Fire Pump Circuit Breaker Sizing

[Sajid khan]

According to NEC Reference 695.4 (B2) regarding the sizing of Fire pump circuit breaker we have to add the Lock rotor current of Fire pump and Jocky pump to get the size of breaker. My Question is, at the time of designing we do not know the code letter of Motor (code A to Code V Table 430.7(B) ) which correspond to the lock rotor current of the motor, so how could i calculate the lock rotor current of the motor and hence not the size of the main circuit breaker.
Is there any Rule of thumb or any other method of calculating the size of the main circuit breaker for the fire pump??
I have a 195kW fire pump motor whose code letter i donot know?? so what should be the size of main breaker??

Can any one Help Please...

Sajid Khan

 

[Gregg Harris]

According to NEC Reference 695.4 (B2) regarding the sizing of Fire pump circuit breaker we have to add the Lock rotor current of Fire pump and Jocky pump to get the size of breaker. My Question is, at the time of designing we do not know the code letter of Motor (code A to Code V Table 430.7(B) ) which correspond to the lock rotor current of the motor, so how could i calculate the lock rotor current of the motor and hence not the size of the main circuit breaker.
Is there any Rule of thumb or any other method of calculating the size of the main circuit breaker for the fire pump??
I have a 195kW fire pump motor whose code letter i donot know?? so what should be the size of main breaker??

Can any one Help Please...

Sajid Khan

If the locked rotor is not known, use table 430.251(B) for your calculations.

 

[Sajid khan]

Thank you very much Mr. Gregg Harris for great help...

There is another question, As per NEC 695.4(B2) by adding the Lock rotor current of Jocky pump and Fire pump we have a circuit breaker much higher in rating.
We know that the starting current of the motor will last for only 4 or 5 seconds, so If we select our circuit breaker based on the Full load current of Fire pump and not according to the lock rotor current of the motor and make adjustment in circuit breaker that will withstand between this period of time ( 4 or 5 second) then we can reduce the size of our circuit breaker.... is this correct?? is there any clause related to this in NEC????


Regards,
Sajid Khan

 

[iwire]

The breaker for a fire pump must handle the locked rotor current continuously. See 695.4(B)(1)

 

[cdslotz]

Since you are in the design stage, is there a way you can design it where the feed comes underground from the utility to the fire pump controller? No OCPD required.

 

[Sajid khan]

No, i Cannot run the cable inside the trench or underground or encased in concrete, i have to run the cable on cable tray...
If i Can run the cable inside the trench or underground then how and why we donot need an OCPD... Which Aritcle state than no OCPD needed..

Regards,
Sajid Khan

 

[Sajid khan]

The breaker for a fire pump must handle the locked rotor current continuously. See 695.4(B)(1)

Sorry i didnot find in the article you referred to carry the lock rotor current continuously... what i think is that the LCR lasts for 4 or 5 seconds only it will not be continuous, after 4 or 5 second it will start decaying and motor will be on his Normal Full load current... So my question was that if we make adjustment in our circuit breaker to carry the spike for 4 or 5 second without tripping instead of using a very big circuit breaker..??

 

[iwire]

Sorry i didnot find in the article you referred to carry the lock rotor current continuously... what i think is that the LCR lasts for 4 or 5 seconds only it will not be continuous, after 4 or 5 second it will start decaying and motor will be on his Normal Full load current... So my question was that if we make adjustment in our circuit breaker to carry the spike for 4 or 5 second without tripping instead of using a very big circuit breaker..??

You are missing the point, the breaker is required to supply locked rotor current indefinitely, not just the start up time or spike. I will post the section in a little while.

 

[iwire]

I do not know what code cycle you are on so I will give you the 2011 and 2002 NEC.


From the 2011 NEC

695.4 Continuity of Power. Circuits that supply electric
motor-driven fire pumps shall be supervised from inadvertent
disconnection as covered in 695.4(A) or (B).

(B) Connection Through Disconnecting Means and Overcurrent
Device.

(2) Overcurrent Device Selection. Overcurrent devices
shall comply with (a) or (b).

(a) Individual Sources. The overcurrent protective device(
s) shall be rated to carry indefinitely the sum of the
locked-rotor current of the fire pump motor(s) and the pressure
maintenance pump motor(s) and the full-load current
of the 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).

From the 2002 NEC

695.4 Continuity of Power. Circuits that supply electric
motor-driven fire pumps shall be supervised from inadvertent
disconnection as covered in 695.4(A) or (B).

(B) Supervised Connection. A single disconnecting
means and associated overcurrent protective device(s) shall
be permitted to be installed between a remote power source
and one of the following:

(1) Overcurrent Device Selection. The overcurrent protective
device(s) shall be selected or set to carry indefinitely
the sum of the locked-rotor current of the fire pump motor(
s) and the pressure maintenance pump motor(s) and the
full-load current of the associated fire pump accessory
equipment when connected to this power supply.

Even if the fire pump is mechanically jammed and will not turn the breaker is not supposed to open.

 

[Sajid khan]

I do not know what code cycle you are on so I will give you the 2011 and 2002 NEC.


From the 2011 NEC



From the 2002 NEC



Even if the fire pump is mechanically jammed and will not turn the breaker is not supposed to open.

Thank you very much now got your point...

 

[GoldDigger]

Even if the fire pump is mechanically jammed and will not turn the breaker is not supposed to open.

Does that then carry over to requiring that the conductors also be sized to the value of the overcurrent protection, i.e LRA?
Or will the conductor size still be based only on the FLA figure? (on the assumption that there will be more urgent things to worry about than overloading the conductors if it ever comes down to that?)

 

[Sajid khan]

Does that then carry over to requiring that the conductors also be sized to the value of the overcurrent protection, i.e LRA?
Or will the conductor size still be based only on the FLA figure? (on the assumption that there will be more urgent things to worry about than overloading the conductors if it ever comes down to that?)

As per NEC 695.6(B) the conductor will be sized according to the 125% of FLA of the motor. see the extract from the article

(B) Conductor Size.
(1) Fire Pump Motors and Other Equipment. Conductors
supplying a fire pump motor(s), pressure maintenance pumps,
and associated fire pump accessory equipment shall have a
rating not less than 125 percent of the sum of the fire pump
motor(s) and pressure maintenance motor(s) full-load current(
s), and 100 percent of the associated fire pump accessory
equipment