Hah, George, at least someone is staying on target.
Tripp,
Voltage drop depends upon current. Since we don't know the actual current flow in the conductors that you are proposing, we can't calculate the voltage drop.
The equation that you posted does not calculate voltage drop; it takes as its _input_ the desired voltage drop, and the distance, and gives as its output the size of conductor that you need. Of course, this equation is simply the voltage drop equation re-arranged.
In other words, if you use the simple voltage drop equation E = I R, you can re-arrange by dividing by I, and get R = E/I, which tells you what resistance is required to get a particular desired voltage drop.
Your equation goes one step further, and takes the inherent resistivity of the conductor into account, to give you the conductor size. You will need to double check; some of these equations use as their constants values that take into account 'round trip distance', others use 'one way' distances. Same equation, different constants, and you need to match the constants that you use to the question that you are answering.
The key point to keep in mind is that voltage drops that are electrically in series simply add up. If you have a 100 foot length of aluminium wire, with a voltage drop of 1V, and a 200 foot length of copper with a voltage drop at 1.5V, then the total voltage drop is 2.5V.
So with the normal E = I R equation, you would first find the resistance of your aluminium conductor, and calculate the voltage drop, and then you would find the resistance of the copper conductor, calculate it's voltage drop, and add the two up.
With your reverse direction equation, you will need to first 'distribute' the total allowed voltage drop to each portion of the circuit. Say you want a total of 3% voltage drop, with 45 feet of Al followed by 87 feet of Cu. You might arbitrarily assign 1% voltage drop to the Al portion, and 2% to the Cu portion. Or could be 0.5% to the Al portion and 2.5% to the Cu portion. Whatever; you just pick two reasonable looking numbers that add up to your desired total.
Then you simply work your equation with the individual section voltage drops, for the length of the individual sections only. So you would run the equation with 45 feet of Al and a 1% voltage drop, and separately run the equation for 87 feet of Cu and a 2% voltage drop, and the two conductors electrically in series will give a 3% voltage drop at the design current.
-Jon
