[RE Marketing] 400 ft DC voltage drop [RE-wrenches]

[Tom DeBates]

good day Wrenches,
  I have used NEC Chapter 9, Table 8 for DC conductor resistance and 
have noticed that the data is for conductors @ 75C. Since most of these 
"runs" would be below grade, wouldn't the conductor temp be lower (~20C 
?) I see that Note #2 has a correction factor, but I've always been too 
lazy to try it. Has anyone done this and, if so, has it made much of a 
difference?
thanks,
Tom
Jeff Clearwater, Village Power Design wrote:
> 
> Hi Steve,
> 
> William' approach is exactly it.  We use to do 1% at Vmp but as 
> copper prices increased we did a more careful analysis of the peak vs 
> average issue and now use 1.5%  But as he says make sure that's from 
> panel to inverter, not just from combiner to disconnect.  I do take a 
> look at Vmin (the inverter's MPPT min) for the worst case scenario 
> but that usually doesn't go over 2% if Vmp is at 1.5% or better.
> 
> That said if the run is short or the pull easy I tend to bump up a 
> size to maximize those precious PV watts which usually puts me back 
> in the 1% range at Vmp - but if the run is long and the cost starts 
> going up - William's perspective is what we use.
> 
> Hope that helps!
> 
> Jeff Clearwater
> Village Power Design
> 
> 
> >Steve:
> >
> >I'd say E because high temp Emp will occur at lower output amperage. 
> >I doubt the difference between D or E will be significant and Voc 
> >does not count.
> >
> >Consider this concept:  Rated current occurs rarely, if ever. 
> >Assume, though, it occurs at noon.  The rest of the day presents 
> >varying degrees of lesser current.  What is the area under the 
> >curve?  My research indicates it is 50% of the peak amperage.
> >
> >What does this mean?  It means that if you design a PV feeder for 2% 
> >voltage drop at peak output, it will average 1% over the day.  Or, 
> >design criteria for PV feeders is only half as important as we think 
> >it is.  (Don't forget to quantify cumulative loses throughout all 
> >feeders and components, however.)
> >
> >William Miller
> >
> >
> >At 10:18 PM 4/30/2008, you wrote:
> >>I have a 400 foot run on an output circuit to combiner, 23.5 amps.
> >>Should the DC VOLTAGE that is used as the system voltage to calculate
> >>voltage drop be:
> >>A) Voc at low temperature
> >>B) Voc at high temperature
> >>C) Nameplate Voc
> >>D) Vmp at high Temperature
> >>E) Nameplate Vmp
> >>
> >
> 
> 
> -- 
> ~~~~~~~~~~~~~
> Jeff Clearwater
> Village Power Design
> Sustainable Energy & Water Solutions for Home & Village
> http://www.villagepower.com
> gosolar at villagepower.com
> NABCEP (tm) Certified Solar PV Installer
> 
> Voice: 413-259-3776
> Fax: 413-825-0703
> 65 Schoolhouse Rd
> Amherst, MA 01002
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~`~


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