Standby Generator Sizing for a Fire Pump (long)
Standby Generator Sizing for a Fire Pump (long)
(OP)
My client wants a back-up generator for his 125HP, 480V, 3phase fire pump motor. The fire pump controller is an 'across-the-line' starter type.
I have gone through some sizing software provided by generator manufacturers and I came up with a 200kW unit. The factory guy tells me I need a 500kW unit because of
NFPA 20 6-2
"Power Source(s). Power shall be supplied to the electric motor-driven fire pump by a reliable source or two or more approved independent sources, all of which shall make compliance with Section 6-4 possible."
NFPA 20 6-4
"Voltage Drop. The voltage at the controller line terminals shall not drop more than 15 percent below normal (controller-rated voltage) under motor-starting conditions. The voltage at the motor terminals shall not drop more than 5 percent below the voltage rating of the motor when the motor is operating at 115 percent of the full-load current rating of the motor."
Then I look at NFPA 20 6-6 'On-Site Power Generator Systems'
6-6.1
Where on-site generator systems are used to supply power to fire pump motors to meet the requirements of 6-2.3, they shall be of sufficient capacity to allow normal starting and running of the motor(s) driving the fire pump(s) while supplying all other simultaneously operated load(s). A tap ahead of the on-site generator disconnecting means shall not be required.
6-6.2*
These power sources shall comply with Section 6-4 and shall meet the requirements of Level 1, Type 10, Class X systems of NFPA 110, Standard for Emergency and Standby Power Systems. The fuel supply capacity shall be sufficient to provide 8 hours of fire pump operation at 100 percent of the rated pump capacity in addition to the supply required for other demands."
NFPA 110
2-2.2 Type.
Type defines the maximum time, in seconds, that the EPSS will permit the load terminals of the transfer switch to be without acceptable electrical power.
Type 10: 10 seconds
I'm confused about the 15% vs. the ten seconds. Am I going to have to get this huge generator?
I have gone through some sizing software provided by generator manufacturers and I came up with a 200kW unit. The factory guy tells me I need a 500kW unit because of
NFPA 20 6-2
"Power Source(s). Power shall be supplied to the electric motor-driven fire pump by a reliable source or two or more approved independent sources, all of which shall make compliance with Section 6-4 possible."
NFPA 20 6-4
"Voltage Drop. The voltage at the controller line terminals shall not drop more than 15 percent below normal (controller-rated voltage) under motor-starting conditions. The voltage at the motor terminals shall not drop more than 5 percent below the voltage rating of the motor when the motor is operating at 115 percent of the full-load current rating of the motor."
Then I look at NFPA 20 6-6 'On-Site Power Generator Systems'
6-6.1
Where on-site generator systems are used to supply power to fire pump motors to meet the requirements of 6-2.3, they shall be of sufficient capacity to allow normal starting and running of the motor(s) driving the fire pump(s) while supplying all other simultaneously operated load(s). A tap ahead of the on-site generator disconnecting means shall not be required.
6-6.2*
These power sources shall comply with Section 6-4 and shall meet the requirements of Level 1, Type 10, Class X systems of NFPA 110, Standard for Emergency and Standby Power Systems. The fuel supply capacity shall be sufficient to provide 8 hours of fire pump operation at 100 percent of the rated pump capacity in addition to the supply required for other demands."
NFPA 110
2-2.2 Type.
Type defines the maximum time, in seconds, that the EPSS will permit the load terminals of the transfer switch to be without acceptable electrical power.
Type 10: 10 seconds
I'm confused about the 15% vs. the ten seconds. Am I going to have to get this huge generator?





RE: Standby Generator Sizing for a Fire Pump (long)
The 15% is for the minimum voltage drop while starting, while the 10s appears to be for the amount of time that it takes the ATS to:
1. start the generator upon loss of normal power
2. allow the generator to come up to normal speed/voltage
3. transfer to generator power
The 15% and the 10s really aren't related to each other, but are simply different requirements. The transfer time is not related to the motor start, since the only time the generator is used for the fire pump motor is when normal power is lost and the ATS has already transfered to generator power.
