Circuit Breaker Sizing

I need help understanding the sizing criteria for circuit breakers. What is the determining factor in selcting the Frame size of a Circuit Breaker in and electrical Panel. It seems the trip ratings are overlapping so what determines the frame size?

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There should be table in the N.E.C. book. I have a table in front of me. The things to consider are Range of Trip Rating (what your breaker will trip at), Interrupting Ratings (very important, that maximum current your breaker can interrupt, it should be stamped right on your breaker), and Short-tme Ratings - 0.5 Second (30 cycle) delay. Sorry I'm not to familiar with the last one.

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When sizing a circuit breaker the first thing that needs to be considered is the maxium current availiable this is the interrupting current of the circuit breaker. The settings are determined by the coordination study. This insures that the proper breaker trips during a fault condition. Most options on the it breaker are determined by the features required by the application. In some applications the NEC requires ground fault detection.

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