Gen bkr size fire pump

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Motor and pumps often operated longer than 3 hours, but for bc wire is sized 125% per art 430 regardless of how long it runs. I don’t have code book handy, please look in examples and see if there is one for motor circuit sizing
 
tom baker said:
Motor and pumps often operated longer than 3 hours, but for bc wire is sized 125% per art 430 regardless of how long it runs. I don’t have code book handy, please look in examples and see if there is one for motor circuit sizing
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Right but the minimum emergency generator breaker size is the question for fire pump Not the conductor.

Fire pump is consider continuous load so how come minimum breaker size is not 125% of full load amps? Wont the generator breaker trip if minimum is sized based on full load amp such as in the case below if 150A generator breaker is provided?:

Fire pump 100HP 480V three phase 124A full load on generator size 124x1.25=155A.
 
hhsting said:
Right but the minimum emergency generator breaker size is the question for fire pump Not the conductor.

Fire pump is consider continuous load so how come minimum breaker size is not 125% of full load amps? Wont the generator breaker trip if minimum is sized based on full load amp such as in the case below if 150A generator breaker is provided?:

Fire pump 100HP 480V three phase 124A full load on generator size 124x1.25=155A.
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Everyone in this thread is saying that it looks like that breaker is undersized. Inverse time circuit breakers feeding motors can be sized to 250% of FLA. That would be a 300A circuit breaker in your scenario.

As has been stated numerous times, the circuit breaker has to allow the motor to start. It is unlikely that a 150A circuit breaker would allow your motor to start, even if you are using a reduced starting method like a wye-delta starter. Maybe if you used a VFD and ramped up slowly you could get the 150A circuit breaker to hold, but I would still have my doubts.
 
d0nut said:
Everyone in this thread is saying that it looks like that breaker is undersized. Inverse time circuit breakers feeding motors can be sized to 250% of FLA. That would be a 300A circuit breaker in your scenario.

As has been stated numerous times, the circuit breaker has to allow the motor to start. It is unlikely that a 150A circuit breaker would allow your motor to start, even if you are using a reduced starting method like a wye-delta starter. Maybe if you used a VFD and ramped up slowly you could get the 150A circuit breaker to hold, but I would still have my doubts.
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Problem is 695.4(B)(2)(a) says minimum size is based on FLA of fire pump. So Lets say assume motor does start can it even run for more than 3 hours which is continuous load on full load amps of 124A and 150A breaker not trip?
 
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I got it to work with a 150A circuit breaker! Well actually an 800A frame circuit breaker with a 250A trip unit turned down to 150A, but I can start a 100hp motor across-the-line with a 150A circuit breaker feeding it.
 
d0nut said:
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I got it to work with a 150A circuit breaker! Well actually an 800A frame circuit breaker with a 250A trip unit turned down to 150A, but I can start a 100hp motor across-the-line with a 150A circuit breaker feeding it.
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Yes you got it to start. Now Can the 100HP at 480V three phase run continuously at full load amps for more than three hours and not trip 150A breaker?
 
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hhsting said:
Yes you got it to start. Now Can the 100HP at 480V three phase run continuously at full load amps for more than three hours and not trip 150A breaker?
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Yes. It is an electronic trip breaker, not a thermal magnetic breaker. It will only trip based on the magnitude of the current flowing through the circuit breaker, not on the temperature of the conductors or ambient environment.
 
hhsting said:
Right but the minimum emergency generator breaker size is the question for fire pump Not the conductor.

Fire pump is consider continuous load so how come minimum breaker size is not 125% of full load amps? Wont the generator breaker trip if minimum is sized based on full load amp such as in the case below if 150A generator breaker is provided?:

Fire pump 100HP 480V three phase 124A full load on generator size 124x1.25=155A.
Click to expand...
It is extremely unlikely that a 150 amp breaker will ever trip with a forever load of 155 amps. And just because the code requires a 125% multiplier, the load does not change...it is still 124 amps for that motor.

As I said before, since there is no specific rule in 695, and am sizing the conductors at 155 amps and the OCPD at 300 or 350 amps as permitted by Article 430.
 
d0nut said:
Yes. It is an electronic trip breaker, not a thermal magnetic breaker. It will only trip based on the magnitude of the current flowing through the circuit breaker, not on the temperature of the conductors or ambient environment.
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Are all electronic unit 100% no 80% rated?

What if engineer installs non electronic trip unit?

Also by sizing the breaker 125% of FLA you are essentially providing overload protection as well since 430.32 defines what overload protection is and how to size it. Fire pump 695 says you cannot provide overload protection. So then engineer would have to upsize it to provide short circuit protection only no?
 
don_resqcapt19 said:
It is extremely unlikely that a 150 amp breaker will ever trip with a forever load of 155 amps. And just because the code requires a 125% multiplier, the load does not change...it is still 124 amps for that motor.

