Nick,
Yes, the wire prices I used were way out there, but I wanted to use a
currently published value that everyone has access to. It only takes a
few minutes to update wire prices in a spreadsheet so I look at the
value of larger wire for every job. I'm not trying to promote large
wire losses -- on many grid connect jobs, I end up running less than 1%
loss on the dc side. Higher values usually make sense for long
circuits to pole top structures.
I think every wrench that reads this list has an intuitive notion that
larger wire at some point becomes impractical. But no one yet has been
able to provide any support for the notion that 1% is the best choice.
Or why 1/2% or 2% isn't better. The fact is that forcing the wire
losses down to 1% is sometimes past the point of any economic return
and folks are pushing designs to that standard without looking at the
costs and benefits.
Kent Osterberg
Blue Mountain Solar, Inc.
Nick Soleil wrote:
I
haven't checked my e-mail for days, but have to add to this topic.
Hi Kent:
In a few isolated situations, a cost analysis may be done to justify
not designing at 1-2% DC Voltage drops, but for most systems the cost
of the wire is less than the cost of the PV. I buy my wire for about
1/4 the cost of your analysis. I would ask you to look closer at your
next design, and consider designing at a lower voltage drop than 3%.
Nick Soleil
Project Manager
Advanced Alternative Energy Solutions, LLC
PO Box 657
Petaluma, CA 94953
Cell: 707-321-2937
Office: 707-789-9537
Fax: 707-769-9037
From:
Bob-O Schultze <[email protected]>
To: RE-wrenches
<[email protected]>
Sent: Sat, April 10,
2010 10:26:56 AM
Subject: Re:
[RE-wrenches] DC wire sizing
Of course it's not a practical solution, which is why we strived for
(but rarely got) 2% VD on low voltage situations as I said in my first
post. The obvious reply to this post is that your scenario isn't real
world anymore. MPPT controllers fixed that. Or perhaps you advocate
that MPPT controllers are just too expensive too? I could easily come
up with a scenario where a C40 and big wire is less expensive than
using MPPT and smaller wire. But is that good design? I don't think so.
My point -and one which you apparently dismiss out of hand- is
just that cheaper is not always better. Good design and best practices
rarely go hand in hand with cheaper.
Waste is waste. Along those same lines, I'll repeat something a
friend of mine recently said, "If you like getting your oil from Saudi
Arabia, you're gonna love getting your PV from China. " It's cheaper,
right?
Ya know?
Bob-O
On Apr 10, 2010, at 9:22 AM, Kent Osterberg wrote:
Here's my challenge to all of you that want to design for 1% or 1.5%
voltage drop all the time:
Take a 50-ft circuit carrying 40 amps at 12 volts, going to a C40
charge controller, select your wire size for 1% or 1.5% loss. You know
that's not a practical solution. I know that it makes more sense to
replace the charge controller and run the PV circuit at a higher
voltage.
At % loss did it become impractical? Ray's answer and my
answer
is "when there is a cheaper alternative -- lower cost per watt out or
lower cost per kWh over the project life." And that answer works even
when you are considering a 400-kW PV array that is 1000 feet away.
Kent Osterberg
Blue Mountain Solar, Inc.
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