[RE-wrenches] 120% rule applying to conductors
Dave Click
daveclick at fsec.ucf.edu
Fri Sep 28 07:02:27 PDT 2012
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 is—the 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. 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.
**
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 <reosolar at gmail.com
> <mailto:reosolar at 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
> (970)596-3744 <tel:%28970%29596-3744>
> reosolar at gmail.com <mailto:reosolar at gmail.com>
> www.reosolar.com <http://www.reosolar.com>
>
>
>
>
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>
>
>
>
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>
> 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
>>>>
>>>> *"Yo solo sé que no sé nada." -Sócrates
>>>> *
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> --
> Shine On!
>
> Kris Schmid
> Legacy Solar, LLC
> 864 Clam Falls Trail
> Frederic, WI 54837
> www.legacysolar.com <http://www.legacysolar.com/>
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