Locked Rotor Current of Fire Pumps

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A question was asked about a breaker tripping for a fire pump. Its a 1600amp breaker and its my understanding unless there is a short circuit, it should not be tripping, it is supposed to handle the locked rotor current indefinitely plus 100% of the other pump related accessories 2014 NEC 695.4(2). The fire pump is a 200hp 460volt Code G motor. I know usually the lrc is around 6 times the flc of the motor in most cases. Its been a while since I had to do these calculations, but isn't something wrong with the breaker tripping like it is when there is no direct short found? The overload amps should have never been reached to tripped the breaker in my opinion.
 

GoldDigger

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A question was asked about a breaker tripping for a fire pump. Its a 1600amp breaker and its my understanding unless there is a short circuit, it should not be tripping, it is supposed to handle the locked rotor current indefinitely plus 100% of the other pump related accessories 2014 NEC 695.4(2). The fire pump is a 200hp 460volt Code G motor. I know usually the lrc is around 6 times the flc of the motor in most cases. Its been a while since I had to do these calculations, but isn't something wrong with the breaker tripping like it is when there is no direct short found? The overload amps should have never been reached to tripped the breaker in my opinion.
You are correct. If the 1600A breaker is properly sized, then it should not trip unless there is a fault in the pump motor or the wiring.
Can you actually measure the amps that the pump motor is drawing?
If it is a three phase motor, it could be that there is a problem with one phase winding that is causing the current in that phase to be even higher than the locked rotor current.
Or if the instantaneous trip is misconfigured the starting surge (lasting only a cycle or two and potentially higher than the locked rotor current) might be causing the trip.
The breaker should not have any kind of residual current or ground fault trip element.
 

Besoeker

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UK
A question was asked about a breaker tripping for a fire pump. Its a 1600amp breaker and its my understanding unless there is a short circuit, it should not be tripping, it is supposed to handle the locked rotor current indefinitely plus 100% of the other pump related accessories 2014 NEC 695.4(2). The fire pump is a 200hp 460volt Code G motor. I know usually the lrc is around 6 times the flc of the motor in most cases. Its been a while since I had to do these calculations, but isn't something wrong with the breaker tripping like it is when there is no direct short found? The overload amps should have never been reached to tripped the breaker in my opinion.

I don't know what Code G means but my calcs give about 1420A for the locked rotor current for a typical cage induction motor assuming 6xFLC. It could be more........but not likely to be much less.
 

don_resqcapt19

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See table 420.7(B) for the KVA per horsepower with a locked rotor. The value for Code letter "G" is 5.6 to 6.29.
 

Lost_RFTech

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Look at the instantaneous trip settings detailed in table 430.52 (2008) and supporting text. Off the top of my head - 1100% of FLA for Nema design B high efficiency motors, all others at 800%. Exceptions allow settings up to 1700/1300% respectively, with proper engineering considerations in mind. I do not know if any other code sections place any other requirements on fire pumps.

I work in a very large and very old facility, the increased in-rush of the design B (replacement) motors makes the conscientious fairly familiar with 430.52.
 

gadfly56

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We're on the 2007 edition of NFPA 20 which is the standard dealing with fire pumps. In table 9.5.1.1, the LRC for a 200HP motor listed for fire pump service is given as 1450 amps at 460VAC (three phase). You shouldn't have an issue unless you're getting a short somewhere.
 

kwired

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I don't know what normal practices are for fire pumps, but are you starting across the line?

Some instances a motor this large needs reduced voltage starting methods as a POCO requirement to cut down on starting current. If that is the case maybe make sure you don't have malfunctioning reduced voltage starting controls. Locked rotor current and instantaneous current the first few cycles when energizing for across the line are not necessarily the same thing.
 

Canton

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I don't know what normal practices are for fire pumps, but are you starting across the line?

Some instances a motor this large needs reduced voltage starting methods as a POCO requirement to cut down on starting current. If that is the case maybe make sure you don't have malfunctioning reduced voltage starting controls. Locked rotor current and instantaneous current the first few cycles when energizing for across the line are not necessarily the same thing.

