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Most of the inspectors I've dealt with are at least somewhat reasonable.
A few are fundamentalists for their own interpretations. I'd at
least show the inspector the email with Bill Brook's statement and
discuss the logic of the situation. It is obvious that the intent
was to protect a wire that was double fed and could overload. The AHJ has
the responsibility for interpretation of the code, so can allow what s/he
sees fit. <br><br>
At 10:06 AM 9/28/2012, you wrote:<br>
<blockquote type=cite class=cite cite="">I agree with Dave, as do most
AHJ's around Southwest Florida.<br><br>
We're stuck with the strict reading of the code, despite the
craziness.<br><br>
Pull new, larger wire.<br><br>
Jason Szumlanski<br>
Fafco Solar<br><br>
<br>
On Fri, Sep 28, 2012 at 10:02 AM, Dave Click
<<a href="mailto:daveclick@fsec.ucf.edu">daveclick@fsec.ucf.edu</a>
> wrote:<br>
<dl>
<dd>Mr. Brooks had an email on this topic on 5/8, 12:58pm ET. NEC
officially says that the conductor needs to be upsized but the 2014 will
fix it because that is dumb. I've copied some of Bill's email
below.<br><br>
<dd>**<br>
<dd>The key distinction was used in my proposal to the 2014 NEC that
removed the statement “and conductor” in 705.12(D) since conductors are
treated very differently in the NEC. We in 690 are the ones that got this
messed up. The issue with conductors are taps. With two sources feeding a
tap, the sum of the feeder breakers would have to be taken into account
in sizing the tap. This does NOT mean that the tap is a full size
conductor. The tap rule determines the size and the new proposal simply
requires you to use both the feeder breaker and the PV breaker in sizing
the tap. This assumes that both breakers are feeding the tap in the event
of fault on the tap and that there would be no problem clearing that
fault. If fault current was used as an argument for oversizing (it is
wrong), it only has relevance in the tap scenario. A fault in a feeder
with no taps does not allow the sum of the currents to flow anywhere but
where the fault is—the rest of the conductor is undamaged in a
fault.<br><br>
<dd>...<br><br>
<dd>Sizing a conductor for the sum of two breakers on opposite ends of a
feeder seems to be what the code says, but it is totally ABSURD from a
technical point of view. John’s articles were merely pointing out that
the code language seems to be telling us to do this, regardless of
whether it makes technical sense. The 2014 NEC will do away with this
craziness.<br>
<dd>**<br><br>
<br>
<dd>On 2012/9/28 9:46, Kristopher Schmid wrote:<br>
<dl>
<dd>It seems to me that the conductor should not be subject to the 120%
rule<br>
<dd>despite what the code says. The potential safety issue here
is<br>
<dd>overloading the neutral bus, right? While feeding currents
could be<br>
<dd>additive in the panel, they would be subtractive on the feeder,
no? I<br>
<dd>seem to remember seeing this discussion on the list before.<br><br>
<dd>Kris<br><br>
<dd>On Fri, Sep 28, 2012 at 8:26 AM, Aaron Mandelkorn
<<a href="mailto:reosolar@gmail.com">reosolar@gmail.com</a><br>
<dd><<a href="mailto:reosolar@gmail.com" eudora="autourl">
mailto:reosolar@gmail.com</a>>> wrote:<br><br>
<dd> I agree. With a 150A bus being fed by 100A
from the grid leaves 80<br>
<dd> additional amps (120% of 150A) to feed the bus
from outside sources.<br>
<dd> It seems to me that 40A of PV being
back fed will be just fine.<br><br>
<dd> Aaron Mandelkorn<br>
<dd> NABCEP Certified PV Installer<br>
<dd> Renewable Energy Outfitters<br>
<dd> Box 65 Salida, CO. 81201<br>
<dd> <a href="tel:%28970%29596-3744">(970)596-3744</a>
<tel:%28970%29596-3744><br>
<dd>
<a href="mailto:reosolar@gmail.com">reosolar@gmail.com</a>
<<a href="mailto:reosolar@gmail.com" eudora="autourl">
mailto:reosolar@gmail.com</a>><br>
<dd>
<a href="http://www.reosolar.com">www.reosolar.com</a>
<<a href="http://www.reosolar.com/" eudora="autourl">
http://www.reosolar.com</a>><br><br>
<br>
<dd> On Sep 28, 2012, at 6:32 AM, Drake wrote:<br><br>
<dl>
<dd> Are you sure you can't? Since the bus has a
rating of 150 A and<br>
<dd> is protected by a 100 A breaker, there is plenty
of room to not<br>
<dd> over amp the bus from the two sources of
power.<br><br>
<dd> The amperage from the inverter will cancel
amperage coming from<br>
<dd> the utility in the feeder. The wire will
never supply over the<br>
<dd> 100 A. The theoretical max the inverter
could backfeed would be<br>
<dd> 40 Amps in the 100 Amp cable if no loads were
being supplied. It<br>
<dd> would certainly not be a safety issue. Am I
missing something in<br>
<dd> the code?<br><br>
<br>
<dd> At 12:51 AM 9/28/2012, you wrote:<br>
<dl>
<dd> Mac:<br><br>
<dd> Nope.<br><br>
<br>
<dd> William Miller<br><br>
<dd> PS: It's pretty straight forwards, the code
says "bus or conductor."<br><br>
<dd> wm<br><br>
<dd> PPS:<br><br>
<dd> Can you customer live with a smaller feeder
breaker, say 125<br>
<dd> amps? If so, your gold.<br><br>
<dd> wm<br><br>
<br><br>
<dd> At 06:43 PM 9/27/2012, you wrote:<br>
<dl>
<dd> Hello wrenches,<br><br>
<dd> I am looking for advice on how the 120% rule
applies to feeders<br>
<dd> for a subpanel. I have this scenario:<br>
<dd> 150A bus rating on subpanel with main breaker of
100A. The<br>
<dd> conductors feeding this subpanel are 100A rated
conductors. Can<br>
<dd> I backfeed with a 40A breaker?<br><br>
<dd> Thanks<br><br>
<dd> --<br><br>
<br><br>
<dd> Mac Lewis<br><br>
</dl>
</dl>
</dl>
</dl>
</dl><br>
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