In selecting PTC thermistor Resettable Fuse for overcurrent and overload protection, following 5 factors should be taken into consideration.

1) Maximum operating voltage

2) Rated current ( Non-trip current) and Switching current (Trip current)

3) Maximum current permissible in maximum operating voltage

4) Selection of PTC thermistor Reference Temperature (also called Switch temperature or Curie temperature)

Reference Temperature optional for 80 ℃, 100 ℃, 120 ℃, 140 ℃.

5) PTC thermistor Application environmental effects

More detailed information can be found in AMWEI PTC Thermistor Current Protect Resettable Fuse 

Please find:

An example selecting AMWEI PTC thermistor for transformer overload protection

Transformer Protection 265V, 100C Data
(For PTC fuse installed inside transformer windings coil)

Transformer Protection 265V 120C Data
(For PTC fuse installed outside transformer windings coil)