Generator breaker settings for fire pump

PE (done with training)

Senior Member
Location
Saint Louis
Occupation
Professional Engineer
To clarify, I am doing the power system study for this project.

I've got a project where a 400 kW generator feeds a CAM lock that then feeds both a wireway with life safety equipment and a fire pump. My questions is, how should the generator breaker be set in this scenario? If I'm feeding both life safety and the fire pump from the same generator breaker is that an issue?

My first instinct is to set the generator breaker as high as possible so that nothing would ever trip in this scenario, but I know that the NEC dictates special settings when a generator breaker feeds a fire pump so I would just like some feed back from anyone on what they think.

See attached one line.
 

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hillbilly1

Senior Member
Location
North Georgia mountains
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Owner/electrical contractor
A lot of installs I did, the engineer would spec the fire pump to tap ahead of the generator main, but from what I have gathered over the years, it only has to be sized to start and run the pump, while still able to handle the other loads. (Unless the other loads are shed when pump is called for) Locked rotor current is not required for the generator to supply.
 

PD1972

Member
Location
New York
Occupation
engineer
The one-line appears to be a little suspect to me. A tap ahead of the generator main isn't required, although it is one way to do it. If the designer decides not to utilize a tap, the generator breaker sizing must be in compliance with 695.4(B)(2)(b).

The generator circuit breaker is only required to be large enough to allow for instantaneous pickup of the full fire pump room load per 695.4(B)(2)(b). The type of starter incorporated with the fire pump controller determines how much inrush there is on start-up. However, there is the size limitation that the breaker shall not be larger than as determined by 430.62 for short circuit protection. The 800A circuit breaker exceeds 250% of the 100HP fire pump motor FLC (124A x 2.5 = 310A).

If not using a tap ahead of the generator main, I have typically put fire pumps on a dedicated circuit breaker sized for 250% of the FLC and asked the FP team to provide a minimum of a soft-starter for the fire pump controller.
 

ron

Senior Member
As long as the long time pickup is 800A, everything else is to achieve the balance of reasonable arc flash incident energy (set low) and allowing instantaneous inrush current to pass (set high), typically even a soft start of a fire pump has an across the line bypass starter.
 

PE (done with training)

Senior Member
Location
Saint Louis
Occupation
Professional Engineer
Thanks everyone for the information,

I am going to advise that they put a dedicated fusible disconnect or circuit breaker between the fire pump ATS and the generator on this project.

Also, just curious, but when you guys mention to tap ahead of the generator main breaker, wouldn't this still be an issue as you would have no overcurrent protection for the fire pump coming from the generator?
 

PD1972

Member
Location
New York
Occupation
engineer
695.4(B)(2)(b) does not require you to put an OCPD for generators, only that the OCPD be sized not larger than as determined by 430.62. 695.4(A) allows for direct connections without disconnecting means/OCPD between the power source and the controller.
 

PE (done with training)

Senior Member
Location
Saint Louis
Occupation
Professional Engineer
The one-line appears to be a little suspect to me. A tap ahead of the generator main isn't required, although it is one way to do it. If the designer decides not to utilize a tap, the generator breaker sizing must be in compliance with 695.4(B)(2)(b).

The generator circuit breaker is only required to be large enough to allow for instantaneous pickup of the full fire pump room load per 695.4(B)(2)(b). The type of starter incorporated with the fire pump controller determines how much inrush there is on start-up. However, there is the size limitation that the breaker shall not be larger than as determined by 430.62 for short circuit protection. The 800A circuit breaker exceeds 250% of the 100HP fire pump motor FLC (124A x 2.5 = 310A).

If not using a tap ahead of the generator main, I have typically put fire pumps on a dedicated circuit breaker sized for 250% of the FLC and asked the FP team to provide a minimum of a soft-starter for the fire pump controller.
Does the fire pump circuit have to be separated? In this scenario can I just put a disconnect switch with a circuit breaker down stream of the camlock and size it per 430.62?
 

PD1972

Member
Location
New York
Occupation
engineer
You are limited in the number of disconnecting means you may have between your power source and the fire pump per 695.4(B)(1). I would say that you may not do what you proposed because then you would have (3) disconnecting means for the fire pump generator feeder (generator breaker, MTS, downstream switch). An argument for some flexibility in the number of disconnecting means may be made if you have multiple generators paralleling to a single bus which then feeds the fire pump. Disconnecting one of the generators will not disconnect standby power to your fire pump; however, this is not the case here.

The MTS requirement is a relatively new code requirement introduced in the 2017 NEC so I can't say that I have ever tried doing whatever is shown on your proposed one-line. I would kick it to whoever the EOR is to figure out next steps.
 

PE (done with training)

Senior Member
Location
Saint Louis
Occupation
Professional Engineer
You are limited in the number of disconnecting means you may have between your power source and the fire pump per 695.4(B)(1). I would say that you may not do what you proposed because then you would have (3) disconnecting means for the fire pump generator feeder (generator breaker, MTS, downstream switch). An argument for some flexibility in the number of disconnecting means may be made if you have multiple generators paralleling to a single bus which then feeds the fire pump. Disconnecting one of the generators will not disconnect standby power to your fire pump; however, this is not the case here.