RE: Standby Generator Sizing for a Fire Pump (long)
By the way, the FLA for the motor is 156 and the LRA is 908...appears that you were looking under the 230V column, DanDel. That's ok, your analysis of the situation was sound and I appreciate your input.
Now to break the bad news to the client...
RE: Standby Generator Sizing for a Fire Pump (long)
This eliminates the generator and probably reduces the size of the engine (a little). I'd talk with the fire pump manufacturer about it.
This does require a bit of extra space at the pump.
RE: Standby Generator Sizing for a Fire Pump (long)
RE: Standby Generator Sizing for a Fire Pump (long)
dpc has a great idea....
If you have the space, the best soultion may be to install a diesel driven pump in addition to the electric pump. The cost of the diesel pump may be less than the new generator..... and the electric + diesel combination is found freqently in industry. Both types are described in detail in NFPA-25
There are many certified "used" diesel pumps available for sale by brokers on the internet....These are typically perfectly acceptable.
PEERLESS pump has a lot of good info on fire pumps
Let us know what you decide.......
MJC
RE: Standby Generator Sizing for a Fire Pump (long)
Is there a way to be able to use this 500kW generator to provide power to more than the fire pump? My client wants to power the lights and exhaust fans also. He thinks he can use two transfer switches and use some fancy load sharing scheme to avoid upsizing the generator for the extra loads.
If you will recall, I had to go to the 500kW generator due to the 15% voltage drop requirement for fire pumps.
RE: Standby Generator Sizing for a Fire Pump (long)
RE: Standby Generator Sizing for a Fire Pump (long)
I think the generator could be used for other purposes provided the other loads are shunt-tripped if the fire pump needs to run and you have a connection to utility power for the fire pump.
RE: Standby Generator Sizing for a Fire Pump (long)
I agree erosa, a reduced voltage starter is what my client needs but the fire pump controller is brand new...once I learned what they had ordered I tried to get them to change it but after the cost increase and the delay that would be involved, they decided to keep the full voltage one coming. Now they are paying the price...in peak KVA charges from the utility and this huge generator...that extra $8,000 isn't so much now...but I digress.
I'm stuck with the full voltage FPC.
RE: Standby Generator Sizing for a Fire Pump (long)
You might want to consider an oversized alternator on your generator. Starting currents are reactive and have little impact on the kW rating of the engine; it's the upsized kVA rating of the alternator and the associated increased starting currents that you're after. An upsized alternater might just enable you to start across the line might be helpful if reduced voltage starting seems borderline.
Reduced voltage starters make sense too as previously mentioned.
Don't overlook the current limiting effect of your supply conductors. If your generator is any distance from the motor, and it sounds like it is, take a good look at the feeder sizing.
In general, any time the rating of a motor approaches the rated capacity of its source (be it a transformer, generator, whatever), you need to take a very close look at voltage drop issues.
RE: Standby Generator Sizing for a Fire Pump (long)
RE: Standby Generator Sizing for a Fire Pump (long)
As others and you mentioned, reduced starting may be one way to reduce the starting current, however, as you may be aware, the fire pump, in the event of emergency, can also mechanically be operated. In that case, regardless of the method of starting, the fire pump will be starting at full voltage (across the line). Furthermore, reduced voltage started pumps may impact the required water pressure in the line since the reduce voltage means less torque.
RE: Standby Generator Sizing for a Fire Pump (long)
Please clarify "can also mechanically be operated". Do you mean from a diesel engine? What's that got to do with electrical starting?
Also, regarding "reduced voltage started pumps may impact the required water pressure in the line since the reduce voltage means less torque", well, there's no motor out there that will start instantaneously, providing 100% pressure at time=0sec. What's your point?