lenght of conductor

The resistance of any part of conductor:

R = ρ₀.L /S (@20 deg.C as Ω)

Increase at resistance due to temp. will be Δρ = α.ρ₀.ΔT and final resistance: ρ_T = ρ₀ + Δρ ....i

Here;

ρ₀ : Specific resistance of conductor @20 degrees centigrade (Ω.mm²/m)

ρT : Specific resistance of conductor @ final temperature.

α : Chance of sp. resistance per degrees centigrade.

L : Length of the conductor as m.

S : Cross-section of conductor as mm²

ΔT : Temperature difference based 20⁰C

Now voltage drop at the line will be:

ε = 200.L.ρ_T.P / S.U² .......ii here;

ε : Max. allowable voltage drop on the line (by regulation in your country) as percent

L : Lenth of line (return excluded)

U : Supply voltage of your installation.

You can find the desired cross-sect. of conductor from eq. ii.

The shape of conductor has no practical effect unless working at high frequencies.

The ρ₀ = 0,028 Ω.mm² / m for al @20 ⁰C. It is better you obtain this value from mfgr of Al conductor that you will use.

The α = 0,0039 ... 0,0043 ⁰C^-1 for aluminium. You could dig in mfgr for this value too.