Voltage drop musings [RE-wrenches]

[William Miller]

Drake:

I must not have articulated very well the whole point of this dilemma.
Plus, I have to respectfully disagree with some points you have made:

Due to the cost of purchasing and installing PV equipment, I believe we
need to build systems that are much more efficient than the NEC would
require.  Consider that a standard panel, say a Siemens SR100, costs as
much as $580, and a 12 panel tracking rack costs about $1,750.00.  Add in
the cost of an excavation contractor, 6" steel pipe, concrete and labor.
The cost of installing 2400 watts of PV could be as high as $22,000 (at
least here in California).  If you design a system with a 5% voltage drop,
you are wasting almost $1,100.00 in installed PV costs in heating your
undersized wire.  In other words, 5% of your PV output never reaches the
charge controller.

If, however, your wiring is sized for 1% voltage drop, you are consuming
only $216 in installed PV costs.  All of these figures are based on your
system is operating at rated output.

The point of this discussion is to attempt to determine how often the
system is operating at rated output.  If the system never operates at rated
output, then designing for 1% may well be over-kill.

Others have spoken about winter charging as being no less robust than
summer charging.  While we do not have snow in my region, the description
of some of the scenarios lead me to imagine a situation many of my clients
may experience:  Our winter is really a rainy season.  The storms come in
waves from the southwest.  Between storms we can have one or more sunny
days.  I can imagine that after the rain washes the air clear, and with the
ambient temperature cooler, the panels may charge closer to rated output
than in the summer.  And after days of rain, my clients my need all of the
charge they can possible get in a short time period.  This is leading me to
believe that a design with a 1% voltage drop may be justified.

You also mention that, "Current charges batteries, not voltage."  I have to
disagree.  Power charges batteries.  Power is the factor that does any type
of work, be it create light, lift water or create a chemical change in a
battery (charging).  Power has two equally important components: voltage
and current.  If you are heating undersized wire, you are losing power.

True, in a series circuit, current remains the same in all legs.  But if
you drop voltage in your wire, you need more current to create the same
power.  These losses are know as "I squared R losses."

I thank all of those who commented on these questions.  I value my
connection to those who do this same work and wrestle with these same
dilemmas.


William Miller


At 05:15 PM 5/1/01 -0600, you wrote:
>You don't need to limit voltage drop to 1%.   Various Code interpretations 
>would limit it to no more than 2 or 3%, or even 5%.y.
>
>The difference in current will not be 
>great.   Current charges batteries, not voltage.
>
>Drake
>


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William Miller
SLO Communications: Communications and Power Systems Consulting
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email: wrmiller at slonet.org
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