After using them for many years, I use the smallest hold rating that will work for running. On my mains where I run passenger trains (they use the most amps) I use PTCs with hold rating 8 or 9 amps and never trip from an overload. Freight trains seldom use more than 5 amps so a hold rating of 5 amps is fine.
The beauty of the PTCs is they're cheap enough so you can test for the lowest hold rating for your conditions. The closer you are to the correct hold rating the quicker a fault will trip the circuit. Caution! Do not touch the PTCs to see if they're working. They run hot under a full load.
