How do I size a disconnect (circuit breaker or fused) when installed upstream of a fire pump controller?


Question:
 How do I size a disconnect (circuit breaker or fused) when installed upstream of a fire pump controller?

Method:

  1.     The disconnecting means should be sized with a rating higher than the locked-rotor current of the fire pump motor.
  2.     In our example, the fire pump only is being fed from the 480V power source.  The locked-rotor current for our 100HP motor is 725 amps.  A disconnect must be selected that is rated for a current higher than this.  The nearest common size disconnect of 800A would be suitable.

References:

a)     NFPA 20 9.3.2.2.3.1 states that a disconnecting means and associated overcurrent protective device(s) is allowed if desired.  It is not required, however.

b)     NFPA 20 9.3.2.2.3.2(A) states that this disconnecting means and its overcurrent protective device must be able to carry indefinitely the sum of the locked-rotor current of the fire pump motor and the

pressure maintenance pump along with the full-load current of all other equipment fed from that disconnect.

c)     Locked-rotor current is determined from NFPA 70 Tables 430.251(A) & (B).

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