ATS48 coordination chart has option to choose breaker with breaking capacity (Icu) of 36KA to 150KA. How to decide the Icu (KA) rating of the breaker to be selected?

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The Breaker should be selected based on the calculated short circuit current rating of supply feeder to the ATS48 soft starter. You can calculate the short circuit current rating by considering the transformer impendence + bus bar trunking impendence and supply voltage. In some cases, the tender specification is clearly provided with the panel short circuit current rating. So you can select the breaker breaking capacity (KA rating) should be equal to or higher than the calculated value.

For example, if the calculated short circuit current rating is 50KA for the supply feeder, you can use NSX630N (630A, 50KA) with ATS48C41Q soft starter without any problem.