True...the last couple of fire pumps I have worked on were not full voltage/across the line starters. A few have been wye start/delta run, especially when is comes to that size of a motor.
 

iwire

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I don't know what normal practices are for fire pumps, but are you starting across the line?

Many do and others do not, however all that I can recall had a manually operable override that would start the pump across the line.

It seems to me any reduced voltage starters would be used strictly to please the power company or reduce voltage sags in the building during starting when doing the required testing.

The code requires the circuit to hold on the locked rotor current.

Locked rotor current and instantaneous current the first few cycles when energizing for across the line are not necessarily the same thing.

My understanding is they would be the same thing, can you expand on that?
 

kwired

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Many do and others do not, however all that I can recall had a manually operable override that would start the pump across the line.

It seems to me any reduced voltage starters would be used strictly to please the power company or reduce voltage sags in the building during starting when doing the required testing.

The code requires the circuit to hold on the locked rotor current.



My understanding is they would be the same thing, can you expand on that?
First few cycles of across the line starting you do not yet have any coil saturation to provide additional impedance that limits current. It is like closing the motor controller into a short circuit for a few cycles.

Locked rotor the field is built already and the winding has more current limiting features. They are not the same thing though many treat them as the same thing.

Now throw in a reduced voltage starting method and the whole idea behind it is to bring that field up with a lower voltage which will limit how much current is drawn during those first few cycles. It only lasts for a very short time then the reduced voltage controller transers to provide direct supply voltage to the motor. If you bypass the reduced voltage portion of the starting scheme - you likely do trip the overcurrent protection in most applications, and in some instances maybe even overcurrent protection that is even further upstream.
 

iwire

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First few cycles of across the line starting you do not yet have any coil saturation to provide additional impedance that limits current. It is like closing the motor controller into a short circuit for a few cycles.

I agree.

Locked rotor the field is built already and the winding has more current limiting features. They are not the same thing though many treat them as the same thing.

So you are saying locked rotor current does not include the inrush of saturating the coils?

That is interesting but it seems to me the NEC treats them as the same thing regardless.

Or do you believe a compliant installation would allow the coil saturation current to trip out the OCPD?



If you bypass the reduced voltage portion of the starting scheme - you likely do trip the overcurrent protection in most applications, and in some instances maybe even overcurrent protection that is even further upstream.

I believe that would be a code violation, the OCPD has to be able to hold the LRC of the motor reduced voltage starter or not.
 

kwired

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I agree.



So you are saying locked rotor current does not include the inrush of saturating the coils?

That is interesting but it seems to me the NEC treats them as the same thing regardless.

Or do you believe a compliant installation would allow the coil saturation current to trip out the OCPD?





I believe that would be a code violation, the OCPD has to be able to hold the LRC of the motor reduced voltage starter or not.
I guess I don't know what NEC intent is. For other then a fire pump application IMO locked rotor current is the steady state current the motor would pull if the rotor is not turning and full voltage is applied. Initial surge of current when first energizing can be and often is higher yet for a few cycles. AFAIK the requirement for fire pumps for the overcurrent device to hold locked rotor current indefinitely is just that, the steady state current it would draw if the motor stalls.

Motor starting is another issue and IMO 695 doesn't really address it. Some instances the source may not even hold if a reduced voltage starting scheme is not used (and I'm talking large motors in general here not just fire pumps).

Does NEC define locked rotor current anywhere? It certainly doesn't in art 695.

What about a motor with a "soft start" or VFD controller? Locked rotor current will be what those applications draw if the motor stalls, and they will never draw the same instantaneous current as they would for the first few cycles of across the line starting. If such control methods are used for a fire pump all 695 requires is for no overcurrent device to open when the motor is stalled, once there is a break down in the motor and a short circuit or ground fault develops though we possibly have more current flowing then locked rotor current value is and it is then acceptable to open the device.- JMO.
 

iwire

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Does NEC define locked rotor current anywhere? It certainly doesn't in art 695.


Table 430.7(B) tells us the locked rotor currents and I am willing to bet they have enough fluff built into them to cover any winding inrush.

