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Jason,<br>
<br>
Now you are opening up a can of worms. <br>
<br>
It's bad enough that 705.12(D) doesn't say anything about a panel that
can't (or shouldn't, or won't) have anything connected but interactive
inverters. But when that is the case, a bus rating of 100% of the
source circuits should apply. For PV systems a 125% factor will already
apply to each inverter circuit.<br>
<br>
While it seems logical that the conductors are an extension of the bus
bar and should be treated the same way, 705.12(D) isn't about the
conductors - it's about the bus bars. The conductors are protected by
the breakers -- 80-amp breakers and wire with 80-amp ampacity. <br>
<br>
Getting into the example further, I see flaws in it. If the inverters
are 7500-watt 240-volt, the output current would be 7500/240 = 31.25
amps and 40-amp breakers would be adequate. Then there would have been
no issues, even going directly into the main panel. If the inverters
are 7500-watt 208-volt, the output current is 7500/208 = 36 amps and
the 50-amp breakers make sense. That means the bus bars and feeder
conductor have a continuous current of 72 amps. That means neither the
80-amp breaker nor 80-amp wire is sufficient because 72 x 1.25 = 90
amps. Now the example doesn't resolve the limitation of backfeeding at
the main. Opps!<br>
<br>
Kent Osterberg<br>
Blue Mountain Solar<br>
<br>
<br>
<br>
<br>
<br>
<br>
Jason Szumlanski wrote:
<blockquote
cite="mid:8EA9619B465C95438B31C92F854FB79672F18F@exchange.fafcosolar.local"
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<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);">Another
key is to remember that this discussion also applies to the conductor
between the main panel and subpanel. In a large PV system, this could
result in a pretty large wire between the two panels, and a significant
cost that is often overlooked. In some cases it makes sense to locate
the subpanel close to the main panel and run multiple sets of smaller
wires from the inverters to the subpanel.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);">And
because the calculation is based on the first OC protection connected
to the inverters, adding a main breaker (theoretically 80A in this
example) in the subpanel doesn’t change things. Even though this wire
would be theoretically protected by an 80A breaker at each end, you
can’t size the wire for 160A / 1.2 = 133.3A. You have to size for
180A/1.2 = 150A. (not that it makes much of a difference in this
example, but it still must be considered)<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);">At
least that’s how I understand it…<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<div>
<p class="MsoNormal"><span
style="font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);">Jason
Szumlanski<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);">Fafco
Solar</span><span
style="font-size: 4pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);">
<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
</div>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<div>
<div
style="border-style: solid none none; border-color: rgb(181, 196, 223) -moz-use-text-color -moz-use-text-color; border-width: 1pt medium medium; padding: 3pt 0in 0in;">
<p class="MsoNormal"><b><span
style="font-size: 10pt; font-family: "Tahoma","sans-serif"; color: windowtext;">From:</span></b><span
style="font-size: 10pt; font-family: "Tahoma","sans-serif"; color: windowtext;">
<a class="moz-txt-link-abbreviated" href="mailto:re-wrenches-bounces@lists.re-wrenches.org">re-wrenches-bounces@lists.re-wrenches.org</a>
[<a class="moz-txt-link-freetext" href="mailto:re-wrenches-bounces@lists.re-wrenches.org">mailto:re-wrenches-bounces@lists.re-wrenches.org</a>] <b>On Behalf Of </b>Mark
Frye<br>
<b>Sent:</b> Wednesday, March 30, 2011 1:33 AM<br>
<b>To:</b> 'RE-wrenches'<br>
<b>Subject:</b> Re: [RE-wrenches] sizing a sub-panel used to
combinemultipleinverter outputs<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span
style="font-size: 10pt; font-family: "Arial","sans-serif"; color: blue;">Opps!</span><o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><span
style="font-size: 10pt; font-family: "Arial","sans-serif"; color: blue;">My
bad, I was thinking of a single phase system, not the three phase
system shown in the article.</span><o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><span
style="font-size: 10pt; font-family: "Arial","sans-serif"; color: blue;">For
