[RE-wrenches] DC Wire Sizing (Ethics & VDrop)
Ryan LeBlanc
ryan at naturalenergyworks.com
Mon Apr 12 20:25:44 PDT 2010
Kent,
Nice to meet you I've enjoyed your past posts and I like what you wrote
below, and I definitely understand that you're just roughing it.
Sometimes I hear people say we should "Add more PV," even when it doesn't
necessarily solve our problem. Most often it's not that easy to add more
PV, or we likely would've. We are typically either at the clients budget
limit, the inverters limits (P,V,A,Strings, etc.), we could be at a Megawatt
PTC, so no more rebate on that PV, or we have used up all the unshaded space
on the roof, etc. And if that's no trouble, with more PV comes more
racking, install time, and with more power at a ceiling voltage (Inv
Input/NEC 600V)... we end up with more amps, which means bigger wire
anyways. Then of course, the whole thing costs more.
For the chart below, would you think that instead of the straight module
price of $3.00W or whatever we SELL them for, not bought them for... Would
you think a more fair number would be closer to the installed cost of
$8.00/W or whatever we sell it for? In which case, #8 would make sense.
If one sells a system at the NEC minimum recommendations, with significant
voltage drop, I say fine, as long as you gave the client an accurate
production estimate and the financial numbers still work for them, and as
long as those conductors last as long and perform as you estimated. Once we
tell a client, "Your system will produce X, and cost X." We have to
ethically and, often contractually, give them that. If you have to pull
that wire in a few years, due to module voltage degradation and exacerbated
excessive Vdrop, with an inverter that isn't starting up, and maybe after a
few inverter replacement trips until the guys figure out that the Vrop is
too much with the modules settled or degraded voltage, etc, it should be
done for free. If not, that original deal with the client is not being
honored and therefore was an unethical sale, and another unsatisfied
customer. That is if the inverter repairs/replacements didn't upset them
enough. :o)
LCOE
To really compare this, I would say you would have to do a few designs with
the proposed wire sizes and Vdrops, calc the costs of each system, including
larger conduit and fittings, etc. Then run production estimates, with
PVSyst, don't forget that year 2 should be about .5% less, and on and on,
for 20,25 or 30 years, depending on how aggressive you are, and calculate
every lifetime cost to the best of your ability, to get the Levelized Cost
of Energy (LCOE) and compare the one that yields the lowest generated cost
per kWhr. Anything less is Swiss cheese...holes all through it.
I do have to wonder though about WHEN that power is made, I'm sure that
conductor loading throughout the curve can factor in somewhere. Maybe
because actual amp loading will grow, peak and shrink throughout the day,
with relatively constant voltage, maybe our Vdrop calculations are too
rudimentary using fixed Vmp & Imp numbers.
Is Imp variable enough for a conductor with 5.5%Vdrop where it only spends a
small amount of time with that high of a drop, and in under STC conditions
be within a reasonable Vdrop?
Thanks to all who participate on here, the education is greatly appreciated.
Look forward to other opinions, comments, etc.
Sorry about the length,
Ryan LeBlanc
Natural Energy Works
> Nick,
>
> Advocating for an economic comparison between the cost of wire and the
> energy saved by larger wire is not the same as advocating for high voltage
> drops, or low ones either. Even with the present low prices for PV
modules
> and high prices for copper wire, a 100-ft long 350-volt dc input to a 3-kW
> inverter should have around 1% voltage drop. Now consider a 350-volt
> 10-amp PV circuit that's 500 feet long. Using 12 AWG copper the dc
voltage
> drop would be 5.5%. Sounds like that might be a poor wire choice, right?
> Look what happens as the wire size is increased:
>
> Conductor Power $ per
> AWG $/ft Cost ---- Loss ---- watt saved
> 12 0.62 $620 193W (5.5%) --
> 10 0.95 $950 123W (3.5%) $4.71
> 8 1.54 $1540 77W (2.2%) $12.83
> 6 2.37 $2370 49W (1.4%) $29.64
> 4 3.73 $3730 32W (0.9%) $80.00
>
> It would be reasonable to use 10 AWG copper, but before going up to 8 AWG,
> I'd consider buying more PV instead. Why buy a watt of power at $12.83
when
> it cost less to buy a watt of PV? The conductor price used here, just for
> illustration, is from Southwire's price list for THHN/THWN wire dated 7
> April 2010. In the column of conductor costs I only considered the cost
of
> two current carrying wires. The cost of the equipment ground wire,
conduit,
> connectors, etc all go up too. That makes the dollars per watt saved look
> even worse.
>
> Kent Osterberg
> Blue Mountain Solar, Inc.
>
>
> Nick Soleil wrote:
>
> I feel that it is best to maintain a 1.5% voltage drop on the AC and DC.
> However, I was just sizing conductors for a 400 KW project, with the array
> 1000' from the main service panel. With AC modules, I would have needed
> 5-Parallel runs of 700MCM at 208VAC (20 wires at 700MCM for 1.5%VD!) The
> cost would have been over 100K, which was cost prohibitive. However, by
> running DC wiring, and utilzing AL, we were able to maintain 1.5 VDC drop
> without being too expensive (yet still expensive.)
>
> 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
>
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Message: 2
Date: Fri, 9 Apr 2010 11:46:28 -0600
From: R Ray Walters <ray at solarray.com>
To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
Subject: Re: [RE-wrenches] DC wire sizing
Message-ID: <54B514A9-A671-489B-B2E0-D86684A7B5B4 at solarray.com>
Content-Type: text/plain; charset="us-ascii"
All our economic analysis is based on a 20 to 25 year life.
