Okay guys - just when i thought the brainiacs had forgotten i even had a question! [Linked Image] The truth is, yes, i really am trying to figure out what size wire to run. I know what voltage drop i want, because i follow NEC guidelines - 3% for feeder or branch circuit, 5% for total. I also know my Vd formulas (or thought i did till i became acquainted with Diver [Linked Image] ), and know that i can transpose any algebraic equation to solve for whatever unknown i want. Hence,

Vd = (2 x K x I x D) divided by cmils

becomes

Cmils = (2 x K x I x D) divided by Vd.

And when solving for cmils, you use what's called "approximate K" values; for aluminum this value is 21.2; for copper it is 12.6.

Distances are 45' alum; 82' copper. In the above formula, D id for one-way distance. (Note: for three-phase calcualtions, the "2" in the above equation would be replaced with "1.73").

In my case I am pulling a feeder to a subpanel, for a 2-pole 50 amp. My intent so far is to pull SE #4 alum, spliced to THHN copper stranded #6. In this situation, either method i chose (whether calculating, say, the alum [or cop] using just the distance of that material's run [45' or 82'] or using the total distance [127'] in each calculation) gave me the same answer because regardless or what answer the Vd formula gives you, you still have to abide by ampacities in T.310.16 for minimum. In some other scenario my answers may have not matched up, depending on how i computed distance, so MY QUESTION REMAINS: which way do i compute distance for this Vd formula: when calculating the alum do i use the distance of just the alum portion of the feeder; or do i use the distance of the entire feeder? same question for the copper.
Thanks.