[RE-wrenches] DC wire sizing

[Warren Lauzon]

The original rationale was probably due more to solar panel costs more than anything else - at a time when solar was $10-15 a watt, and copper was cheap it made sense.

But now that solar panels cost 1/5th as much and copper wire is 4x as much, probably not so much now. Nowadays it is quite often much more economical on systems with long wire runs to add more panel than to use bigger wire.

Also, when that "standard" was adopted, there were still a lot of panels made in the 15.5 to 16.5 volt range, and 2/10th to 4/10th of a volt drop was more important to battery life and charging in hot weather, so that may have been a factor also.

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Northern Arizona Wind & Sun - Electricity From The Sun Since 1979
Solar Discussion Forum: http://www.wind-sun.com/ForumVB/
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  ----- Original Message ----- 
  From: Kent Osterberg 
  To: RE-wrenches 
  Sent: Monday, April 12, 2010 10:51 AM
  Subject: Re: [RE-wrenches] DC wire sizing


  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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