the three phase system Kent is correct in counting 180A of supply per
bar.</span><o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><span
style="font-size: 10pt; font-family: "Arial","sans-serif";">Mark Frye</span>
<br>
<span style="font-size: 10pt; font-family: "Arial","sans-serif";">Berkeley
Solar Electric Systems</span> <br>
<span style="font-size: 10pt; font-family: "Arial","sans-serif";">303
Redbud Way</span> <br>
<span style="font-size: 10pt; font-family: "Arial","sans-serif";">Nevada
City, CA 95959</span> <br>
<span style="font-size: 10pt; font-family: "Arial","sans-serif";">(530)
401-8024</span> <br>
<a moz-do-not-send="true" href="http://www.berkeleysolar.com/"><span
style="font-size: 10pt; font-family: "Arial","sans-serif";">www.berkeleysolar.com</span></a><span
style="font-size: 10pt; font-family: "Arial","sans-serif";"> </span> <o:p></o:p></p>
<div>
<p class="MsoNormal"> <o:p></o:p></p>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<div class="MsoNormal" style="text-align: center;" align="center">
<hr align="center" size="2" width="100%"></div>
<p class="MsoNormal" style="margin-bottom: 12pt;"><b><span
style="font-size: 10pt; font-family: "Tahoma","sans-serif";">From:</span></b><span
style="font-size: 10pt; font-family: "Tahoma","sans-serif";">
<a class="moz-txt-link-abbreviated" href="mailto:re-wrenches-bounces@lists.re-wrenches.org">re-wrenches-bounces@lists.re-wrenches.org</a>
[<a class="moz-txt-link-freetext" href="mailto:re-wrenches-bounces@lists.re-wrenches.org">mailto:re-wrenches-bounces@lists.re-wrenches.org</a>] <b>On Behalf Of </b>Mark
Frye<br>
<b>Sent:</b> Tuesday, March 29, 2011 10:17 PM<br>
<b>To:</b> 'RE-wrenches'<br>
<b>Subject:</b> Re: [RE-wrenches] sizing a sub-panel used to combine
multipleinverter outputs</span><o:p></o:p></p>
<p class="MsoNormal"><span
style="font-size: 10pt; font-family: "Arial","sans-serif"; color: blue;">I
think Kent and I agree. For the case where the subpanel is not
dedicated a PV sub-panel he is calculating for 2 - 50A breakers and I
calculated for 3 - 50A breakers.</span><o:p></o:p></p>
<p class="MsoNormal"><br>
<span style="font-size: 10pt; font-family: "Arial","sans-serif";">Mark
Frye</span> <br>
<span style="font-size: 10pt; font-family: "Arial","sans-serif";">Berkeley
Solar Electric Systems</span> <br>
<span style="font-size: 10pt; font-family: "Arial","sans-serif";">303
Redbud Way</span> <br>
<span style="font-size: 10pt; font-family: "Arial","sans-serif";">Nevada
City, CA 95959</span> <br>
<span style="font-size: 10pt; font-family: "Arial","sans-serif";">(530)
401-8024</span> <br>
<a moz-do-not-send="true" href="http://www.berkeleysolar.com/"><span
style="font-size: 10pt; font-family: "Arial","sans-serif";">www.berkeleysolar.com</span></a><span
style="font-size: 10pt; font-family: "Arial","sans-serif";"> </span> <o:p></o:p></p>
<div>
<p class="MsoNormal"> <o:p></o:p></p>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<div class="MsoNormal" style="text-align: center;" align="center">
<hr align="center" size="2" width="100%"></div>
<p class="MsoNormal" style="margin-bottom: 12pt;"><b><span
style="font-size: 10pt; font-family: "Tahoma","sans-serif";">From:</span></b><span
style="font-size: 10pt; font-family: "Tahoma","sans-serif";">
<a class="moz-txt-link-abbreviated" href="mailto:re-wrenches-bounces@lists.re-wrenches.org">re-wrenches-bounces@lists.re-wrenches.org</a>
[<a class="moz-txt-link-freetext" href="mailto:re-wrenches-bounces@lists.re-wrenches.org">mailto:re-wrenches-bounces@lists.re-wrenches.org</a>] <b>On Behalf Of </b>Kent
Osterberg<br>
<b>Sent:</b> Tuesday, March 29, 2011 9:26 PM<br>
<b>To:</b> RE-wrenches<br>
<b>Subject:</b> Re: [RE-wrenches] sizing a sub-panel used to combine
multiple inverter outputs</span><o:p></o:p></p>
<p class="MsoNormal">Per 705.12(D) the sub-panel could be any
distribution equipment on the premises. So the question becomes: is the
sub-panel capable of supplying branch circuits or feeder loads? If yes,
then the sum of the breakers (potentially) feeding the bus is 180 amps
so a 150-amp rating is required and the inverters would have to feed
the opposite end of the bus bars. If no, the code is not clear on the
requirement, but obviously the 80-amp breaker in the main panel limits
the maximum current flowing through the sub-panel. <br>
<br>
Kent Osterberg<br>
Blue Mountain Solar<br>
<br>
<o:p></o:p></p>
</div>
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