Being DC, it's a round trip for the electrons. Fat pipe one way and thin
pipe the other will work. Calculate the drop for 250' of 3/0 and add the
drop for 250' of twice the circular mils of 3/0 and you have it. Offhand
I'll say it is 1.875%.
As for the code issue, I don't see one, as long as the cable is rated
for the conditions.
Hilton Dier
On 5/17/2012 1:37 PM, re-wrenches-requ...@lists.re-wrenches.org wrote:
Wrenches,
I am doing a job for a customer installing a ground mounted array about 250'
from his existing battery bank. Array is:
- Vmp: 87.9 V
- Imp: 47.1 A
- Classic 150 controller
The customer already owns (hence the choice of the 150 over the 200 or 250
controller) sufficient length for the array transmission of a heavy duty direct
burial rated cable, 3 (total) conductor 3/0 AWG copper. He also happens to own
sufficient length of #2 AWG USE-2 copper.
My question is: is there a problem (code or practical or both) with using two
of the 3/0 conductors paralleled for either the positive or negative, and one
of the 3/0 conductors by itself for the other side? In this case I would use
the USE-2 as the grounding jumper.
The max voltage drop on the circuit using only single conductor 3/0 over this
distance (and also accounting for stepping down the conductor size and array
circuit conductors, etc.) is about 2.5% max, so we could live with this. That
said, the customer already owns all of this wire and if I can safely shave a
couple tenths of a percent voltage drop, every bit helps when you are off grid.
It seems to me that using doubled cable on one side only is effectively exactly
the same as having home run cables (of the same AWG) from a series strings of
modules where either the positive or the negative is twice as long as the
other, which happens all the time at the array.
Any thoughts most appreciated!
-Nathan
--
Hilton Dier III
Renewable Energy Design
Partner, Solar Gain LLC
453 East Hill Rd.
Middlesex, VT 05602
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