Custom Search. For the purpose of calculating voltage drop within a cable, the table below gives the reactance and resistance values for Copper and Aluminium cables: Values for Copper Cables. Cable Size, S (mm2) Single Core Cable. Two Core/Three Core Cables. R (Ω/km) 80°C. X (Ω/km) 80°C. R (Ω/km) 80°C.