How to Choose the Size of Varistor

The volume of varistor is mainly reflected in the amount of energy that can be absorbed. The current product is generally zinc oxide varistor, it is a volume type of varistor, the larger the volume, the more energy shock can be withstood (allow through the varistor pulse (peak) current value). Varistor actions are repeatable and can be recovered immediately after impact within a range not exceeding its tolerance. If there is an energy shock greater than it can withstand will completely break through the damage and can no longer recover on its own, then it is broken.

So the size of the choice is based on what happens when there's different energy in the circuit you're protecting. If one or two thunder strikes a month will introduce the circuit, then the best choice is 20-40D. If there is high voltage nearby, choose 14-20D. The average 5-14D can handle it. Choose on your own needs.