[RE-wrenches] 120% Rule - applying to multiple load centers etc

[August Goers]

Hi Andrew and Bill,



Thanks for the responses - very interesting indeed. I think Andrew and I
basically came to the same conclusion about what the code says and I agreed
with his points. 705.12(D) seems to makes logical sense when applying it to
any given electrical panel/load center. It is great to hear Bills thoughts
because I was wondering how the rule, worded as it is in the 2008/2011 NEC,
applied in particular to two breakers on opposite ends of a feeder.



This issue particularly comes up, as Andrew pointed out, when we're trying
to tie PV into an existing subpanel somewhere remote from the main service.
Feeder wiring for these existing subpanels is seldom oversized and if we
followed the wording in the code it would leave that as a non-option. I
wonder how other code savvy installers are dealing with this; or maybe they
aren't.



Best, August



*From:* re-wrenches-bounces at lists.re-wrenches.org [mailto:
re-wrenches-bounces at lists.re-wrenches.org] *On Behalf Of *Bill Brooks
*Sent:* Tuesday, May 08, 2012 9:58 AM
*To:* 'RE-wrenches'
*Subject:* Re: [RE-wrenches] 120% Rule - applying to multiple load centers
etc



Andy, August, and Eric,



There will likely be significant changes in the 2014 NEC to clarify the
situation you are discussing. Fault current has very little to do with this
issue.



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.



As was pointed out, in a fault, the PV inverter will shut down in a few
cycles leaving no contribution from the inverter anyway. Don’t even bother
thinking about high impedance faults—the NEC does little to deal with these
types of faults other than to require ground fault protectors on all
services 1000A and up.



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.



Bill.





*From:* re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] *On Behalf Of *Andrew
Truitt
*Sent:* Monday, May 07, 2012 11:24 AM
*To:* RE-wrenches
*Subject:* Re: [RE-wrenches] 120% Rule - applying to multiple load centers
etc



August / Eric - My understanding of this requirement is that all equipment
in the circuit must be rated to handle the maximum fault current that could
flow at a given point.  So, if your power source / overcurrent protection
scheme is: 50A (inverter 1) + 50A (inverter 2) + 100A (utility grid via
fused disconnect) then 200A is the max fault current at any point in that
circuit (including conductors and switches) and should be used in your 120%
rule calculation (as J.W. does).  This can definitely present issues,
especially when attempting to interconnect at existing subpanels with
feeders that were not sized with the future addition of PV in mind, but I
think one intent of this article is to ensure that if there were a fault in
those feeders, and the PV inverters continued to operate (unlikely), that
the conductor could handle the sum of the fault currents (PV + utility).





For a brighter energy future,


Andrew Truitt
NABCEP Certified PV Installer™ (ID# 032407-66)

Principal
Truitt Renewable Energy Consulting

(202) 486-7507

http://www.linkedin.com/pub/andrew-truitt/8/622/713


"Don't get me wrong: I love nuclear energy! It's just that I prefer fusion
to fission. And it just so happens that there's an enormous fusion reactor
safely banked a few million miles from us. It delivers more than we could
ever use in just about 8 minutes. And it's wireless!"

~William McDonough





On Mon, May 7, 2012 at 10:55 AM, August Goers <august at luminalt.com> wrote:

Hi Wrenches,



Please see question below forwarded from one of my fellow engineers.
Thanks, August



*From:* Eric Schoonbaert [mailto:eric at luminalt.com]
*Sent:* Monday, May 07, 2012 9:38 AM
*To:* August Goers
*Subject:* 120% Rule



The 120% rule, and how it is applied to panel board bus size has been
widely discussed. There is one part of the rule (2008 and 2011 NEC quoted
below) that gets much less attention and is the subject of this email. That
is, how and when is the rule applied to a conductor? The heading and text
both clearly say bus or *conductor *rating [emphasis added].

* *
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