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

Steve Johnson stevejohnson at comcast.net
Thu May 1 13:21:11 PDT 2008


Thanks for moving this over from marketing whoever did.  I already know 
how marketing calculates voltage drop!!

Great feedback. I meant to say from combiner to inverter but I think 
everyone got that.

I'm core drilling roof (after x-ray,tensioned steel cable construction) 
of 5 story concrete building and running through conditioned space to 
basement.

Vmp is 328V.  Imp is 23.5 x 1.56 is 36.6 amps
So #2 in 1-1/4" EMT is 1.73%vd.

But it seems to me the 1.56 mutiplier isn't really necessary for this 
voltage drop calculation.  The wire is way oversized already for either 
23.5 amps or 36.6 amps.  
This calculation is about resistance.
So if I use 23.5 amps at 328 Vmp I can use #2 for 1.1%vd.

The AC side isn't a problem.

Steve


Joel Davidson wrote:
> 
> Hello Steve,
> 
> Bob-O Schultze and Bill Brooks are right. Granted, supply shortages and 
> the 
> dollar/Euro value are important business factors, but minimizing voltage 
> 
> drop is important because a 1% increase in voltage drop means a 
> cumulative 
> 1% decrease in the total output of your PV system over its lifetime.
> 
> I do not know of a PV system sale that was lost to a competitor because 
> of 
> the price difference between 2% and 5% I2R loss wire. On the other hand, 
> I 
> know jobs that were won because the customer selected the system with 
> the 
> highest peak (not average) inverter efficiency. PV customers want 
> efficient 
> systems.
> 
> In the 1970s, some PV system designers referenced NEC 215 and 
> rationalized 
> 5% wire loss because 36- and even 33-cell solar modules could charge 
> batteries under most conditions. By 1984 when NEC 690 was first 
> published, 
> many designers had switched from 5% to 3% and some designers were used 
> 2% 
> wire loss to squeeze every watt out of the system. In 2002, John Berdner 
> got 
> designers' attention by recommending 1% to 1.5% loss on the AC side. 
> Since 
> then some wrenches have posted that higher than 2% wire losses are 
> acceptable (maybe to them but not to me).
> 
> A Ferengi Solar Rule of Acquisition states that money today is worth 
> more 
> than money tomorrow and its corollary states that first-cost is most 
> important. Thus, it is believed by some beings in this galaxy that the 
> lowest price PV system is best. However, I say design PV systems with 2% 
> or 
> less wire loss, invest in copper wire, and reap the kWh rewards for 
> decades.
> 
> Joel Davidson
> 
> "Conventional energy economics is a value system masquerading as 
> mathematics. At its heart is one key assumption: the future is worthless 
> and 
> the environment doesn't matter. Fie on future generations, who needs 
> 'em? 
> What have my grandchildren done for me? For 80 years, our culture has 
> had 
> cheap power on a pedestal. In most contexts, cheap means 'shoddy' or 
> 'second-rate.' Cheap is schlock, cheap is shunned. Think of your own 
> purchasing behavior. Do you buy the cheapest ice cream, put powdered 
> dairy 
> creamer in your coffee, drive a Yugo? Of course not. But when it comes 
> to 
> electricity, cheap is best. If similar thinking prevailed in the 
> underwear 
> industry, Calvin Klein would sell only burlap bras and boxer shorts. 
> Scratchy sure, but cheaper than cotton. A Public Underwear Commission 
> would 
> ensure that he didn't try to blend some pricey silk garments with the 
> burlap 
> ones. If consumers complained of a rash, the PUC would say, 'Quit 
> itching. 
> Americans want cheap undies. Burlap is best.'" - from The New Frontier: 
> Grid-Connected PV, James R. Udall, 1998.
> 
> 
> 
> 
> ----- Original Message ----- 
> From: "Bob-O Schultze, Electron Connection"
> To: <RE-Markets at topica.com>
> Sent: Thursday, May 01, 2008 6:55 AM
> Subject: Re: [RE Marketing] 400 ft DC voltage drop
> 
> 
> > I'd go with D if possible, but E would be fine too. Losing expensive  PV 
> > 
> > watts in wire losses is bad design and economically foolhardy when  you 
> > consider the lifespan of the system.
> > Bob-O
> >
> > On Apr 30, 2008, at 10:18 PM, Steve Johnson 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
> 



Steve Johnson
LightWave Solar Electric
Nashville, Tennessee


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