All,
This is all very interesting, but on the dc side, it is primarily an issue of energy loss for large grid-connected systems. My rule is 2% voltage drop for dc side and 1% for ac side. When we talk about dc % voltage drop, most of what has been discussed in this thread is instantaneous voltage drop. That is interesting, but year-round voltage losses are more important. There are two ways to get this: 1. Use a simulation program like PVSyst to calculate the voltage drop every hour of the day and give you an overall loss for the year. 2. Take 80% of Imp for the array and use that current with the nominal voltage of the array (330 Volts for a 600V array). The reason for 80% of Imp is that half of the energy is delivered above this number and half below. It makes an ideal "nominal" current of the array. By using one of these methods, you will calculate the accurate energy loss on the dc side. Bill. From: [email protected] [mailto:[email protected]] On Behalf Of Allan Sindelar Sent: Thursday, July 22, 2010 3:12 PM To: [email protected]; RE-wrenches Subject: Re: [RE-wrenches] calculating DC voltage drop Mike, I would respectfully challenge both reasons ;^)} . The responses already posted today cover them: you don't need to compensate for intermittent high-insolation conditions when sizing for voltage loss, as this is not a Code issue; Code addresses ampacity and OCP issues. And being conservative means you add unnecessarily to wire cost (sometimes substantially at 25% greater current) when that money could be better spent on more PV, with an ultimately greater annualized return of total energy per $$ spent. Ray Walters' recent posts cover this well. Allan Sindelar <mailto:[email protected]> [email protected] NABCEP Certified Photovoltaic Installer EE98J Journeyman Electrician Positive Energy, Inc. 3201 Calle Marie Santa Fe, New Mexico 87507 505 424-1112 www.positiveenergysolar.com <http://www.positiveenergysolar.com/> On 7/22/2010 3:53 PM, Michael Kelly wrote: Allan, I use Imp*1.25 because NEC calls out a 125% adder for unexpected high-insolation conditions (snowscapes, cloud effect, yellow houses next door, etc). And because I am an engineer and like to be conservative :). -Mike --- Michael Kelly Applications Engineer NABCEP Certified Solar PV Installer T toplogo Solectria Renewables, LLC 360 Merrimack St. Building 9, Floor 2 Lawrence, MA 01843 Phone: 978-683-9700 ext. 167 Fax: 978-683-9702 [email protected] www.solren.com <http://www.solren.com/> From: Allan Sindelar [mailto:[email protected]] Sent: Thursday, July 22, 2010 5:26 PM To: [email protected]; RE-wrenches Subject: Re: [RE-wrenches] calculating DC voltage drop Mike, Why do you make this exception, please? Allan Sindelar <mailto:[email protected]> [email protected] NABCEP Certified Photovoltaic Installer EE98J Journeyman Electrician Positive Energy, Inc. 3201 Calle Marie Santa Fe, New Mexico 87507 505 424-1112 www.positiveenergysolar.com <http://www.positiveenergysolar.com/> _______________________________________________ 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
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