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Friends:<br><br>
We are missing some information. We know the mid-string distance,
but what is the distance from the positive and negative ends of the
strings to the inverter? This is all part of the entire string
circuit and must be analyzed.<br><br>
We have spent a lot of time researching voltage drop. One of the
most interesting discoveries we made is when we considered the duration
of peak amperage conditions. If one designs a PV circuit for a
target voltage drop at brochure Imp, one must consider how often this
value actually occurs. We submit it is infrequent and of short
duration. We took the hour-by-hour results available from PV watts
for an entire year and averaged all non-zero (daylight) values. The
result was close to 50%. This means that if you design a system for
1% voltage drop at Imp (which is our standard), the average Vdrop is
actually 0.5%. Knowing this, we sometimes allow one size smaller
wire when distance, cost, conduit fill or other factors indicate this
might make a design more pragmatic.<br><br>
For a free voltage drop calculator, look on our web site
(millersolar.com) in the Resources area. We think it is superior to
most others because: it allows one to use any voltage, not just
standard AC circuit voltages as on the SMA calculator, it allows
one to view a range of wire sizes in one glance and it now compensates
for temperature and conduit fill. Making this resource available is
one way in which we are trying to give back to the industry..<br><br>
<br>
William Miller<br><br>
<br><br>
At 07:13 AM 4/22/2009, you wrote:<br>
<blockquote type=cite class=cite cite>Drake, it's as quick as V=IR. So
taking the 11A from your evergreens, and a #10 interconnect wire at
1.24Ohm/kft (NEC Table 8):<br><br>
Vdrop = (11A) (0.00124Ohm/ft) (36 ft) = 0.5V. If the modules are hot,
you're looking at a Vmp around 265V- so your worst-case Vdrop% in that
jumper wire is just 0.5V/265V = 0.2%.<br><br>
You're not using the voltage of the entire array to figure the drop on
the interconnect; you're just using the string current. You use the array
voltage just to see whether the voltage drop is significant (as David
Katz wrote).<br><br>
That answer your question?<br>
DKC (just to add another Dave to the thread)<br><br>
<blockquote type=cite class=cite cite><blockquote type=cite class=cite cite>David
Palumbo wrote:<br>
<blockquote type=cite class=cite cite>Does the following series string
wiring cause a problem? With (18) Evergreen ES-A 205’s (low voltage
modules 18.4Vmp) in series feeding a grid tie inverter with a long series
connector in the middle of the string. (2) Top Of Pole 9 module
mounts with the series connection from module #8 on the first pole to
module #9 on the second pole. This MC cable connection would be, about,
36 feet in length to go 10’ down the pole below grade 18” for 13’ and
back up the second pole 10’ (in conduit all the way) .<br>
<br>
Will this extra long series wire cause problems?<br>
<br>
I could use other modules of course but that would raise the cost per
watt by 70 cents.<br>
<br>
Dave<br>
<br>
/David Palumbo,
President</font></blockquote></blockquote></blockquote></blockquote></body>
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