Once I have fulfilled NEC min. requirements, I use a spreadsheet to analyze the cost of larger wire vs. the cost of power lost. Going under 2% is usually not worth it, if copper prices are high, and PV cost is low enough (current market). Sizing for under 2% was good economics a few years back, when PV was high, and copper was low, though.
For NEC 2011, I agree: while I readily welcome development of DC AFI, implementing code before the technology is ready, is a bad idea. But that may be the only way to get the technology in place..... Ray On Apr 6, 2010, at 11:15 AM, Kent Osterberg wrote: > Ray, > > Considering that we design PV wiring to be efficient with voltage (and power) > loss typically less than 2%, the wire size is nearly irrelevant to arcing > issues. Essentially all the energy available from the PV array can be > dissipated in the dc arc. And since the current is limited by the nature of > the IV curve, breakers alone usually won't clear the fault. The best > combiner breakers can do (if you have enough parallel circuits) is isolate > the fault to one string in the PV array. With one string being 1 or 2 kW in > many systems there is still the potential for a lot of heat. > > With the 2011 code just around the corner and no dc arc fault protection on > the horizon, it looks like our industry is again going to have a code > requirement that no one can fulfill. > > Kent Osterberg > Blue Mountain Solar, Inc. > > > > R Ray Walters wrote: >> >> I agree that we don't want to create the code first, and try and develop the >> product after. On the other hand, if a DC AFI can be developed that could >> stop some of the problems I've seen breakers not help, I'm installing them, >> and pushing for code requirements. >> AC GFIs were gimmicky too at first, but now have gone on to save countless >> lives; usually kids, but a few of us wet booted contractors too. >> As far as running things at 100%, I do agree with you, but I also think that >> 156% over rating in many cases is too much. If the wire and breaker are that >> oversized, it is less likely to trip when you want it to. >> My most recent damage I saw, the cable a few inches back was in no way >> damaged, as it only saw array short circuit current, but the connector that >> arced burned up a whole circuit board. >> More oversizing would have only increased the arc potential, and reduced >> the chances of a breaker tripping. >> Proper sizing (not too big, not too small) is the way. >> >> R. Walters >> r...@solarray.com >> Solar Engineer >> >> >> >> >> On Apr 6, 2010, at 9:16 AM, robert ellison wrote: >> >>> I have seen info from independent tests that convinces me that AFCI's >>> probably don't work for AC and i hate to think what they would do for DC, >>> if anything. Besides drive the costs up. >>> This was a few years ago and maybe they have gotten it together by now. >>> >>> Anyone remember the original ground faults form Trace (?) after the code >>> change requiring them in 96? Expensive and prone to catching fire comes to >>> mind, if i remember correctly. Lets not encourage more if that type of >>> experimentation in the industry. >>> >>> Just the same i am not a believer of running anything at 100%, it will >>> always have a higher failure rate than something run at a lower capacity, >>> be it a generator, lawn mower or a circuit breaker. >>> >>> Bob >>> >>> >>> >>> Mon, Apr 5, 2010 at 5:36 PM, William Miller <will...@millersolar.com> >>> wrote: >>> Ray: >>> >>> It is my analysis that combiner breakers (if present) will protect only >>> wiring upstream of the combiner -- that is, the individual string circuits. >>> This protection would happen if there is a fault in one individual string >>> (in the wiring or the modules) that allows current from other strings, in >>> excess of the breaker rating, to be supplied through the breaker feeding >>> the faulted string. >>> >>> There are two scenarios at play here: >>> >>> 1. Any fault between the combiner and the feeder destination will not trip >>> any circuit breakers. The breakers are sized such that the current from >>> each individual string is less than the breaker rating (by more than 1.56 >>> times) and they will not open. >>> >>> 2. PV GFDI protection at the destination end of a feeder will not help. >>> PV GFDI circuits will not remove power from a feeder and they will open the >>> ground-to-grounding conductor bond. >>> >>> Analyzing this further: Fault conditions are made more likely given that >>> PV string circuits are no longer protected by conduit. Faults are then >>> more likely in individual string circuits (those circuits without conduit >>> protection). This is most problematic at installations with two or fewer >>> strings, where there is no combiner, i.e. residential installations. >>> Statistically, residential installations offer greater exposure to >>> electrical fires because: occupancy occurs for more hours per year, fire >>> alarms and sprinklers are often not installed, children are more often >>> present and standards are more lenient for residential wiring systems. >>> >>> These two facts are PVs dirty little secrets. Further innovation is >>> needed... >>> >>> William Miller >>> >>> >>> >>> At 12:22 PM 4/5/2010, you wrote: >>>> I think the 100% rating exception is an interpretation issue. I consider >>>> the assembly to be defined as the breaker mounted in its listed enclosure. >>>> I agree that the AFIs would add cost, but they might actually offer some >>>> protection too. (possibly one AFI unit could offer protection for multiple >>>> circuits?) >>>> I've never had a PV circuit breaker actually trip, except some nuisance >>>> tripping due to faulty breakers. >>>> PV breakers seem to only offer protection for very limited situations ie, >>>> a short in a PV wire being backfed by enough other PV circuits to trip the >>>> breaker. >>>> It could happen, but I've never actually seen it. Even completely >>>> shattered modules still have enough internal resistance to limit the short >>>> circuit current to >>>> a value below the breaker trip point. >>>> >>>> Ray >>>> >>>> On Apr 4, 2010, at 11:47 PM, Kent Osterberg wrote: >>> >>> _______________________________________________ >>> List sponsored by Home Power magazine >>> >>> List Address: RE-wrenches@lists.re-wrenches.org >>> >>> Options & settings: >>> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org >>> >>> List-Archive: >>> http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org >>> >>> List rules & etiquette: >>> www.re-wrenches.org/etiquette.htm >>> >>> Check out participant bios: >>> www.members.re-wrenches.org >>> >>> >>> >>> _______________________________________________ >>> List sponsored by Home Power magazine >>> >>> List Address: RE-wrenches@lists.re-wrenches.org >>> >>> Options & settings: >>> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org >>> >>> List-Archive: >>> http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org >>> >>> List rules & etiquette: >>> www.re-wrenches.org/etiquette.htm >>> >>> Check out participant bios: >>> www.members.re-wrenches.org >>> >> >> >> _______________________________________________ >> List sponsored by Home Power magazine >> >> List Address: RE-wrenches@lists.re-wrenches.org >> >> Options & settings: >> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org >> >> List-Archive: >> http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org >> >> List rules & etiquette: >> www.re-wrenches.org/etiquette.htm >> >> Check out participant bios: >> www.members.re-wrenches.org >> >> >> >> >> No virus found in this incoming message. >> Checked by AVG - www.avg.com >> Version: 9.0.800 / Virus Database: 271.1.1/2794 - Release Date: 04/05/10 >> 23:32:00 >> >> > _______________________________________________ > List sponsored by Home Power magazine > > List Address: RE-wrenches@lists.re-wrenches.org > > Options & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > List-Archive: > http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out participant bios: > www.members.re-wrenches.org >
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