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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:
<blockquote cite="mid:7BE0A6BC01704780B46BA557CA101DC5@Warren"
type="cite">
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<div><font face="Calibri" size="2">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>
<div> </div>
<div>..................................................................................................<br>
Northern Arizona Wind & Sun - Electricity From The Sun Since 1979<br>
Solar Discussion Forum: <a moz-do-not-send="true"
href="http://www.wind-sun.com/ForumVB/">http://www.wind-sun.com/ForumVB/</a><br>
..................................................................................................</div>
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<div
style="font-family: arial; font-style: normal; font-variant: normal; font-weight: normal; font-size: 10pt; line-height: normal; font-size-adjust: none; font-stretch: normal;">-----
Original Message ----- </div>
<div
style="background: rgb(228, 228, 228) none repeat scroll 0%; font-family: arial; font-style: normal; font-variant: normal; font-weight: normal; font-size: 10pt; line-height: normal; font-size-adjust: none; font-stretch: normal; -moz-background-clip: -moz-initial; -moz-background-origin: -moz-initial; -moz-background-inline-policy: -moz-initial;"><b>From:</b>
<a moz-do-not-send="true" title="kent@coveoregon.com"
href="mailto:kent@coveoregon.com">Kent Osterberg</a> </div>
<div
style="font-family: arial; font-style: normal; font-variant: normal; font-weight: normal; font-size: 10pt; line-height: normal; font-size-adjust: none; font-stretch: normal;"><b>To:</b>
<a moz-do-not-send="true" title="re-wrenches@lists.re-wrenches.org"
href="mailto:re-wrenches@lists.re-wrenches.org">Wrenches</a> </div>
<div
style="font-family: arial; font-style: normal; font-variant: normal; font-weight: normal; font-size: 10pt; line-height: normal; font-size-adjust: none; font-stretch: normal;"><b>Sent:</b>
Saturday, April 10, 2010 9:22 AM</div>
<div
style="font-family: arial; font-style: normal; font-variant: normal; font-weight: normal; font-size: 10pt; line-height: normal; font-size-adjust: none; font-stretch: normal;"><b>Subject:</b>
[RE-wrenches] DC wire sizing</div>
<div><br>
</div>
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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