Finding cable impedance, to determine RS485 line termination
Impedance on cables is defined by the ratio of distributed reactance of L/C as follows.

\$Z_o=\sqrt {\frac{L}{C}}\$  for L= 0.16 uH/ft for each line and C= 44 pF/ft, line to line thus;
\$Z_0= \sqrt{\frac{0.16uH}{44pF}}=60 Ω\$ for each line and thus differential Zo = 120  Ω