William,
That is exactly what I said in the first response to this thread,
(quote)"You should calculate your voltage drop for the entire round
trip distance of this circuit and size the wire accordingly. It does
not matter where the wire is located in the series circuit, just the
total length (of the circuit)." I believe this is what Dave was
asking; "Will this extra long series wire cause problems?"
Sometimes our threads wander off track and don't stay focused on the
substance of the question. In the interest of cohesive threads, could
we please keep it on track or change the subject line? Thanks:)
Kindest Regards,
Larry Crutcher
Starlight Solar
(928) 941-1660
Renewable Energy Products, Service and Installation
Mailing Address (NO SHIPPING):
11881 S Fortuna Rd.
#210
Yuma, AZ 85367
Shipping and retail store (NO MAIL):
2998 Shari Ave
Yuma, AZ 85365
On Apr 22, 2009, at 10:07 PM, William Miller wrote:
Friends:
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.
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.
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..
William Miller
At 07:13 AM 4/22/2009, you wrote:
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):
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%.
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).
That answer your question?
DKC (just to add another Dave to the thread)
David Palumbo wrote:
Does the following series string wiring cause a problem? With
(18) Evergreen ES-A 205s (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) .
Will this extra long series wire cause problems?
I could use other modules of course but that would raise the
cost per watt by 70 cents.
Dave
/David Palumbo, President
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