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-boun...@lists.re-wrenches.org [mailto: re-wrenches-boun...@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-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@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 <aug...@luminalt.com> wrote: Hi Wrenches, Please see question below forwarded from one of my fellow engineers. Thanks, August *From:* Eric Schoonbaert [mailto:e...@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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