As I said before, since there is no specific rule in 695, and am sizing the conductors at 155 amps and the OCPD at 300 or 350 amps as permitted by Article 430.
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Did some research on my own see the link according to the article interpretation is that for Fire Pump Generator Breaker sizing Minimum is greater than overload protection stated in 430.32 since 695 says no overload protection only short circuit protection
Agree or disagree?

 
hhsting said:
Did some research on my own see the link according to the article interpretation is that for Fire Pump Generator Breaker sizing Minimum is greater than overload protection stated in 430.32 since 695 says no overload protection only short circuit protection
Agree or disagree?

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The breaker does not provide "overload protection" for most motor circuits...it is only provides short-circuit and ground-fault protection. A separate device provides the overload protection for most motors, but that protection is not permitted for fire pump motors.
 
don_resqcapt19 said:
The breaker does not provide "overload protection" for most motor circuits...it is only provides short-circuit and ground-fault protection. A separate device provides the overload protection for most motors, but that protection is not permitted for fire pump motors.
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Fire pump is not provided overload protection from manufacturer but sizing the gen breaker 1.25xFLA or under does provide the overload protection and this not in compliance with 695 and 430?
 
hhsting said:
Fire pump is not provided overload protection from manufacturer but sizing the gen breaker 1.25xFLA or under does provide the overload protection and this not in compliance with 695 and 430?
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As I said a number of times I would size the breaker at the maximum size permitted by 450.52.
 
Looking at this and related threads we seem to be going in circles. The NEC gives you a numeric maximum size for your generator supply breaker (250%) but is not as definitive on the minimum size stating only: 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..... In your case an designer (engineer) has selected the breaker size and there is some agreement that the 150 is questionable and as a plans examiner (or inspector) I would question him concerning the size. I agree with don that I would tend to size it at maximum allowed but I would definitely run it by the designer considering this is a fire pump install (and, as mentioned in another thread. .document it!).
 
Oh dear! Their is actually an exception to overload protection Fire Pump! It is NEC 2017 Section 695.6(C) exception no. 2 see below. And guess what it involves onsite generators! Just my luck!

Generator FLA feeding fire pump plus all three branches life safety, optional standby and standby is 481A. Fire pump FLA is 124A. So then that exceeds 225% as stated in exception no. 2 below.


I dont quite get it. Is it saying I dont need overcurrent protection at all? Does it apply to generator feeder breaker? Also the last sentence protection shall be provided shall be in accordance with the shirt circuit current rating of the combination fire pump transfer switch controller or separately derived mounted transfer switch what does that mean?

Anyone please please help


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augie47 said:
Looking at this and related threads we seem to be going in circles. The NEC gives you a numeric maximum size for your generator supply breaker (250%) but is not as definitive on the minimum size stating only: 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..... In your case an designer (engineer) has selected the breaker size and there is some agreement that the 150 is questionable and as a plans examiner (or inspector) I would question him concerning the size. I agree with don that I would tend to size it at maximum allowed but I would definitely run it by the designer considering this is a fire pump install (and, as mentioned in another thread. .document it!).
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Ok I want to run it by the engineer but say what ensure that breaker doesn’t trip when fire pump is running continuously as i think breaker is too small and sized per overload which 6954(C) says should not be any?
 
augie47 said:
1.25 is for the circuit conductor. 695 and 430.62 would allow a lot larger than 150 or 175, closer to 300, assuming it's the stand-buy source. It being a fire pump, I'd let an engineer decide..
If your desitgn shows 150, I'd question he designer.
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Yes to letting engineer decide this.

Most the 100HP motors I get involved with are on 175 amp RK5 fuses. I know of at least one that is on a 200 amp thermal mag breaker though art 430 allows maybe 350 amp thermal mag breaker and up to 1000 for instantaneous trip.

But since this is for a fire pump reliability in an emergency situation is important and is consideration for some the requirements in 695.

Also need to consider the starting surge won't necessarily be same when starting on utility as it may be when running from a generator - source impedance and ability of prime mover are factors in what that current will be.
 
tom baker said:
I was working with a EE on a emergency generator for a fire pump, and he sized the breaker like HHStings, as it was a 100 hp fire pump. I questioned his breaker size, it was never resolved, and the project dropped. Shouldn't the generator breaker be sized to carry locked rotor current?
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I believe yes it should be required to carry locked rotor current indefinitely. Magnetic trip setting upon initial energizing still needs to be higher than locked rotor current.

Listed fire pump controller is easiest way to go, problem here is OP has a breaker on the genset that is maybe good for protecting the genset but probably doesn't comply with fire pump requirements. Does genset need to be listed/rated to supply the fire pump? IDK, guessing at very least maybe needs to be able to provide locked rotor current indefinitely.
 
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