I have a question regarding overcurrent protection for fire pumps. I was teaching a code class and pulled a question out of a book regarding fire pump overcurrent protection for a 25 HP 208 three phase code letter "C" fire pump.
I know that 695.4(b)(1) says that the overcurrent device shall be selected to indefinetely carry the locked rotor current of the motor. However it does not tell me where to get the locked rotor current. I know that I can go to table 430.251(b) and get a locked rotor current of 404 amps. I would then be allowed to go to the next standard size of 450 amp overcurrent device. But above table 430.251(b) it tells me what code sections to use this table with and article 695 isn't one of them. The second choice is to go to table 430.7(b) the high end for code letter "C" is 3.99 kva per HP. 3.99 x 1,000 x 25 / 208 x 1.732 = 277 amps and I would go with a 300 amp overcurrent device. I am wondering why these two tables do not correspond with each other. I have seen similar questions and I see table 430.251(b) being used. Which one is to be used if any. I talked about this with my students and told them the best thing to do is get the information from the manufacturer. But if you had this question on a test how is the correct way to proceed? The answer in the book I was using was 450 amp overcurrent device based on table 430.251(b).