Also consider that most people are figuring volt drop at a wire temperature of 167 deg F (75C). That's really hot, and most of the time the wire will be much cooler, and therefore the volt drop will be less. (use the fine print note #2, NEC table 8 to adjust down for cooler temperature) William's time analysis is very interesting as well: when considering wire losses for a continuous 24/7 load, it is roughly 4 times more important to reduce the losses than the PV wiring. Likewise a load that is only on 5 min/ day doesn't make near as much sense to oversize the wire. I added this time factor to my spreadsheet for calculating economics.
R. Walters r...@solarray.com Solar Engineer On Apr 9, 2010, at 8:44 AM, William Miller wrote: > Friends: > > Forgive me if this point has already been made: > > When designing PV feeders, one typically looks at the Impp of an array for > current specifications. Impp is rarely achieved, however -- only a handful > of days per year and for only a short period of time per day. This is > particularly true for a fixed array. > > I took the hour-by-hour data from the PV watts tool, subtracted out the zero > values (night time) and averaged the remaining values. The result was about > 50%. > > Therefore, a 2% VD at Impp is actually a 1% VD on average. This gives a bit > more headroom in any PV feeder design. > > William Miller > > > At 04:25 AM 4/9/2010, you wrote: >> IMHO it seems like we're all really on the same page here...... trying to >> make >> the the most out of the the resources we have available to work with......sun >> hours, pv, copper, labor, dollars, etc. Job specifics come into play. There >> are >> definitely times when a detailed cost/ benefit analysis is called for. Many >> other times, let's not miss the forest for the trees, an increase in wire >> size >> to avoid waste of other aforementioned resources, is low hanging fruit. As a >> percentage of the overall system cost, the decision is a no -brainer. >> >> Dick Ratico >> Solarwind Electic > _______________________________________________ > 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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