[RE-wrenches] DC wire sizing

[Bob-O Schultze]

Guys,
Is it just me being dense or are none of you folks advocating for higher VD looking at the savings over time?
If we assume that Kent's wire costs are correct (and even assuming a 33% mark-up, he's paying WAY, WAY too much for wire) , the difference in delivered watts between #10 and # 4 wire in this situation is 91W. If I were installing this in Southern Oregon, which is pretty average as far as peak sun hours/day go, we'd be looking at 91 x 4.5 (peak sun hours) x 365days/yr x 25yrs = 3736 KW/H. Even at $0.10/KWH that's about $375 AT TODAY'S POWER RATES. Anyone think those rates are going to stay the same or go down over the next 25 years? Anybody think they won't go up by 5X? 10X? 20X?
So... for what and for whom are we designing these systems?
Bob-O

On Apr 8, 2010, at 12:40 PM, Kent Osterberg wrote:

Nick,

Advocating for an economic comparison between the cost of wire and the energy saved by larger wire is not the same as advocating for high voltage drops, or low ones either.  Even with the present low prices for PV modules and high prices for copper wire, a 100-ft long 350-volt dc input to a 3-kW inverter should have around 1% voltage drop.   Now consider a 350-volt 10-amp PV circuit that's 500 feet long.  Using 12 AWG copper the dc voltage drop would be 5.5%.  Sounds like that might be a poor wire choice, right?  Look what happens as the wire size is increased:

                  Conductor         Power             $ per
AWG   $/ft       Cost         ---- Loss ----     watt saved
 12     0.62      $620        193W (5.5%)         -- 
 10     0.95      $950        123W (3.5%)        $4.71        
  8     1.54      $1540         77W (2.2%)      $12.83
  6     2.37      $2370         49W (1.4%)      $29.64
  4     3.73      $3730         32W (0.9%)      $80.00

It would be reasonable to use 10 AWG copper, but before going up to 8 AWG, I'd consider buying more PV instead.  Why buy a watt of power at $12.83 when it cost less to buy a watt of PV?  The conductor price used here, just for illustration, is from Southwire's price list for THHN/THWN wire dated 7 April 2010.  In the column of conductor costs I only considered the cost of two current carrying wires.  The cost of the equipment ground wire, conduit, connectors, etc all go up too.  That makes the dollars per watt saved look even worse.

Kent Osterberg
Blue Mountain Solar, Inc.


Nick Soleil wrote:
> 
> I feel that it is best to maintain a 1.5% voltage drop on the AC and DC.  However, I was just sizing conductors for a 400 KW project, with the array 1000' from the main service panel.  With AC modules, I would have needed 5-Parallel runs of 700MCM at 208VAC (20 wires at 700MCM for 1.5%VD!)  The cost would have been over 100K, which was cost prohibitive.  However, by running DC wiring, and utilzing AL, we were able to maintain 1.5 VDC drop without being too expensive (yet still expensive.)
>  
> 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
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