The current needed for the RS-485 tranceiver?
As Umar says, most of those transceivers have built-in short circuit protection. So that'll save the transceiver from damage, but it won't necessarily prevent it from drawing more current than you want.

A PTC fuse is a great solution to this, as long as you are OK with its behavior - once it shuts off due to overcurrent protection, it will stay off until you power down the device. To use one, putting it in-line with the transceiver's VDD should work just fine. 

I would size it by looking at the recommended maximum single-leg output current and then double it, and add on the device's quiescent current. Then find the PTC size that's the same or bigger.