Safety first, but then good design is to spend the customer's money where it
does the most good.
No matter how big the wire, you will always have losses. It is an
exponential curve that never reaches zero.
It just costs more and more for each extra watt saved.
Nobody would install a 4/0 cable for a 50 watt panel, even though the losses
would be lower.
As soon as the design wire starts getting big, I start looking at
alternatives: raise the array voltage, relocate the array closer, etc.
Also, this discussion concerns the DC losses, we've all agreed that AC
losses can cause shutdowns that greatly exceed the wire loss itself.
R. Walters
ray at solarray.com
Solar Engineer
On Apr 9, 2010, at 11:31 AM, Bill Hoffer wrote:
> Ray
>
> I think that the point is not the cost, but what is good electrical
design!
>
> Voltage Drop in a wire is still undesirable and equates to an unneeded
"heat" load on the wire. Are we advocating that if your water pipe is too
small just increase the pressure so you get the same output you desired.
Sure as long as you are well within the operational limits of what the pipe
was designed to do (over the lifetime expectancy of the product). Maybe for
a short period of time that is fine, but we are talking about a system that
we want to perform for 25+ years. I am sure we have all seen 25+ year old
wiring in a house that has become brittle due to operating right at the
limit, not enough to pop the breaker, but enough over a long period of time
to deteriorate the copper. Heat is not our friend.
>
> The worse use of solar electricity is for heating, and I really do not
want to install extra PV just to heat my wire. Nobody said that good
electrical design was cheap! I will continue to design my voltage drop at
1.5% and as always attempt to meet that goal as inexpensively as possible.
NEC is a minimum we need to meet and is not necessarily the best electrical
design practice! When we are talking about Mega-watt commercial
installations, It would be pretty silly to have a system shut down (ie lost
production = lost investment money) because of saving some money on wire.
>
> Bill Hoffer PE
> Sunergy Engineering Services PLLC
>
> On Thu, Apr 8, 2010 at 11:41 PM, R Ray Walters <ray at solarray.com> wrote:
> Over the same amount of time a similar investment in PV would save even
more money.
>
> R. Walters
> ray at solarray.com
> Solar Engineer
>
>
>
>
> On Apr 8, 2010, at 4:28 PM, Bob-O Schultze wrote:
>
>> Guys,
>> Is it just me being dense or are none of you folks advocating for higher
VD looking at the savings over time?
>> If we assume that Kent's wire costs are correct (and even assuming a 33%
mark-up, he's paying WAY, WAY too much for wire) , the difference in
delivered watts between #10 and # 4 wire in this situation is 91W. If I were
installing this in Southern Oregon, which is pretty average as far as peak
sun hours/day go, we'd be looking at 91 x 4.5 (peak sun hours) x 365days/yr
x 25yrs = 3736 KW/H. Even at $0.10/KWH that's about $375 AT TODAY'S POWER
RATES. Anyone think those rates are going to stay the same or go down over
the next 25 years? Anybody think they won't go up by 5X? 10X? 20X?
>> So... for what and for whom are we designing these systems?
>> Bob-O
>>
>> On Apr 8, 2010, at 12:40 PM, Kent Osterberg wrote:
>>
>> Nick,
>>
>> Advocating for an economic comparison between the cost of wire and the
energy saved by larger wire is not the same as advocating for high voltage
drops, or low ones either. Even with the present low prices for PV modules
and high prices for copper wire, a 100-ft long 350-volt dc input to a 3-kW
inverter should have around 1% voltage drop. Now consider a 350-volt
10-amp PV circuit that's 500 feet long. Using 12 AWG copper the dc voltage
drop would be 5.5%. Sounds like that might be a poor wire choice, right?
Look what happens as the wire size is increased:
>>
>> Conductor Power $ per
>> AWG $/ft Cost ---- Loss ---- watt saved
>> 12 0.62 $620 193W (5.5%) --
>> 10 0.95 $950 123W (3.5%) $4.71
>> 8 1.54 $1540 77W (2.2%) $12.83
>> 6 2.37 $2370 49W (1.4%) $29.64
>> 4 3.73 $3730 32W (0.9%) $80.00
>>
>> It would be reasonable to use 10 AWG copper, but before going up to 8
AWG, I'd consider buying more PV instead. Why buy a watt of power at $12.83
when it cost less to buy a watt of PV? The conductor price used here, just
for illustration, is from Southwire's price list for THHN/THWN wire dated 7
April 2010. In the column of conductor costs I only considered the cost of
two current carrying wires. The cost of the equipment ground wire, conduit,
connectors, etc all go up too. That makes the dollars per watt saved look
even worse.
>>
>> Kent Osterberg
>> Blue Mountain Solar, Inc.
>>
>>
>> Nick Soleil wrote:
>>>
>>> I feel that it is best to maintain a 1.5% voltage drop on the AC and DC.
However, I was just sizing conductors for a 400 KW project, with the array
1000' from the main service panel. With AC modules, I would have needed
5-Parallel runs of 700MCM at 208VAC (20 wires at 700MCM for 1.5%VD!) The
cost would have been over 100K, which was cost prohibitive. However, by
running DC wiring, and utilzing AL, we were able to maintain 1.5 VDC drop
without being too expensive (yet still expensive.)
>>>
>>> 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
>> _______________________________________________
>> List sponsored by Home Power magazine
>>
>> List Address: RE-wrenches at lists.re-wrenches.org
>>
>> Options & settings:
>> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
>>
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http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
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>
>
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