How are molded-case circuit breaker trip ratings determined?
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Table 7.1.2.2.1 of UL 489 lists the maximum automatic tripping time at 200% of the rated current.

For example, breakers rated 101-150 Amps must trip within 8 minutes (480 seconds) at 200% of their rating. Since at 480 seconds this breaker has interrupted about 300 Amps, this breaker is probably rated 150 or 125 Amps.