|
August, Thanks for remembering which SolarPro had that article. As an installer you can always claim that larger wire will make the system more efficient. The question is: does it make sense to put in larger wire? There is always a point where the extra copper is more valuable the extra energy. Knowing where that break even point is an important aspect of the system design. The techniques shown in this article are what I use. Those that strive blindly to get 1% voltage drop in a PV circuit when it would be cheaper to buy more PV modules are wasting money and natural resources. Kent Osterberg Blue Mountain Solar, Inc. August Goers wrote: Hi All - It seems to me that there is indeed a rather simple set of calculations that we can run to determine costs and benefits of wire sizing. Solar Pro 3.2 (Feb/Mar 2010) has a good article on this on page 16.http://solarprofessional.com/issue/?backissues=1 -August August Goers Luminalt Energy Corporation O: 415.641.4000 M: 415.559.1525 F: 650.244.9167 [email protected] -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Bob-O Schultze Sent: Wednesday, April 07, 2010 1:51 PM To: RE-wrenches Subject: Re: [RE-wrenches] DC wire sizing Ray, Sorry, I'm not buying that argument over a 25+ year design lifespan. Also, -and perhaps something many folks don't consider- the NEC requires a MAX loss of ≤5% over the ENTIRE circuit. That means all the way to the mains panel. Best practices would require no more than 1.5% VD between the inverter and the mains so as to avoid any potential of overvoltage disconnection by the inverter. As it tries to push the current into the grid, it also pushes up the voltage on that line and on it's own sensors. Obviously, the larger the current flow and the higher the VD, the worse the situation could be. Bear in mind that the grid is not held to the same over and under voltage specifications as your inverter is. 3% + 1.5% is pushing it just a bit too close for comfort and I still think wasting watts in wire losses is bad design. We agree that orienting PVs to an azimuth other than 180° and a bit less than latitude elevation is less than ideal, but you "run what you brung" in terms of the orientation and roof pitch of a structure. Better to take a hit on production if we're talking about the backasswards method of incentivizing by the installed watt, than not installing PV at all. That said, we DO have control over wire sizing. IMO, throwing away watts forever just to cheapen it up a very little bit upfront is poor economy and as the price of gird supplied power increases over the years, the waste and lost revenue is even more acerbated. Best, Bob-O On Apr 6, 2010, at 6:04 PM, R Ray Walters wrote: Just run the numbers sometime. Compare the cost difference of #6 vs. #4 wire say, and then look at how many more watts you're actually saving, then multiply that additional wattage by the installed cost per watt. Very simply, once you've satisfied code requirements, there is a point at which it is cheaper to add more panels than use bigger wire. Also, that point is a moving target that fluctuates with PV and wire costs. I've found recently for our projects, that that point is falling at about 3% loss. I also include a cost factor for oversizing the conduit, extra labor (bigger wire is harder to handle), and any connectors needed to land the larger wire. I've got very well designed systems working for decades, using this method. This is how large commercial systems are designed as well. You can't simply pull 1%, and then call us bad designers because we actually do an economic analysis for each wire run. It used to be unheard of to install PV facing anything but due south at latitude tilt, but now we know to add a few more modules. Same concept. R. Walters [email protected] Solar Engineer On Apr 6, 2010, at 1:37 PM, Bob-O Schultze wrote: |
_______________________________________________ List sponsored by Home Power magazine List Address: [email protected] 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
