[RE-wrenches] DC wire sizing

[R Ray Walters]

Also consider that most people are figuring volt drop at a wire temperature of 167 deg F (75C). That's really hot, and most of the time the wire will be much cooler, and therefore the volt drop will be less.
(use the fine print note #2, NEC table 8 to adjust down for cooler temperature)
William's time analysis is very interesting as well: when considering wire losses for a continuous 24/7 load, it is roughly 4 times more important to reduce the losses than the PV wiring. Likewise a load that is only on 5 min/ day doesn't make near as much sense to oversize the wire. I added this time factor to my spreadsheet for calculating economics.

R. Walters
ray at solarray.com
Solar Engineer




On Apr 9, 2010, at 8:44 AM, William Miller wrote:

> Friends:
> 
> Forgive me if this point has already been made:
> 
> When designing PV feeders, one typically looks at the Impp of an array for current specifications.  Impp is rarely achieved, however -- only  a handful of days per year and for only a short period of time per day.   This is particularly true for a fixed array.
> 
> I took the hour-by-hour data from the PV watts tool, subtracted out the zero values (night time) and averaged the remaining values.  The result was about 50%.
> 
> Therefore, a 2% VD at Impp is actually a 1% VD on average.  This gives a bit more headroom in any PV feeder design.
> 
> William Miller
> 
> 
> At 04:25 AM 4/9/2010, you wrote:
>> IMHO it seems like we're all really on the same page here...... trying to make
>> the the most out of the the resources we have available to work with......sun
>> hours, pv, copper, labor, dollars, etc. Job specifics come into play. There are
>> definitely times when a detailed cost/ benefit analysis is called for. Many
>> other times, let's not miss the forest for the trees, an increase in wire size
>> to avoid waste of other aforementioned resources, is low hanging fruit. As a
>> percentage of the overall system cost, the decision is a no -brainer.
>> 
>> Dick Ratico
>> Solarwind Electic
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