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.
At 10:06 AM 9/28/2012, you wrote:
I agree with Dave, as do most AHJ's around Southwest Florida.
We're stuck with the strict reading of the code, despite the craziness.
Pull new, larger wire.
Jason Szumlanski
Fafco Solar
On Fri, Sep 28, 2012 at 10:02 AM, Dave Click
<<mailto:davecl...@fsec.ucf.edu>davecl...@fsec.ucf.edu> wrote:
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.
**
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 isthe rest of the
conductor is undamaged in a fault.
...
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. Johns 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.
**
On 2012/9/28 9:46, Kristopher Schmid wrote:
It seems to me that the conductor should not be subject to the 120% rule
despite what the code says. The potential safety issue here is
overloading the neutral bus, right? While feeding currents could be
additive in the panel, they would be subtractive on the feeder, no? I
seem to remember seeing this discussion on the list before.
Kris
On Fri, Sep 28, 2012 at 8:26 AM, Aaron
Mandelkorn <<mailto:reoso...@gmail.com>reoso...@gmail.com
<mailto:reoso...@gmail.com>> wrote:
I agree. With a 150A bus being fed by 100A from the grid leaves 80
additional amps (120% of 150A) to feed the bus from outside sources.
It seems to me that 40A of PV being back fed will be just fine.
Aaron Mandelkorn
NABCEP Certified PV Installer
Renewable Energy Outfitters
Box 65 Salida, CO. 81201
<tel:%28970%29596-3744>(970)596-3744 <tel:%28970%29596-3744>
<mailto:reoso...@gmail.com>reoso...@gmail.com <mailto:reoso...@gmail.com>
<http://www.reosolar.com>www.reosolar.com <http://www.reosolar.com>
On Sep 28, 2012, at 6:32 AM, Drake wrote:
Are you sure you can't? Since the bus has a rating of 150 A and
is protected by a 100 A breaker, there is plenty of room to not
over amp the bus from the two sources of power.
The amperage from the inverter will cancel amperage coming from
the utility in the feeder. The wire will never supply over the
100 A. The theoretical max the inverter could backfeed would be
40 Amps in the 100 Amp cable if no loads were being supplied. It
would certainly not be a safety issue. Am I missing something in
the code?
At 12:51 AM 9/28/2012, you wrote:
Mac:
Nope.
William Miller
PS: It's pretty straight forwards, the code says "bus or conductor."
wm
PPS:
Can you customer live with a smaller feeder breaker, say 125
amps? If so, your gold.
wm
At 06:43 PM 9/27/2012, you wrote:
Hello wrenches,
I am looking for advice on how the 120% rule applies to feeders
for a subpanel. I have this scenario:
150A bus rating on subpanel with main breaker of 100A. The
conductors feeding this subpanel are 100A rated conductors. Can
I backfeed with a 40A breaker?
Thanks
--
Mac Lewis
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