[RE-wrenches] calculating DC voltage drop

[Dana]

Ray, 

 

The $ could be making power I agree on a large scale project. 

 

The point here is where is the cutoff point?

 

Let’s take a very small cabin system:

410 watts or 13 amps  [inc.1.25%]@ 44VOC, over a 90’ one-way run.

Utilizing  and Electro-calc with 2008 NEC installed

 

2.6%VD – 1.1vd requires a pair #6 Thhn @ $0.66/ft = $118 for 180’wire @ whole sale price.

 

1.6%VD – 0.7vd requires a pair #4 Thhn @ $0.78/ft = $140 for 180’wire @ whole sale.

 

0.4%VD – 0.4vd requires a pair #1 Thhn @ $1.55/ft = $279for 180’ wire @ whole sale.

 

I grant you the cost savings up front, and MPPT covers a lot errors these days, but where do we say what is better?

 

So my question is-

In MPPT what is .3volt drop worth VS 0.7vd VS 1.1vd worth in MPPT charge capabilities over the life of the system? 

Weather it is a utility scale project or a small cabin.

 

 

Thanks,  Dana Orzel

 

Great Solar Works, Inc

E - dana at solarwork.com

V - 970.626.5253

F - 970.626.4140

C - 970.209.4076

web - www.solarwork.com

 

"Responsible Technologies for Responsible People since 1988"

 

From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of R Ray Walters
Sent: Thursday, July 22, 2010 3:30 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] calculating DC voltage drop

 

Hi Dana;

 

I'd say that's a bit overkill for today's market. That money could be making more power, not just preventing losses. 

Definitely a law of diminishing returns on wire sizing. 

If you allocated 50% of the budget to wire, and 50% to PV, your losses would be very low, (immeasurable but unfortunately still there) but your total system production vs. the money spent would be terrible.

I'm only using this ridiculous example to show, that at some point, we all spend money on more PV, not bigger wire. 

I pick that point based on sound economic analysis, not some over applied rule of thumb. As copper costs rise, and PV gets cheaper, that point moves up. 

If copper was still at 20th century prices, and PV were $10/ watt, your 1% might very well be the right answer.

 

( this is also the point where Bob-O starts hammering me, so I'd better run. ??)

 

R. Walters

ray at solarray.com

Solar Engineer

 

 





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