[RE-wrenches] DC wire sizing

[Nick Soleil]

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.
    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. 

 Nick Soleil
Project Manager
Advanced Alternative Energy Solutions, LLC
PO Box 657
Petaluma, CA 94953
Cell:   707-321-2937
Office: 707-789-9537
Fax:    707-769-9037




________________________________
From: Kent Osterberg <kent at coveoregon.com>
To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
Sent: Mon, April 12, 2010 10:51:10 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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