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SIZING BREAKER FOR FIRE PUMP FED FROM GENERATOR

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    SIZING BREAKER FOR FIRE PUMP FED FROM GENERATOR

    Hey Everyone,

    I have a power systems study that I'm working on where there is a 200 HP fire pump being fed from a generator on the emergency side and fed from the Utility transformer on the normal side. The emergency side has a 800 amp LSI adjustable trip breaker at the generator and I'm trying to figure out where to put my settings on the breaker. I was reading that when fed by the generator, the upstream breaker on the emergency side can be sized according to table 430.52 of the NEC. I see that an instantaneous trip breaker is to be set at 800% of the FLA and an inverse time breaker is 250%. Is there any information on what to do for LSI breakers? Should I assume that longtime should be 250% and the instantaneous portion be 800%? If anyone has experience with this please let me know. Also attached is the TCC.

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    Best Regards


    #2
    I think I understand now what to do for this.. I found this snippet from Generac:

    NEC 430.62 requires that the protective device be sized per 430.52 which limits the maximum inverse time breaker size to 250% of the motors nameplate rating. This sets the maximum size of the breaker. What about the minimum size of the breaker? What does “provide short circuit protection only” mean? NEC provides for short circuit, ground fault, and overload protection. Thus, short circuit only protection means no protection against ground fault or overload. NEC 430.32 defines overload protection at a maximum of 125% of the motor nameplate rating. Breakers sized larger than 125% provide no overload protection and thus provide short circuit only protection. The result of all this is that any breakers from the on-site generator source to the fire pump must be sized greater than 125% and less than 250% of the fire pump rating and support the temporary locked rotor amps associated with normal starting.

    Comment


      #3
      Originally posted by engineerintraining12 View Post
      I think I understand now what to do for this.. I found this snippet from Generac:

      NEC 430.62 requires that the protective device be sized per 430.52 which limits the maximum inverse time breaker size to 250% of the motors nameplate rating. This sets the maximum size of the breaker. What about the minimum size of the breaker? What does “provide short circuit protection only” mean? NEC provides for short circuit, ground fault, and overload protection. Thus, short circuit only protection means no protection against ground fault or overload. NEC 430.32 defines overload protection at a maximum of 125% of the motor nameplate rating. Breakers sized larger than 125% provide no overload protection and thus provide short circuit only protection. The result of all this is that any breakers from the on-site generator source to the fire pump must be sized greater than 125% and less than 250% of the fire pump rating and support the temporary locked rotor amps associated with normal starting.
      The provisions of the code are just the minimum requirements for safety and overthinking/ worry on the details of the code provision gets you nowhere, IMO. You could have been missing the forest for the trees there!

      You are allowed to protect the fire pump motor for short-circuit only! You should design your fire pump to run until destruction, because when it is needed, a failure to run a fire pump could mean lives and properties lost. Forget about over current protection when dealing with fire pump controllers.

      Comment


        #4
        The fire pump is sacrificial to the fire. The controller has no overload protection just short circuit protection.
        I suggest a review of Article 695. Part 694.4 2 a 1 states

        Overcurrent protective device(s) shall be rated to carry
        indefinitely the sum of the locked-rotor current of the
        largest fire pump motor and the pressure maintenance
        pump motor(s) and the full-load current of all of the
        other pump motors and associated fire pump accessory
        equipment when connected to this power supply. Where
        the locked-rotor current value does not correspond to a
        standard overcurrent device size, the next standard overcurrent
        device size shall be used in accordance with
        240.6. The requirement to carry the locked-rotor currents
        indefinitely shall not apply to conductors or devices other
        than overcurrent devices in the fire pump motor
        circuit(s). The requirement to carry the locked rotor
        currents indefinitely shall not apply to feeder overcurrent
        protective devices installed in accordance with 695.3(C).
        [20:9.2.3.4]

        If you are not familar with a fire pump controller on the front there is a knob/lever that overrides the short circuit breaker.
        Also Art 695 should be used in conjunction with NFPA 20, but you need to ask the AHJ which edition of NFPA 20 they are enforcing
        Moderator-Washington State
        Ancora Imparo

        Comment


          #5
          More than that is the provisions for over sizing the wiring and such so it can pretty much stall without damage. So no overload but it’s not needed either.

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