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Joined: Jul 2012
Posts: 9
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Hello,

I wanted to inquire about effects on generator sizing, if fire pump motor, soft-starting methods, and required manual electric/mechanical starting of the fire pump (as per NFPA 20) are taken into consideration.

A generator is required to be sized to back-up fire pump motor and controller loads in addition to regular building lighting, power, and motor loads. The fire pump motor is the largest load on the generator. The fire pump controller may have either a manual electric or manual mechanical starting (as required by NFPA 20), in addition to automatic starting. The fire pump controller also is configured for soft-starting the fire pump motor.

I've read that for fire pump application, the generator is permitted for a voltage drop upto 15%, but if the fire pump is operated manually by mechanical means, then the 15% voltage limit is not applicable. Even if the fire pump controller has a soft-starting method, if the fire pump is started mechanically, then it would be started across-the-line.

Considering the above scenario, I have the following queries:

1. What would be an acceptable voltage drop for the generator, when the fire pump is started via mechanical starting?

2. In a case, there the fire pump controller has a soft starting method, manual electric starting, and manual mechanical starting, would the generator be sized for fire pump motor locked rotor amperage calculated for soft-starting or the locked rotor amperage calculated for across-the-line starting. Would the generator size also be affected based on the increased threshold of voltage drop for manual mechanical starting?

Thank you.

Joined: May 2005
Posts: 984
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G
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15% based on the highest possible amperage draw.
You always design for worst case.


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