[RE-wrenches] Very unbalanced DC wiring

[Hilton Dier III]

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-request at 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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