Motor starting is another issue and IMO 695 doesn't really address it. Some instances the source may not even hold if a reduced voltage starting scheme is not used (and I'm talking large motors in general here not just fire pumps).

Wow, I can't even begin to comprehend the NEC allowing a fire pump not to start. :huh:

What about a motor with a "soft start" or VFD controller? Locked rotor current will be what those applications draw if the motor stalls, and they will never draw the same instantaneous current as they would for the first few cycles of across the line starting..

And again in my experience fire pump controllers that employ those types of devices will still have a manually closed override to start the pump across the line.

I think the intent is clear, under no circumstances should the load of the fire pump be able to open an over current device.
 

kwired

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Table 430.7(B) tells us the locked rotor currents and I am willing to bet they have enough fluff built into them to cover any winding inrush.



Wow, I can't even begin to comprehend the NEC allowing a fire pump not to start. :huh:



And again in my experience fire pump controllers that employ those types of devices will still have a manually closed override to start the pump across the line.

I think the intent is clear, under no circumstances should the load of the fire pump be able to open an over current device.

Maybe someone like Jraef can give us more insight on locked rotor vs across the line starting inrush.

Like I said I have little experience with fire pumps, and all those I have worked with were across the line starting. But I have been around some reduced voltage starting. A major reason reduced voltage starting is used is so ensure the motor does start and does not trip an overcurrent device upstream - maybe even ahead of service equipment. NEC doesn't say anything about assurance of starting the fire pump, just that it wants it to run til it burns out so to speak. But I am willing to bet starting reliability is covered in other NPFA publications that apply to the fire pump whether it be electric driven or by other means.

If the intent was that a fire pump never be able to open an overcurrent device, why even bother requiring an overcurrent device? Why not just a switch to disconnect for maintenance purposes but no overcurrent protection - though POCO will still have a limitation of some sort somewhere upstream - at very least a fuse in the primary of their service transformer.
 

kwired

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On top of what I have already said, if locked rotor current is what I described it as - the sustained current drawn if the rotor stops turning while full voltage is still applied - that current should remain same from one install to the next assuming the supply is able to supply full voltage when that much current is being drawn - plus any other loads.

But same motor at a different location may not have same instantaneous current the first few cycles of across the line starting depending on things like size of source, other loads on the source, impedance of the source, size and length of conductors between the source and the motor in question.

I see that kind of difference all the time with same size and even same model number of motor from one location to another. One place you may be able to start a motor on a particular fuse or breaker go to another site and same motor/driven load will not hold because it is closer to the source, the source is larger, things of that nature allow more current to flow during starting.
 

big john

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...But same motor at a different location may not have same instantaneous current the first few cycles of across the line starting depending on things like size of source, other loads on the source, impedance of the source, size and length of conductors between the source and the motor in question....
That's a good point.

I don't know enough to discuss how accurate it may be, but I see what you're saying that because inrush is much higher than LRA it would tend to depend more on the impedance of the supply than LRA might.

I've seen transformers trip instantaneous settings on breakers during startup due to inrush. I've never seen it happen with a motor, though. And I do also get iwire's point that if this were a very critical concern I would expect it to be specifically addressed separately of LRA.
 
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GoldDigger

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Instantaneous surge is more likely to be a problem when restarting a motor that has not yet coasted to a full stop or in shorter terms has not had time for the induced field to decay.
 

iwire

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On top of what I have already said, if locked rotor current is what I described it as - the sustained current drawn if the rotor stops turning while full voltage is still applied - that current should remain same from one install to the next assuming the supply is able to supply full voltage when that much current is being drawn - plus any other loads.

But same motor at a different location may not have same instantaneous current the first few cycles of across the line starting depending on things like size of source, other loads on the source, impedance of the source, size and length of conductors between the source and the motor in question.

I see that kind of difference all the time with same size and even same model number of motor from one location to another. One place you may be able to start a motor on a particular fuse or breaker go to another site and same motor/driven load will not hold because it is closer to the source, the source is larger, things of that nature allow more current to flow during starting.

And I maintain the LRCs provided by the NEC take this into account just like the FLC tables in 430 are based on worst case scenario.

The LRC table would be worthless in sizing OCPDs if that was not so.
 
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