Locked Rotor Current of Fire Pumps

[kwired]

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.

I kind of agree with you - but only when sizing an OCPD for across the line starting, throw in reduced voltage starting or a soft starter and you likely find lower OCPD setting in most installations then you would see if it were across the line started. I just am not seeing the NEC addressing the issue of a reduced voltage starting method for a fire pump being required to be able to have overcurrent protection sized to be able to start across the line should the reduced voltage method happen to fail. The switches and contactors maybe still need to meet worst case scenario, but trip setting could be anywhere as far as art. 430 tables are concerned.

 

[kwired]

We kind of went on a different tangent, yet still do not know if the OP even has reduced voltage starting at his application

 

[iwire]

I kind of agree with you - but only when sizing an OCPD for across the line starting, throw in reduced voltage starting or a soft starter and you likely find lower OCPD setting in most installations then you would see if it were across the line started. I just am not seeing the NEC addressing the issue of a reduced voltage starting method for a fire pump being required to be able to have overcurrent protection sized to be able to start across the line should the reduced voltage method happen to fail. The switches and contactors maybe still need to meet worst case scenario, but trip setting could be anywhere as far as art. 430 tables are concerned.

I disagree.

We are directed to design based on the published LRCs not the reduced currents soft starts, VFDs or any other method employed.

 

[kwired]

I disagree.

We are directed to design based on the published LRCs not the reduced currents soft starts, VFDs or any other method employed.

And as I said, that means the switches, contactors, etc, will be same size regardless of overcurrent device trip setting, but trip settings will vary depending on starting methods employed.

 

[don_resqcapt19]

And as I said, that means the switches, contactors, etc, will be same size regardless of overcurrent device trip setting, but trip settings will vary depending on starting methods employed.

It is my understanding (but I don't have a copy) that NFPA 20 requires the circuit to have the ability to support accross the line start even where the fire pump controller uses some type of reduced current starting. NFPA 20, and not NFPA 70 is the controlling document for fire pumps.

 

[iwire]

And as I said, that means the switches, contactors, etc, will be same size regardless of overcurrent device trip setting, but trip settings will vary depending on starting methods employed.

I still cannot agree, the trip setting has to be able to handle a manual start with a soft start functioning or not.

The soft starts, the wye delta starts, are only in place to appease the power company and reduce voltage drop to the building under normal conditions.

We are not talking about a grain elevator or roller coaster, it's a fire pump and it is life safety. It must run, we can't have a breaker tripping.

 

[kwired]

We are not talking about a grain elevator or roller coaster, it's a fire pump and it is life safety. It must run, we can't have a breaker tripping.

Correct, and that starting reliability is likely covered in more detail in other NFPA publications then it is in NFPA 70.

NFPA 20 (?) surely has reliability starting requirements for more then just electrically operated pumps as well as requiring other pump types to continue to run even if overloaded.


Art 695 mentions holding @ LRC indefinitely, doesn't say much at all about starting methods.

 

[iwire]

Correct, and that starting reliability is likely covered in more detail in other NFPA publications then it is in NFPA 70.

NFPA 20 (?) surely has reliability starting requirements for more then just electrically operated pumps as well as requiring other pump types to continue to run even if overloaded.


Art 695 mentions holding @ LRC indefinitely, doesn't say much at all about starting methods.

I have no idea what you are trying to say here.

695 says the OCPD is based on the LRC of the motor, not the LRC of a motor and soft start combined.

695 does not tell us we can adjust that LRC just becuse we happen to install a soft start.


I don't know how to make it any clearer. :happysad:

 

[kwired]

I have no idea what you are trying to say here.

695 says the OCPD is based on the LRC of the motor, not the LRC of a motor and soft start combined.

695 does not tell us we can adjust that LRC just becuse we happen to install a soft start.


I don't know how to make it any clearer. :happysad:

I don't know how to make it any clearer that LRC and across the line first few cycles inrush are not the same thing, nor are they necessarily the same value for a particular application.

