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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Andy, August, and Eric,<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>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. <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>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.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>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.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>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.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Bill.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> re-wrenches-bounces@lists.re-wrenches.org [mailto:re-wrenches-bounces@lists.re-wrenches.org] <b>On Behalf Of </b>Andrew Truitt<br><b>Sent:</b> Monday, May 07, 2012 11:24 AM<br><b>To:</b> RE-wrenches<br><b>Subject:</b> Re: [RE-wrenches] 120% Rule - applying to multiple load centers etc<o:p></o:p></span></p></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>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).<o:p></o:p></p><div><div><p class=MsoNormal><o:p> </o:p></p><div><p class=MsoNormal><o:p> </o:p></p></div><div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='color:#006600'>For a brighter energy future,<br><br><br>Andrew Truitt <br>NABCEP Certified PV Installer</span><span style='font-size:10.0pt;font-family:"Arial","sans-serif";color:#006600'>™</span><span style='color:#006600'> (ID# 032407-66)</span><o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='color:#006600'>Principal<br>Truitt Renewable Energy Consulting</span><o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='color:#006600'><a href="tel:%28202%29%20486-7507" target="_blank"><span style='color:#1155CC'>(202) 486-7507</span></a></span><o:p></o:p></p></div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><a href="http://www.linkedin.com/pub/andrew-truitt/8/622/713" target="_blank"><span style='color:#147DBA'>http://www.linkedin.com/pub/andrew-truitt/8/622/713</span></a><br><br><span style='color:#147DBA'><br></span>"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!"<br><br>~William McDonough<o:p></o:p></p><div><div><p class=MsoNormal><o:p> </o:p></p></div></div></div><div><p class=MsoNormal style='margin-bottom:12.0pt'><o:p> </o:p></p><div><p class=MsoNormal>On Mon, May 7, 2012 at 10:55 AM, August Goers <<a href="mailto:august@luminalt.com" target="_blank">august@luminalt.com</a>> wrote:<o:p></o:p></p><div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Hi Wrenches,</span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'> </span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Please see question below forwarded from one of my fellow engineers. Thanks, August</span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'> </span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> Eric Schoonbaert [mailto:<a href="mailto:eric@luminalt.com" target="_blank">eric@luminalt.com</a>] <br><b>Sent:</b> Monday, May 07, 2012 9:38 AM<br><b>To:</b> August Goers<br><b>Subject:</b> 120% Rule</span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-family:"Tahoma","sans-serif"'>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 <i>conductor </i>rating [emphasis added].</span><o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><b><span style='font-family:"Tahoma","sans-serif"'> </span></b><o:p></o:p></p></div></div></div></div><p class=MsoNormal><o:p> </o:p></p></div></div></div></div></body></html>