Re: [RE-wrenches] DC wire sizing

[Kent Osterberg]

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. My question is: what is the rational for using 1% loss as the design objective.  Why not 1/2%?  Why not 2%? So far no one has offered an answer to that question; despite many claiming that 1% or 1.5% is always their design objective. 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. Kent Osterberg Blue Mountain Solar, Inc. Warren Lauzon wrote: 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.   .................................................................................................. Northern Arizona Wind & Sun - Electricity From The Sun Since 1979 Solar Discussion Forum: http://www.wind-sun.com/ForumVB/ .................................................................................................. ----- Original Message ----- From: Kent Osterberg To: Wrenches Sent: Saturday, April 10, 2010 9:22 AM Subject: [RE-wrenches] DC wire sizing Here's my challenge to all of you that want to design for 1% or 1.5% voltage drop all the time: 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. At % loss did it become impractical?  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. Kent Osterberg Blue Mountain Solar, Inc.

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