Gen bkr size fire pump

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hhsting

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I have 480V three phase 100HP fire pump. The generator breaker feeding it is sized 150A. However i dont see how since FLA of pump is 124A and 124x1.25= 155A. Should it not be 175A?
 
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d0nut

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The breaker is permitted to be 250% of FLA for an inverse time breaker per Table 430.52. Realistically, the breaker just has to be large enough to start the motor.
 
augie47

augie47

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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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petersonra

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engineer
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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yea. A fire pump only has to work once, but when it has to work it has to work and protecting the wiring or motor just does not matter. Plus, it is not like there is any danger of it starting a fire because if the fire pump is running there already is a fire. :)
 
tom baker

tom baker

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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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petersonra

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tthh said:
Brings to mind "Battle Short"

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Among other things I design control systems for test stands to test diesel engines. They often have a feature that disables shutting down the engine on alarms meant to protect the engine. They call it "battle override". The ship's computer tells the engine controller to enable battle override and the controller no longer cares about protecting the engine.
 
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hhsting

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Glen bunie, md, us
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Looking for minimum size. Well looking at NEC 2017 section 695.4(b)(2)(b) it says shall be selected and sized to allow a fire pump instantaneous pickup of full pump room load but shall not be larger than the value selected to comply with 430.62 to provide short circuit protection only.

So fire pump full load is 124A.

Confusing part is 695.4(b)(2)(b) does Not say continuous so i dont see how 125% can be used to size breaker. Correct or incorrect?

Also no where in 695.4(b)(2)(b) it says to size breaker per locked rotor amps

So if i am reading correctly breaker can be as small as 125A. I dont see how others are saying to question 150A breaker?
 
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d0nut

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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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The normal side has to be sized for locked rotor per 695.4(B)(2)(a). Per 698(B)(2)(b), the generator size OCP has to allow full pickup of the pump room load, but shall not be larger than the value selected to comply with 430.62 to provide short-circuit protection only.
 
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d0nut

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hhsting said:
Looking for minimum size. Well looking at NEC 2017 section 695.4(b)(2)(b) it says shall be selected and sized to allow a fire pump instantaneous pickup of full pump room load but shall not be larger than the value selected to comply with 430.62 to provide short circuit protection only.

So fire pump full load is 124A.

Confusing part is 695.4(b)(2)(b) does Not say continuous so i dont see how 125% can be used to size breaker. Correct or incorrect?

Also no where in 695.4(b)(2)(b) it says to size breaker per locked rotor amps

So if i am reading correctly breaker can be as small as 125A. I dont see how others are saying to question 150A breaker?
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The breaker has to allow the motor to start. That is the only requirement. People are questioning the 150A breaker because that breaker feels like it would be too small to allow a 100hp motor to start, but you wouldn't know for sure without comparing the motor start curve to the breaker trip curve.

Unasked in this discussion so far is if the generator itself can start the 100hp motor.
 
tom baker

tom baker

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We had a trailer generator we used with the 100 hp fire pump, seems like it was 250 kw engine with an oversized alternator
 
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hhsting

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Glen bunie, md, us
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don_resqcapt19 said:
What is the size of the breaker on the utility source to the fire pump?
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The utility service conductors lands on all in on service rated fire pump controller/ATS. So On the normal side their isnt any breaker.

But the question is about fire pump emergency generator side breaker sizing. See post #1. I dont follow
 
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hillbilly1

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d0nut said:
The breaker has to allow the motor to start. That is the only requirement. People are questioning the 150A breaker because that breaker feels like it would be too small to allow a 100hp motor to start, but you wouldn't know for sure without comparing the motor start curve to the breaker trip curve.

Unasked in this discussion so far is if the generator itself can start the 100hp motor.
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It’s probably a delta-wye starter to reduce the starting current, usually a 100 HP is not started straight across the line.
 
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d0nut

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Omaha, NE
hillbilly1 said:
It’s probably a delta-wye starter to reduce the starting current, usually a 100 HP is not started straight across the line.
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Right. If the person who designed the system knew what they were doing when they chose the breaker size, everything is probably fine. If they didn't and the breaker is undersized, slapping a bigger breaker in the generator won't help anything if the generator itself isn't large enough to start the fire pump.

The point I was making is that the breaker might not be the only part of the system that is undersized.
 
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hhsting

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Glen bunie, md, us
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Junior plan reviewer
d0nut said:
Right. If the person who designed the system knew what they were doing when they chose the breaker size, everything is probably fine. If they didn't and the breaker is undersized, slapping a bigger breaker in the generator won't help anything if the generator itself isn't large enough to start the fire pump.

The point I was making is that the breaker might not be the only part of the system that is undersized.
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Well the generator is 400kW but it not only powers fire pump it is emergency generator for high rise building so also powers three ATS one life safety, one standby and another legally required standby.
 
don_resqcapt19

don_resqcapt19

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hillbilly1 said:
It’s probably a delta-wye starter to reduce the starting current, usually a 100 HP is not started straight across the line.
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However NFPA 20 requires that the breaker support across the line starting even if a reduced voltage starting means is provided.
10.4.3.3.1.1*
The circuit breaker shall not trip when starting a motor from rest in the across-the-line (direct-on-line) mode, whether or not the controller is of the reduced inrush starting type.
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acrwc10

acrwc10

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CA
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petersonra said:
yea. A fire pump only has to work once, but when it has to work it has to work and protecting the wiring or motor just does not matter. Plus, it is not like there is any danger of it starting a fire because if the fire pump is running there already is a fire. :)
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Don't forget the scheduled testing, unless the building catches fire right after it is built, the pump may need to run more than once. You will sure know if the breaker is undersized in both situations. ;)
 
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hhsting

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Glen bunie, md, us
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Junior plan reviewer
We all know that fire pump must remain on even if it means days no matter what under emergency.

So then fire pump must be looked at as continuous load more than 3 hours. I think everyone would agree with that.

So then fire pump at 100HP for 480V three phase full load amp is 124A. To be continuous load

124x1.25= 155A

So then would not the 150A generator breaker trip?

Anyone know? Also i am surprised that NEC 2017 section 695 calls for generator breaker to be sized based on full load amps but not larger then 430. No mention of continuous load for fire pump as minimum size. Why? Anyone know or have any knowledge?
 
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