BACnet MSTP (RS485), Cable selection does make a difference.

All cables offer impedance (resistance). Some cables are designed so that the impedance is relatively independent of distance. You want one of these cables. A clue to knowing if you selected one is to look at the cable’s Nominal Impedance. If they quote a number such a 100Ohms you have a good cable. If they quote an impedance per meter/foot you have chosen the wrong kind. Wrong in the sense – to determine the value of terminating resistors now requires measurements and calculations. Choose low capacitance cables.

Can you use Cat5 cable ? Yes. Use one pair for Tx,Rx and a conductor from another pair for the ground reference signal.

We recommend these two cables.

Belden 3106A

Belden 3106A - RS485 Cable 

Multi-Conductor – EIA Industrial RS-485 PLTC/CM
22 AWG stranded (7×30) tinned copper conductors, Datalene® insulation, twisted pairs, overall Beldfoil® shield (100% coverage) plus a tinned copper braid (90% coverage), drain wire, UV resistant PVC jacket. 
 

Belden 3107A

Belden 3107A - RS485 Cable 

Multi-Conductor – EIA Industrial RS-485 PLTC/CM
22 AWG stranded (7×30) tinned copper conductors, Datalene® insulation, twisted pairs, overall Beldfoil® shield (100% coverage) plus a tinned copper braid (90% coverage), drain wire, UV resistant PVC jacket.