The MTS requirement is a relatively new code requirement introduced in the 2017 NEC so I can't say that I have ever tried doing whatever is shown on your proposed one-line. I would kick it to whoever the EOR is to figure out next steps.
If my transfer switch / controller for the fire pump is service entrance rated and contains an OCPD, then wouldn’t that cover me in this scenario? I’m trying to work with the EOR so that we don’t have to change anything as everything is already installed..
 

PE (done with training)

Senior Member
Location
Saint Louis
Occupation
Professional Engineer
You are limited in the number of disconnecting means you may have between your power source and the fire pump per 695.4(B)(1). I would say that you may not do what you proposed because then you would have (3) disconnecting means for the fire pump generator feeder (generator breaker, MTS, downstream switch). An argument for some flexibility in the number of disconnecting means may be made if you have multiple generators paralleling to a single bus which then feeds the fire pump. Disconnecting one of the generators will not disconnect standby power to your fire pump; however, this is not the case here.

The MTS requirement is a relatively new code requirement introduced in the 2017 NEC so I can't say that I have ever tried doing whatever is shown on your proposed one-line. I would kick it to whoever the EOR is to figure out next steps.
I think I'm still going to suggest providing a disconnect between the docking station and the fire pump ATS downstream.

I would install it directly adjacent to the docking station with the conductors tapped from the load bus of the docking station.

See attached picture for an example that I found from another project.

FIRE PUMP.PNG
 

PD1972

Member
Location
New York
Occupation
engineer
If the installation is already built, installed, and inspected, I would not provide additional recommendations. In my opinion, you are only making it worse by introducing additional disconnects between the input of your combination fire pump controller/transfer switch and your generator breaker. The intent of article 695 and NFPA 20 is to have a fire pump installed with as few "things that can go wrong" so that the fire pump may run until failure if the unfortunate catastrophic fire ever occurs. As you are responsible for performing the power system study and not the EOR, just make sure the generator breaker won't trip on fire pump start-up with the settings you're providing and leave it at that.
 

PE (done with training)

Senior Member
Location
Saint Louis
Occupation
Professional Engineer
If the installation is already built, installed, and inspected, I would not provide additional recommendations. In my opinion, you are only making it worse by introducing additional disconnects between the input of your combination fire pump controller/transfer switch and your generator breaker. The intent of article 695 and NFPA 20 is to have a fire pump installed with as few "things that can go wrong" so that the fire pump may run until failure if the unfortunate catastrophic fire ever occurs. As you are responsible for performing the power system study and not the EOR, just make sure the generator breaker won't trip on fire pump start-up with the settings you're providing and leave it at that.
In this scenario I am actually working directly with the engineer of record. We are technically under the same company.

We both are under the opinion that the generator being lost while trying to carry locked rotor current from the fire pump motor is a worst case scenario and that it would be better to have overcurrent protection to trip the fire pump motor and still keep the life safety motor loads running in this situation.
 

Todd0x1

Senior Member
Location
CA
To clarify, I am doing the power system study for this project.

I've got a project where a 400 kW generator feeds a CAM lock that then feeds both a wireway with life safety equipment and a fire pump. My questions is, how should the generator breaker be set in this scenario? If I'm feeding both life safety and the fire pump from the same generator breaker is that an issue?

My first instinct is to set the generator breaker as high as possible so that nothing would ever trip in this scenario, but I know that the NEC dictates special settings when a generator breaker feeds a fire pump so I would just like some feed back from anyone on what they think.

See attached one line.

Some thoughts:
This looks like a stationary generator is feeding a generator dock with manual transfer switch built in (labeled Cam-Lock w/ Manual Transfer on the drawing). Assuming so a rental generator can be connected if the stationary unit is out of service. Some in this thread are suggesting tapping the fire pump feeder ahead of the generator breaker, doing so would lose the ability for the camlock connected portable generator to supply the fire pump.

Some of these generator docks have circuit breakers in them, check that for settings too.

Hopefully the generator dock has provisions for ATS generator start contacts and connections for block heater and battery charger.
 

PE (done with training)

Senior Member
Location
Saint Louis
Occupation
Professional Engineer
Some thoughts:
This looks like a stationary generator is feeding a generator dock with manual transfer switch built in (labeled Cam-Lock w/ Manual Transfer on the drawing). Assuming so a rental generator can be connected if the stationary unit is out of service. Some in this thread are suggesting tapping the fire pump feeder ahead of the generator breaker, doing so would lose the ability for the camlock connected portable generator to supply the fire pump.

Some of these generator docks have circuit breakers in them, check that for settings too.

Hopefully the generator dock has provisions for ATS generator start contacts and connections for block heater and battery charger.
Totally agree with you, tapping ahead of the main in this scenario would be bad. It would essentially by pass the temporary connection and leave us without a fire pump when the temporary connection is in use.

The generator dock does not appear to have any overcurrent protection which is why I'm not treating it as a disconnect.

Our plan is to provide a disconnect between the docking station and the fire pump ATS downstream.
 

Todd0x1

Senior Member
Location
CA
Totally agree with you, tapping ahead of the main in this scenario would be bad. It would essentially by pass the temporary connection and leave us without a fire pump when the temporary connection is in use.

The generator dock does not appear to have any overcurrent protection which is why I'm not treating it as a disconnect.

Our plan is to provide a disconnect between the docking station and the fire pump ATS downstream.
One more slightly OT thought. If the generator dock hasn't been ordered yet, consider a phase rotation monitor on the camlock input.
 
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