For NTC selection, the first criteria is energy. For example, the oscilloscope is reading peak inrush current of 564A with an input voltage of 120VAC, 60Hz. To determine the energy required for the NTC use the formula E = 1/2LI2. Then figure the inductive reactance of the transformer, Xl, which is equal to the input voltage divided by inrush current, 120V divided by 564A is equal to 0.213Ω.  Xl also equals 2πfL (2x3.14x60xL), hence inductance is equal to 565µH. Thus using the completed formula we would come up with 90 joules.