I don't know what is typical for a large fire pump motor - never been around any, been around other larger motors though, and often you will not start them across the line without reduced voltage methods period or you will blow a POCO primary fuse, then what good does a higher setting service/branch circuit/etc. OCPD do you?? I've had primary fuses blow when starting what may be considered not so large of a motor a few times.

 

[big john]

I don't know how to make it any clearer that LRC and across the line first few cycles inrush are not the same thing, nor are they necessarily the same value for a particular application.

But do you agree or disagree that the NEC does not appear to make a distinction between the two, regardless of whether they may be two different values?

I think that's where this discussion is going awry.

 

[kwired]

But do you agree or disagree that the NEC does not appear to make a distinction between the two, regardless of whether they may be two different values?

I think that's where this discussion is going awry.

With a fire pump, I'm not really sure. With other motors, yes your switches, controllers, etc. need be same size no matter what the starting method is, even conductor sizing is same. But short circuit and ground fault trip point is not something that has any absolute value you must adhere to. There are recommendations - most common being 175% for time delay fuses, 250% for inverse time breakers, but there are instances where there are applications both higher as well as lower that are all NEC compliant.

Fire pump - it says we must be able to carry the locked rotor current indefinitely. To me locked rotor current is that current that is sustained should the motor stall - not the instant current the first few cycles when starting across the line, the actual value of the latter can vary from one install to another, where the value of the first will be more consistent from one install to another - assuming same motor and supply voltage.

I have been always told we want that pump to run until it fails when it comes to fire pumps, but nothing in NEC mentions reliability of starting that motor - I believe that requirement comes from other NFPA publications.

Should the fire pump be powered by an on site generator - my understanding that generator needs to deliver locked rotor current indefinitely as well, but very possible it can not deliver the current level of the first few cycles indefinitely, maybe even likely it can't start the pump in some cases without reduced voltage starting methods, my guess is they do need to start these pumps periodically to help ensure they will start if ever needed, to comply with other NFPA documents.

 

[iwire]

LRC and across the line first few cycles inrush are not the same thing, nor are they necessarily the same value for a particular application.

I believe you.

I am saying that the NEC provided LRCs include enough fluff to cover that. If they did not fire pumps might not start and IMO that would be both undesirable and a code violation.

Do you understand that the FLCs in 430 also list currents to cover the worst case scenario and even if you choose a motor with a lower FLC that we still have to design based on what is listed in 430? Why is it so hard to accept that the listed LRCs are also worst case scenarios?

But do you agree or disagree that the NEC does not appear to make a distinction between the two, regardless of whether they may be two different values?

I think that's where this discussion is going awry.

I agree.

 

[iwire]

Fire pump - it says we must be able to carry the locked rotor current indefinitely. To me locked rotor current is that current that is sustained should the motor stall - not the instant current the first few cycles when starting across the line,

And to me that veiw makes no sense whatsoever as that could allow an OCPD to prevent a fire pump from starting.

I have been always told we want that pump to run until it fails when it comes to fire pumps, but nothing in NEC mentions reliability of starting that motor -

As is my way I will be blunt, I find that silly. Your saying it is esential that it keeps running but not esential it starts in the first place. :huh:

I believe that requirement comes from other NFPA publications.

While other codes also cover fire pumps the NEC tells us what rating the OCPDs shall have.

Should the fire pump be powered by an on site generator - my understanding that generator needs to deliver locked rotor current indefinitely as well,

No, things change with a generator.

695.4(B)(2)(b) On-Site Standby Generators. Overcurrent protective devices between an on-site standby generator and a fire
pump controller shall be selected and sized to allow for
instantaneous pickup of the full pump room load, but shall
not be larger than the value selected to comply with 430.62
to provide short-circuit protection only.

 

[don_resqcapt19]

I don't know how to make it any clearer that LRC and across the line first few cycles inrush are not the same thing, nor are they necessarily the same value for a particular application.

