Required back-up generator kVA rating for 300HP Fire Pump

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Rommel

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Hi Guys,

Good day! Please help me to size my back-up generator as alternative supply for a 300 HP fire pump. What would be the kVA rating of my generator?
 
Rommel said:
Hi Guys,

Good day! Please help me to size my back-up generator as alternative supply for a 300 HP fire pump. What would be the kVA rating of my generator?
Click to expand...

Loaded question because generator sizing depends on many factors including motor starting code for the fire pump and voltage dip, frequency shift and starting kva capabilities of the genset. these factors in turn depend on engine and barrel size, fuel type, generator size. all of those depend on the manufacturer. i would suggest contacting one (or more) of the major manufacturers (cummins, cat, kohler, etc) and see what they say when you tell them you are using a standby generator for fire pump service. they will likely want a data sheet of the motor you intend to use.

since we are working with a fire pipe, you must also design the power feed such that, when the motor starts, the voltage at the line side of the motor controller does not fall below 15% of the controller's rated voltage. we must take into consideration that the generator's voltage will drop in response to the motor starting and the resulting current, at the starting power factor, will have a measurable voltage drop between the genset and the controller.
 
If this is really a fire pump for building sprinklers needs to be sized per locked rotor current of the pumps . NEC Article 695. This generator will be large


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Sean.Day72 said:
If this is really a fire pump for building sprinklers needs to be sized per locked rotor current of the pumps . NEC Article 695. This generator will be large


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Click to expand...

OP did not state if the generator was dedicated to the fire pump and was the only power source. if true, the generator must be able to carry locked rotor amps indefinitely according to 695.3(A), as you suggest. if there are multiple power sources, and the utility can supply LRA indefinitely the generator may be sized for normal starting and running of the fire pump while also supplying all other simultaneously operated loads.

upon further reflection, it might make more sense to purchase a diesel powered fire pump on a skid, if the intent is for the generator to be dedicated to the fire pump. either way they will be a maintenance item since you would need to load test the gen or diesel pump regularly. i am unfamiliar with testing and maintenance requirements for standby power sources for fire safety systems.
 
I would recommend a diesel operated pump as well. It is easier to maintain and less work as part of a preventative maintenance schedule.
 
FWIW, locked rotor current is usually estimated at around six to eight times FLC.
 
(edit to add)
Disclaimer: I have quite a bit of experience with generation. Not so much with starting big motors on small generators.

Rommel -
This is not a design, but rather a planning tool:

Assumptions (none of which may fit your application):
  • Generator impedance ~.2
  • Motor impedance at locked rotor ~.2
  • Limit Voltage sag to .85 (15% Vd)
  • Motor is Design B, Code G (LRA = 6xFLA)
  • Gen is really close to motor (Feeder impedance ~zero)


Vstart = V x (Xlr/(Xd' + Xlr))

Shift the impedances to the same base

For a 2000KVA gen, voltage drops to about 84%.

As noted by other posters:
Reduced voltage starting is a really good idea
Diesel drive is a really good idea

Here is a link to a paper on motor starting voltage drop
http://voltage-disturbance.com/power-engineering/voltage-sag-due-to-induction-motor-starting/
 
Last edited:
drktmplr12 said:
....when the motor starts, the voltage at the line side of the motor controller does not fall below 15% of the controller's rated voltage. we must take into consideration that the generator's voltage will drop in response to the motor starting and the resulting current, at the starting power factor, will have a measurable voltage drop between the genset and the controller.
Click to expand...

I don't have a code book to check this, but I believe the voltage drop limitation is at the motor terminals and not the controller. Please correct me if I'm wrong.
 
mayanees said:
I don't have a code book to check this, but I believe the voltage drop limitation is at the motor terminals and not the controller. Please correct me if I'm wrong.
Click to expand...

695.7 has two requirements, one each for starting and running

Motor starting conditions limit VD to 15% of controller rated voltage at line side of motor controller

Motor running limits VD to 5% of the motor voltage rating at the load terminals of the controller when the motor is at 115% of FLA

edit: OP is curiously absent from discussion.. may have been looking for someone to just tell them xxx kW ;)
 
MrJLH said:
I would recommend a diesel operated pump as well. It is easier to maintain and less work as part of a preventative maintenance schedule.
Click to expand...
I would have thought a Diesel engine would have required more preventive maintenance than a cage induction motor. You don't have to routinely change the oil on an electric motor.
 
Bes -
Time for you to read the posts again.
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Okay, now that you have done that:
Compare the maintenance on the generator the OP is asking about to a 300Hp diesel recip to drive the pump.
 
Besoeker3 said:
I would have thought a Diesel engine would have required more preventive maintenance than a cage induction motor. You don't have to routinely change the oil on an electric motor.
Click to expand...
All I know is that, barring any actual fire, these things seldom run except for the required verification tests (depending on the code requirements, diesel-driven fire pumps is required to run weekly versus electric-driven fire pumps are required to be tested monthly). With equipment wear dependent on running time duration, maybe the electric-driven scheme is desirable. But it is also losing to the engine-driven fire pump on the number of failure units--> 3 for the electric-driven (engine + generator + fire pump) against 2 for the engine-driven fire pump (engine + fire pump).

But there are whitepapers that say diesel-driven fire pumps are more viable and reliable.
 
Sean.Day72 said:
If this is really a fire pump for building sprinklers needs to be sized per locked rotor current of the pumps . NEC Article 695. This generator will be large


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Click to expand...

It will not be designed for the locked rotor current. Any disconnects will be sized and fused (if fused) for the locked rotor current. The generator has to maintain the voltage at 85% of running at the controller line side during start up. The in-rush won't necessarily be 6X FLA, especially if the fire pump motor is soft-start (which we don't know). As others have suggested, don't guess, get the fire pump manufacturer and generator manufacturer talking to each other.
 
Rommel said:
Hi Guys,

Good day! Please help me to size my back-up generator as alternative supply for a 300 HP fire pump. What would be the kVA rating of my generator?
Click to expand...

Keep in mind all fire pump controllers come with an emergency run handle or lever that operates to mechanically close the motor-circuit switching mechanism which means that if you have a soft start or wye delta or VFD start then that emergency handle will bypass that starting method and will start across the line. So your generator should be able to handle that inrush. For a 300HP motor your generator will be huge and also when you are sizing that for direct on line you dont need to comply with 15% voltage drop.
 
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