[RE-wrenches] DC wire sizing

[R Ray Walters]

Just run the numbers sometime. Compare the cost difference of #6 vs. #4 wire say, and then look at how many more watts you're actually saving, then multiply that additional wattage by the installed cost per watt. 
Very simply, once you've satisfied code requirements, there is a point at which it is cheaper to add more panels than use bigger wire.
Also, that point is a moving target that fluctuates with PV and wire costs. I've found recently for our projects, that that point is falling at about 3% loss.
I also include a cost factor for oversizing the conduit, extra labor (bigger wire is harder to handle), and any connectors needed to land the larger wire.
I've got very well designed systems working for decades, using this method. 
This is how large commercial systems are designed as well. 
You can't simply pull 1%, and then call us bad designers because we actually do an economic analysis for each wire run.
It used to be unheard of to install PV facing anything but due south at latitude tilt, but now we know to add a few more modules. Same concept.

R. Walters
ray at solarray.com
Solar Engineer




On Apr 6, 2010, at 1:37 PM, Bob-O Schultze wrote:

> Ray,
> A 2% wire loss is the generally accepted metric for battery based systems with relatively low PV voltage input (<150Voc). It's just plain bad design to accept more than a 1% VD on higher voltage systems. PVs ain't THAT cheap.
> Best, Bob-O
> 
> On Apr 6, 2010, at 11:44 AM, R Ray Walters wrote:
> 
> 
> Once I have fulfilled NEC min. requirements, I use a spreadsheet to analyze the cost of larger wire vs. the cost of power lost. Going under 2% is usually not worth it, if copper prices are high, and PV cost is low enough (current market). Sizing for under 2% was good economics a few years back, when PV was high, and copper was low, though.
> 
> For NEC 2011, I agree: while I readily welcome development of DC AFI, implementing code before the technology is ready, is a bad idea. But that may be the only way to get the technology in place.....
> 
> Ray
> 
> On Apr 6, 2010, at 11:15 AM, Kent Osterberg wrote:
> 
>> Ray,
>> 
>> Considering that we design PV wiring to be efficient with voltage (and power) loss typically less than 2%, the wire size is nearly irrelevant to arcing issues.   Essentially all the energy available from the PV array can be dissipated in the dc arc.   And since the current is limited by the nature of the IV curve, breakers alone usually won't clear the fault.  The best combiner breakers can do (if you have enough parallel circuits) is isolate the fault to one string in the PV array.  With one string being 1 or 2 kW in many systems there is still the potential for a lot of heat.
>> 
>> With the 2011 code just around the corner and no dc arc fault protection on the horizon, it looks like our industry is again going to have a code requirement that no one can fulfill.  
>> 
>> Kent Osterberg
>> Blue Mountain Solar, Inc.
> 
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