Bill,
The NEC is a minimum design design standard, but voltage drops are not
part of the code. The spots where voltage drop is discussed in the NEC
are fine print notes that are not code and are not enforceable as
code. Furthermore those notes were written without any consideration
of the range of dc voltages that we use in different PV systems. The
code does specify ampacity limits and those are based on the heat load
on the wire. Voltage drop goes up with the circuit length, and
current, and it goes up for lower voltage circuits too; the heat load
only goes up with the current. So just evaluating voltage drop doesn't
give any indication about the heat load on the wire.
Kent Osterberg
Blue Mountain Solar, Inc.
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 <r...@solarray.com> wrote:
Over the same amount of time a similar investment in
PV would save even more money.
R. Walters
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@lists.re-wrenches.org
Options & settings:
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
List rules & etiquette:
www.re-wrenches.org/etiquette.htm
Check out participant bios:
www.members.re-wrenches.org
_______________________________________________
List sponsored by Home Power magazine
List Address: RE-wrenches@lists.re-wrenches.org
Options & settings:
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
List rules & etiquette:
www.re-wrenches.org/etiquette.htm
Check out participant bios:
www.members.re-wrenches.org
_______________________________________________
List sponsored by Home Power magazine
List Address: RE-wrenches@lists.re-wrenches.org
Options & settings:
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
List rules & etiquette:
www.re-wrenches.org/etiquette.htm
Check out participant bios:
www.members.re-wrenches.org
No virus found in this incoming message.
Checked by AVG - www.avg.com
Version: 9.0.801 / Virus Database: 271.1.1/2802 - Release Date: 04/09/10 23:32:00
|
