Fire Pumps - To tap ahead of the Generator's circuit breaker or not too?

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mmontoya

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This is my first post on this site, so I apologize if I'm not posting this properly.

I’m going to try to keep this story fairly short. I’m new to fire pump installations and our office are trying to finalize a design change. This is a project that is currently in construction and the client wanted to make a change to the original design. The client wanted to add a redundant generator for the already installed on site generator. But instead of adding an actual generator, they want to add a receptacle. Everything seemed fine with this setup, but when looking at the original design, seeing what’s already installed out in the field and what the contractor/generator manufacturer are trying to do, we ran into an issue and the code doesn’t clearly state what the right approach is. The issue is when installing fire pumps you are supposed to tap ahead of everything and connect directly to the generator, but do you tap before the generator’s circuit breaker or ahead.

Also reading a few articles about fire pumps, it seems you want to keep the fire pumps running at all costs. But do you do it at the expense of destroying the generator, or do you still want to protect the generator? Any help is appreciated, thank you.
 
dkidd

dkidd

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From NFPA 20

A.9.6.5 Generator protective devices are to be sized to permit
the generator to allow instantaneous pickup of the full pump
room load. This includes starting any and all connected fire
pumps in the across-the-line (direct on line) full voltage starting
mode. This is always the case when the fire pump(s) is started by
use of the emergency-run mechanical control in 10.5.3.2.
 
gadfly56

gadfly56

Senior Member
Location
New Jersey
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Professional Engineer, Fire & Life Safety
mmontoya said:
This is my first post on this site, so I apologize if I'm not posting this properly.

I’m going to try to keep this story fairly short. I’m new to fire pump installations and our office are trying to finalize a design change. This is a project that is currently in construction and the client wanted to make a change to the original design. The client wanted to add a redundant generator for the already installed on site generator. But instead of adding an actual generator, they want to add a receptacle. Everything seemed fine with this setup, but when looking at the original design, seeing what’s already installed out in the field and what the contractor/generator manufacturer are trying to do, we ran into an issue and the code doesn’t clearly state what the right approach is. The issue is when installing fire pumps you are supposed to tap ahead of everything and connect directly to the generator, but do you tap before the generator’s circuit breaker or ahead.

Also reading a few articles about fire pumps, it seems you want to keep the fire pumps running at all costs. But do you do it at the expense of destroying the generator, or do you still want to protect the generator? Any help is appreciated, thank you.
Click to expand...

The pump is the last thing allowed to die. Design accordingly.
 
T

topgone

Senior Member
mmontoya said:
This is my first post on this site, so I apologize if I'm not posting this properly.

I’m going to try to keep this story fairly short. I’m new to fire pump installations and our office are trying to finalize a design change. This is a project that is currently in construction and the client wanted to make a change to the original design. The client wanted to add a redundant generator for the already installed on site generator. But instead of adding an actual generator, they want to add a receptacle. Everything seemed fine with this setup, but when looking at the original design, seeing what’s already installed out in the field and what the contractor/generator manufacturer are trying to do, we ran into an issue and the code doesn’t clearly state what the right approach is. The issue is when installing fire pumps you are supposed to tap ahead of everything and connect directly to the generator, but do you tap before the generator’s circuit breaker or ahead.

Also reading a few articles about fire pumps, it seems you want to keep the fire pumps running at all costs. But do you do it at the expense of destroying the generator, or do you still want to protect the generator? Any help is appreciated, thank you.
Click to expand...

The essence of the fire code rules tells you to supply your fire pump ahead of the other electrical loads so that any problems with other loads do not impact on the fire pumps ability to supply water for fire extinguishment. If your generator gets damaged while providing power to the fire pumps, you're off the hook. But if your feeder protection to the fire pumps includes a trip other than a short-circuit, that's where you get AHJs attention.

To add to what I just said above, that idea always enters my mind being an electrical man: redundant generator to assure availability! Why can't you recommend to your boss installing an engine-driven fire pump instead? Methinks that would be simpler.
 
Sahib

Sahib

Senior Member
Location
India
So your generator breaker should not have thermal magnetic trip but only magnetic trip...........
 
gadfly56

gadfly56

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Location
New Jersey
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Professional Engineer, Fire & Life Safety
dkidd said:
That's not correct. See http://static.schneider-electric.us/docs/Circuit Protection/0100DB0711.pdf

To further address OPs original question, NEC says

695.3(D)(2) Connection. A tap ahead of the generator disconnecting
means shall not be required. [20:9.6.1.2]

Reference in brackets refers to NFPA 20.
Click to expand...

The OP asked about tapping before the circuit breaker, which is not necessarily the same as "disconnecting means". If the disconnecting means is a fused disconnect, it will need to be sized for the locked rotor current.
 
dkidd

dkidd

Senior Member
Location
here
Occupation
PE
The generator overcurrent device is not sized for locked rotor.

(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. [20:9.6.1.1]
 
dkidd

dkidd

Senior Member
Location
here
Occupation
PE
gadfly56 said:
The OP asked about tapping before the circuit breaker, which is not necessarily the same as "disconnecting means". If the disconnecting means is a fused disconnect, it will need to be sized for the locked rotor current.
Click to expand...

The OP stated that the generator breaker was the disconnecting means.
 
dkidd

dkidd

Senior Member
Location
here
Occupation
PE
topgone said:
Nope. The other guy read the OP correct. Please see the title: "Fire Pumps - To tap ahead of the Generator's circuit breaker or not too?"
Click to expand...

So, the breaker is not the disconnecting means? And there might be a fused disconnect which is the disconnecting means?
 
T

topgone

Senior Member
dkidd said:
So, the breaker is not the disconnecting means? And there might be a fused disconnect which is the disconnecting means?
Click to expand...
Although OP mentioned making his story short, I found it quite long.
In his words--> the problem came when the client wanted to change the original design by adding a "receptacle" instead of an actual "additional generator"!

AFAIK, a receptacle derives its power from a "feeder" breaker, which I think muddled the install. The idea that the installer will "tap ahead of the breaker", I understood it as tapping ahead of the "feeder" breaker of the "receptacle"! I am assuming there is still a main breaker up from where the feeder breaker gets its feed.

Anyways, I doubt the AHJ will give it a go as he described his system to be, imo.
 
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