I thought that I answered that question. You calculate each part entirely separately. The voltage drops add up.
To size the Al section, you use the length of the Al section and the voltage drop apportioned to the Al section.
To size the Cu section, you use the length of the Cu section and the voltage drop apportioned to the Cu section.
If you want to keep the voltage drop below 3%, then you must apportion this 3% between the two sections. If you have 10 sections, then you apportion your voltage drop between all of the components, and then calculate for each component separately.
-Jon
