Fire Pump Automatic Transfer Switch size...

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RobbieR

Member
Location
Atlanta
I have an existing facility with a 480V, 60HP fire pump. This fire pump is fed upstream from the Utility, goes through a 600A rated disconnect and through a 100A Fire Pump ATS. The emergency side of this Fire Pump ATS is fed off of a generator with a 600A breaker.

The Locked Rotor Current of a 60HP motor is approximately 435A per NEC Table 430.251(B).

According to NFPA 20:

10.8.3.4 Horsepower or Ampere Rating.

10.8.3.4.2
Where rated in amperes, the power transfer switch shall have an ampere rating not less than 115 percent of the motor full-load current and also be suitable for switching the motor locked rotor current.

And NEC Article 695:

NEC 2005: Article 695 ? Fire Pumps
695.3 Power Source(s) for Electric Motor-Driven Fire
Pumps. Electric motor-driven fire pumps shall have a reliable
source of power.

(A) Individual Sources. Where reliable, and where capable
of carrying 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,
the power source for an electric motor-driven fire pump shall
be one or more of the following.

It appears that this ATS is undersized. How can an ATS supply power to the Fire Pump under locked rotor conditions, in this case 435A, when the rating of the ATS is only 100A? Would this ATS not fail as a result?

A local rep says that the locked rotor conditions are built into the fire pump controller. But that still doesn't explain how this ATS will continue to operate.

What am I missing here?
 

erickench

Senior Member
Location
Brooklyn, NY
Locked rotor current is about the same as starting current for a motor. The ATS would have to be able to withstand the high current for a limited amount of time. If the LRC exceeds this time limit then the ATS could be damaged.
 

wirenut1980

Senior Member
Location
Plainfield, IN
Interesting...it certainly seems as though the transfer switch is undersized, since it too would have to carry the locked rotor current "indefinitely." Supposedly the pump motor would fail eventually (since overcurrent protection seems like it is supposed to be set up to allow locked rotor current to flow indefinitely as well), but would that be before the transfer switch fails?
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Since it is a listed fire pump ATS, I would assume it is made to carry locked rotor current long enough that the pump burns up first.
 

steve066

Senior Member
No, I think it would be more like long enough for the upstream OPD to open the circuit.

On fire pumps, the OCPD is sized to handle the locked rotor current of the fire pump. So the OCPD generally won't open for any faults other than a dead short circuit. The OCPD will hold forever for a overloaded or locked pump motor.
 

RobbieR

Member
Location
Atlanta
I still think the ATS is undersized.

To me, it just doesn't make sense. Under emergency conditions, the fire pump needs to run until it melts. Having an ATS that doesn't meet the locked rotor current of the motor prevents this.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
I still think the ATS is undersized.

To me, it just doesn't make sense. Under emergency conditions, the fire pump needs to run until it melts. Having an ATS that doesn't meet the locked rotor current of the motor prevents this.

The same argument could be applied to everything supplying the fire pump. But per the NEC, the wiring to the fire pump isn't sized for the locked rotor current, its sized to 125% of the full load motor current. So that argument fails in at least one instance.

Normally, the fire pump ATS is fed directly from the source, without additional OCP. And this is from a spec. for one of Eaton's fire pump ATS's:

The transfer switch shall be provided with locked rotor overcurrent protection. The locked rotor protector shall be calibrated and set to a minimum of 300% of the motor full-load current and have a tripping time between 8
and 20 seconds.

To me, that also indicates that the fire pump ATS is designed without the need to "upsize" it for locked rotor current.

But if the OP is still in doubt, I would suggest contacting the manufacturer of the ATS. I'm sure they can tell you if the application meets the code requirements.

Steve

 
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