WHY TWO DIFERENT RATING FOR FIRE PUMP?

[vaaoc1425]

I have just read someone’s design drawing for a 30 Horse Power Fire Pump. I am not sure why his design calls for 250 amp enclosed circuit breaker on the Normal side of the Fire Pump controller ATS on the Normal side and a 50 amp enclosed circuit breaker to the Emergency side of the Fire Pump controller ATS.

I am not sure why this two different rating for Normal side and Emergency side. Can someone explain this concept?

 

[davidbeach]

Check out the Fire Pump article in the NEC and notice the difference in requirements for the utility source and the alternate source. One has to handle locked rotor current indefinitely and the other doesn't.

 

[wareagle]

The breaker must handle the LRA of the motor
indefinitely. The conductor is rated at 125%
of the FLA.

 

[jraef]

As davidbeach said, the reason behind the two different ratings has to do with what they expect out of the two power systems when feeding the pump that is fighting the fire. When connected to the utility source, they don't want line faults that may occur as other equipment catches on fire to cause the breaker to trip the fire pump off-line just when you need it the most. It's all within the concept of "run to destruction" behind most fire pump rules. The breaker in the backup genset source can be smaller (essentially cheaper) because they recognize that the backup source is limited anyway and not as prone to system wide damage as the fire progresses.

JRaef.com
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[vaaoc1425]

There is no where in the code [NEC] that we can size it differently/lower for the Normal side though I agree all the reasoning for doing so.

 

[vaaoc1425]

I meant size lower for the EMERGENCY side but have LRC rating for the Normal side