[RE-wrenches] DC wire sizing

[Bill Hoffer]

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
>
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