Hey, I've been trying to find the proper varistor (or MOV) for an AC circuit. The voltage ratings in datasheets, such as this one, are not making sense to me. How do I know the clamping voltage, and the max voltage ratings and such?

 

There are a few things to consider, the steady state operating voltage and the transient condition, as well as the energy rating (joules).
The selection process is not easy it is mainly a case of assuming the transient source and energy of such.
First select the voltage rating that is high enough that the MOV is not going to conduct during natural high supply voltage levels that may occur due to off peak periods etc. At least 110% above normal working voltage.
Then the energy rating is harder to predict, because of unknown factors, I use one of the higher rated types in the voltage range.
In the past suppliers such as GE have published selection manuals, fortunately I still have one, they may be still available out there in PDF if lucky.
Max.

 

I generally use Panasonic ZNR varistors when I need one. I attached the data sheet for that line. You will notice that the "varistor voltage" is the voltage where the part draws 100 microamps or 1 milliamp (depending on the size). You can then see two rating that show the max. clamping voltage and max. peak current. These are survivable conditions. Go beyond these values and the varistor will be damaged.

Wikipedia has a good write up. https://en.wikipedia.org/wiki/Varistor