br>
<b><span style='font-weight:bold'>Sent:</span></b> Saturday, August 01, 2009
11:13 PM<br>
<b><span style='font-weight:bold'>To:</span></b> RE-wrenches<br>
<b><span style='font-weight:bold'>Subject:</span></b> Re: [RE-wrenches]
down-sizing main breaker</span></font><o:p></o:p></p>
</div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>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>
<br>
<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>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 color=black><span style='color:black'>“</span></font>It
is my understanding<font color=black><span style='color:black'> </span></font>that
if one is going to install a load side tap, the back feed calculations need to
work for every link of the distribution system upstream to the meter.<font
color=black><span style='color:black'>”</span></font> It seems to me that
there<font color=black><span style='color:black'>’</span></font>s still
no way to exceed main disconnect current rating. <o:p></o:p></span></font></p>
<ol start=1 type=1>
<li class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;
mso-list:l0 level1 lfo1'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>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). <o:p></o:p></span></font></li>
<li class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;
mso-list:l0 level1 lfo1'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>100A + 40A tapped into subpanel ahead of 100A
main breaker: <o:p></o:p></span></font></li>
<ol start=1 type=1>
<li class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;mso-list:l0 level2 lfo1'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>maximum current passing through 100A main
breaker = 100A. Adding 40A of PV only reduces amount through conductors
from main AC panel to 60A. <o:p></o:p></span></font></li>
<li class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;mso-list:l0 level2 lfo1'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>no AC load in daytime = 40A of current back to
main AC load center <o:p></o:p></span></font></li>
<li class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;mso-list:l0 level2 lfo1'><font size=3 face="Times New Roman"><span
style='font-size:12.0pt'>some subpanel loads in daytime reduces 2.b.
above <o:p></o:p></span></font></li>
</ol>
<li class=MsoNormal style='color:black;mso-margin-top-alt:auto;mso-margin-bottom-alt:
auto;mso-list:l0 level1 lfo1'><font size=3 color=black
face="Times New Roman"><span style='font-size:12.0pt;color:windowtext'>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</span></font>’<font color=black><span
style='color:windowtext'>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</span></font>’<font
color=black><span style='color:windowtext'>s all. The subpanel feeders don</span></font>’<font
color=black><span style='color:windowtext'>t need to be upsized. </span></font><o:p></o:p></li>
</ol>
<p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;
margin-left:.5in'><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><br>
What am I missing here, please?<br>
<br>
Allan Sindelar<br>
</span></font><font size=2><span style='font-size:10.0pt'><a
href="mailto:Allan@positiveenergysolar.com">Allan@positiveenergysolar.com</a><br>
NABCEP Certified Photovoltaic Installer<br>
EE98J Journeyman Electrician<br>
</span></font>Positive Energy, Inc.<br>
3201 Calle Marie<br>
<st1:place w:st="on"><st1:City w:st="on">Santa Fe</st1:City>, <st1:State w:st="on">New
Mexico</st1:State> <st1:PostalCode w:st="on">87507</st1:PostalCode></st1:place><br>
505 424-1112<br>
<font size=2 face=Arial><span style='font-size:10.0pt;font-family:Arial'><a
href="http://www.positiveenergysolar.com">www.positiveenergysolar.com</a></span></font><font
color=black><span style='color:black'><o:p></o:p></span></font></p>
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face=Arial><span style='font-size:10.0pt;font-family:Arial'>
<hr size=2 width="100%" align=center>
</span></font></div>
<p class=MsoNormal><b><font size=2 face=Tahoma><span style='font-size:10.0pt;
font-family:Tahoma;font-weight:bold'>From:</span></font></b><font size=2
face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma'>
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><span style='font-weight:bold'>On Behalf Of </span></b>William Miller<br>
<b><span style='font-weight:bold'>Sent:</span></b> Saturday, August 01, 2009
6:11 PM<br>
<b><span style='font-weight:bold'>To:</span></b> RE-wrenches<br>
<b><span style='font-weight:bold'>Subject:</span></b> Re: [RE-wrenches]
down-sizing main breaker<br>
</span></font> <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<o:p></o:p></p>
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