Practice

The calculated available three-phase symmetrical fault current at a 480 V switchgear bus is 21 kA. The owner's specification requires the breakers to carry a 20% interrupting margin for future utility growth. Standard interrupting ratings are 14, 18, 22, 25, 35, 42, 50, 65, 85, and 100 kA. The minimum standard interrupting rating that satisfies the specification is:

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