<html><head><style type="text/css"><!-- DIV {margin:0px;} --></style></head><body><div style="font-family:arial,helvetica,sans-serif;font-size:12pt">The more we write on this subject, the more I realize that we are saying the same thing. I think it is important to start with a design objective of 1% to 2% for PV systems. As you mentioned it could be 1/2%, but that would be impractical to me. On most systems, we will lose 1/2% just on the MC cables. I do not stock #8 USE-2 nor do I stock MC connectors and strain reliefs to manage #8 MC cables, so I accept a bit of loss with the #10 MC cables. That leaves me another 1/2% or more on the THWN-2 wiring.<br> If we did a cost analysis on every project, then we would not need to use a rule of thumb (or design objective.) In the system I was designing recently, with a 900' DC run, the wiring was going to cost $50,000 to keep the loss at 2% during peak
production. The system cost $2.4 million, so each 1% of power would be worth ~$24,000. I ran the numbers a bunch of different ways, and in the end, I did compromise on the wire size, to keep the wiring and conduit costs reasonable. In the end, the wire and conduit did account for 2-3% of the cost of the system. In my design, I designed the system with the lowest voltage the array would ever produce power at, and I used peak power production for my numbers, so I knew that my real world efficiencies would be even higher, so in the end, I am probably near 1.5% voltage drop, which is fine for me. If I can design that system with a 1-2% voltage drop, then I doubt that many systems would justify a 3% loss. <br><div> </div>Nick Soleil<br>Project Manager<br>Advanced Alternative Energy Solutions, LLC<br>PO Box 657<br>Petaluma, CA 94953<br>Cell: 707-321-2937<br>Office: 707-789-9537<br>Fax: 707-769-9037<div><br></div><div
style="font-family: arial,helvetica,sans-serif; font-size: 12pt;"><br><div style="font-family: times new roman,new york,times,serif; font-size: 12pt;"><font size="2" face="Tahoma"><hr size="1"><b><span style="font-weight: bold;">From:</span></b> Kent Osterberg <kent@coveoregon.com><br><b><span style="font-weight: bold;">To:</span></b> RE-wrenches <re-wrenches@lists.re-wrenches.org><br><b><span style="font-weight: bold;">Sent:</span></b> Mon, April 12, 2010 10:51:10 AM<br><b><span style="font-weight: bold;">Subject:</span></b> Re: [RE-wrenches] DC wire sizing<br></font><br>
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I know you wouldn't, Warren. Because it is much more economical (that
means more kWh/$) to use a MPPT controller. But you would run a
100-volt 30-amp PV array to a MPPT charge controller that is 150 feet
away. For that distance, it takes 1/0 wire to get the wire loss down
to 1%. I'd probably settle for using 3 AWG copper with 2% loss. With
the smaller wire, the looses are 29 watts more, but the system cost is
less by at least $700. At $700/29 watts = $24/watt, the 1/0 wire is
not a good choice.<br>
<br>
My question is: what is the rational for using 1% loss as the design
objective. Why not 1/2%? Why not 2%?<br>
So far no one has offered an answer to that question; despite many
claiming that 1% or 1.5% is always their design objective.<br>
<br>
In Home Power issue 104, December 2004 - January 2005, I showed that
striving for low voltage drops in low-voltage systems doesn't mean
you've got a good (economical) design. The spreadsheet I created for
that article is no longer available, but one that is easier to use is
available from Ray Walters' web site. In SolarPro 3.2, February -
March 2010, Blake Gleason shows an example where upsizing the wire size
to get 2.6% loss is not economical.<br>
<br>
Kent Osterberg<br>
Blue Mountain Solar, Inc.<br>
<br>
<br>
Warren Lauzon wrote:
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<div><font size="2" face="Calibri">I would never design such a
system. That is what MPPT controllers are for. 10 years ago we did not
have that choice, but now we do, so there is no reason at all to design
a 40 amp 12 volt array circuit.</font></div>
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<div>..................................................................................................<br>
Northern Arizona Wind & Sun - Electricity From The Sun Since 1979<br><span>
Solar Discussion Forum: <a target="_blank" href="http://www.wind-sun.com/ForumVB/">http://www.wind-sun.com/ForumVB/</a></span><br>
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Original Message ----- </div>
<div style="background: none repeat scroll 0% 50% rgb(228, 228, 228); font: 10pt arial;"><b>From:</b>
<a rel="nofollow" title="kent@coveoregon.com" ymailto="mailto:kent@coveoregon.com" target="_blank" href="mailto:kent@coveoregon.com">Kent Osterberg</a> </div>
<div style="font: 10pt arial;"><b>To:</b>
<a rel="nofollow" title="re-wrenches@lists.re-wrenches.org" ymailto="mailto:re-wrenches@lists.re-wrenches.org" target="_blank" href="mailto:re-wrenches@lists.re-wrenches.org">Wrenches</a> </div>
<div style="font: 10pt arial;"><b>Sent:</b>
Saturday, April 10, 2010 9:22 AM</div>
<div style="font: 10pt arial;"><b>Subject:</b>
[RE-wrenches] DC wire sizing</div>
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Here's my challenge to all of you that want to design for 1% or 1.5%
voltage drop all the time:<br>
Take a 50-ft circuit carrying 40 amps at 12 volts, going to a C40
charge controller, select your wire size for 1% or 1.5% loss. You know
that's not a practical solution. I know that it makes more sense to
replace the charge controller and run the PV circuit at a higher
voltage.<br>
<br>
<b>At % loss did it become impractical?</b> Ray's answer and my
answer is "when there is a cheaper alternative -- lower cost per watt
out or lower cost per kWh over the project life." And that answer
works even when you are considering a 400-kW PV array that is 1000 feet
away.<br>
<br>
Kent Osterberg<br>
Blue Mountain Solar, Inc.<br>
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