I don't know what is typical for a large fire pump motor - never been around any, been around other larger motors though, and often you will not start them across the line without reduced voltage methods period or you will blow a POCO primary fuse, then what good does a higher setting service/branch circuit/etc. OCPD do you?? I've had primary fuses blow when starting what may be considered not so large of a motor a few times.

I worked at a cement mill that started a 4,500 hp motor across the line. What you can start is based on coordinated design between the equipment and the utility.
I would expect that where there are large fire pumps the utility OCPD is designed to permit across the line starting.

 

[kwired]

I worked at a cement mill that started a 4,500 hp motor across the line. What you can start is based on coordinated design between the equipment and the utility.
I would expect that where there are large fire pumps the utility OCPD is designed to permit across the line starting.

Very possible, I'm stuck in this little corner of the world where it is not going to be all that feasible to even have a 4500 hp motor without running at least 50 miles of transmission line to a location sort of close to that load.

 

[AHMED BAYOUMI]

THANK YOU , I HAVE FIRE PUMP 200HP WITH STAR DELTA AS STARTING METHOD,I WANT TO CALCULATE C.B SIZE BY KNOWING LRC BUT CAN I REDUCE THE LRC BECAUSE OF USING STAR DELTA METHOD OR OR NOT ?????????????????

 

[kwired]

THANK YOU , I HAVE FIRE PUMP 200HP WITH STAR DELTA AS STARTING METHOD,I WANT TO CALCULATE C.B SIZE BY KNOWING LRC BUT CAN I REDUCE THE LRC BECAUSE OF USING STAR DELTA METHOD OR OR NOT ?????????????????

That pump will have less instantaneous current at startup then it would if started across the line.

It will still draw same current should it stall sometime after a successful start, no matter what starting method was used.

It will still draw same current should it fail to accelerate after the initial surge to magnetize the field is over with no matter what starting method is used.

IMO those statements are true whether NEC recognizes those facts or not.

 

[iceworm]

You guys have already hashed this out pretty well. This discussion is not about fire pumps specifically and I'm leaving out Irrigation and HVAC.

Here are a few notes on KVA, inrush, locked rotor.

As noted, table 430.7.B gives the Code Letters, indicating locked rotor kva and implies a calculation for LRC. The only places this is used is in 430.82.A, Controller design - capable of interupting LRC and, 430.110.C.1, Disconnect rating, Combination Loads. Interesting note: Single motor disconnects are not required to interupt LRC (430.110.A)

Motor Branch Circuit Short Circuit protection (430, Part IV) is not set by the Code Letter, but rather by the Design Letter and energy efficiency rating. Regardless of the Code Letter, the NEC allows an Instantaneous to be set up to 1300% or even 1700% - more than double the Code Letter G normal motor LRC.

A few years back, I had a new installation, 60 Hp, Design B, Code G, , occasionally tripping the instantaneous set to 800%. Information from the vendor indicated the motor met MG-1 energy efficient criteria. We connected an analyzer and recorded starting currents. There was the cutest little 40ms bump to ~10X FLA right at contactor closure. Then the current settled down to ~6X FLA and dropped off to FLA as the motor came up to speed.

My conclusion are:
The NEC does not specifically recognize "Inrush", but they try to account for it in 430, part IV.

Interesting Note: I did not see this mentioned above, but may have well missed it. 2014, 695.4.B.2 adds:

"The overcurrent protective device shall not open with a re-start transient of 24 times the full-load current of the fire pump motor(s). "​

As for designing per code and having a marginal system - yep

ice

 

[AHMED BAYOUMI]

The starting method is star delta. Which reduce the current by 58 % can i reduce the lrc by this factor to reduce c.b size????

 

[gadfly56]

The starting method is star delta. Which reduce the current by 58 % can i reduce the lrc by this factor to reduce c.b size????

No.

If a rock comes flying down the suction line and jams the impeller, you're going to see the full locked rotor current (LRC) even if you use a magic jinn to start the pump.