Sizing of Fire pump's Circuit Breaker according to NEC

[Ahmed Magdy]

hello everybody,
I Want to ask about the method of sizing the Circuit breaker of fire pump According to NEC.
I want to know the following points specially:
1 ) The specifications of the circuit breaker _ Thermal magnetic Circuit breaker or magnetic release only circuit breaker _ in case the fire pump fed from service utility and in case the fire pump fed from an on site generator.
2 ) when i should size the circuit breaker according to locked rotor current , I mean if i choose a magnetic release only circuit breaker to fed the fire pump , should i size the circuit breaker according to the locked rotor current too ? .. if the answer yes , please clarify why i should size the circuit breaker according to the locked rotor current although i select a magnetic release only circuit breaker .
3 ) what about the feeder, Contactors and Transfer switch if i size the circuit breaker according to locked rotor current ? should is size them by the same method too ? and why ?

Please i need the answers of these points from the NEC

Many Thanks

 

[Dennis Alwon]

The motor must have some specifications. What are they?

There is this

Overcurrent protective device(s) shall be rated to carry
indefinitely the sum of the locked-rotor current of the
largest fire pump motor and the pressure maintenance
pump motor(s) and the full-load current of all of the
other pump motors and associated fire pump accessory
equipment when connected to this power supply. Where
the locked-rotor current value does not correspond to a
standard overcurrent device size, the next standard overcurrent
device size shall be used in accordance with
240.6. The requirement to carry the locked-rotor currents
indefinitely shall not apply to conductors or devices other
than overcurrent devices in the fire pump motor
circuit(s). The requirement to carry the locked rotor
currents indefinitely shall not apply to feeder overcurrent
protective devices installed in accordance with 695.3(C).
[20:9.2.3.4]

 

[Ahmed Magdy]

Thanks Eng. Dennnis for your Valuable reply,
This section starts by " Over current protective device(s) "
so what about if i select a magnetic release only circuit breaker, in this case there is no over current protection , the circuit breaker will trip instantaneously in case of short circuit only, so why i need to size the circuit breaker according to locked rotor current in this case ?

Thanks

 

[Dennis Alwon]

That question is out of my knowledge so hopefully someone else can answer that

 

[paulengr]

You only provide short circuit protection for fire pumps.

The whole thing is rated to handle stall conditions by oversizing instead of tripping.

Even MCPs (motor circuit protectors) which is the NEC term aka magnetic only breakers that are actually technically not circuit breakers have a frame rating and that’s what you go by for continuous rating.

 

[tom baker]

I have only installed one electric fire pump, and much prefer gravity! The fire pump service had no disconnecting means, short circuit protection was built into the controller, there was no overload protection. (A big knob you pull down overrides the short circuit breaker as the fire pump is sacrificial in a fire) If you have a circuit breaker ahead of the fire pump controller, it has to be able to carry locked rotor current, for the 100 HP 460 volt fire pump, that would of required an 800 amp circuit breaker. I would also recommend reviewing what every version of NFPA 20 is enforced by your AHJ. It and the NEC go hand in hand. What I like about NFPA 20 is the nice explanations at the end, kind of like a handbook.

 

[gadfly56]

I have only installed one electric fire pump, and much prefer gravity! The fire pump service had no disconnecting means, short circuit protection was built into the controller, there was no overload protection. (A big knob you pull down overrides the short circuit breaker as the fire pump is sacrificial in a fire) If you have a circuit breaker ahead of the fire pump controller, it has to be able to carry locked rotor current, for the 100 HP 460 volt fire pump, that would of required an 800 amp circuit breaker. I would also recommend reviewing what every version of NFPA 20 is enforced by your AHJ. It and the NEC go hand in hand. What I like about NFPA 20 is the nice explanations at the end, kind of like a handbook.

^^^^What Tom said. The fire pump controller has everything needed for motor protection. I wouldn't try to "roll your own" in this matter.

 

[kwired]

Isn't this primarily an issue if you have a stand alone disconnect of some sort? Otherwise magnetic only breaker will have to be part of a listed controller or other assembly and not a general use type enclosed breaker or safety switch. Listed fire pump controller will have an overcurrent device that will comply. Should you have a general use service disconnect then a feeder to the controller you need a device that will hold locked rotor current indefinitely.

 

[jim dungar]

While many fire pump controllers are service entrance rated, there are times that the fire pump circuit is actually a feeder.

You must make sure you are dealing with the proper code section for the type of circuit you are providing. Most people do not have experience with fire pump feeder circuits so it is easy to get misdirected.

I agree with the advice Tom Baker provided.

 

[kwired]

While many fire pump controllers are service entrance rated, there are times that the fire pump circuit is actually a feeder.

You must make sure you are dealing with the proper code section for the type of circuit you are providing. Most people do not have experience with fire pump feeder circuits so it is easy to get misdirected.

I agree with the advice Tom Baker provided.

I have only done one fire pump installation myself, controller was also the service disconnect in that one. I can see how some might be reluctant to use say an 800 amp service disconnecting means and supply only 100-150 amp feeder conductors, but should be allowed in the circumstances.

 

[tom baker]

This fire pump I worked on was orginally feed from a 200 amp disconnect with 1970s aluminum wire. My company took over the operation of the fire pump and started weekly testing. Shortly thereafter, the wire and 200 amp disconnect exploded. When I went to pull an electrical permit the AHJ rolled his eyes and handed me an article on fire pumps. So for the OP, I am rolling my eyes, get your self an article (check at IAEI).

 

[Jraef]

If you are wanting to size a feeder breaker in a distribution board to feed a fire pump circuit, then you use the

Thanks Eng. Dennnis for your Valuable reply,
This section starts by " Over current protective device(s) "
so what about if i select a magnetic release only circuit breaker, in this case there is no over current protection , the circuit breaker will trip instantaneously in case of short circuit only, so why i need to size the circuit breaker according to locked rotor current in this case ?

Thanks

As kwired has pointed out, you cannot use a magnetic-only circuit breaker on your own as if it is a feeder breaker. It CAN be part of a listed assembly, but if that were the case, YOU would not be the one selecting it, that would be done by the assembly manufacturer.

In the field, you must use a thermal-magnetic circuit breaker if this is a feeder situation, and you would size it per what Dennis Alwon posted; LRC of the largest fire pump motor plus FLC of any others and any ancillary equipment.

 

[Shazam76]

So to summarize what was said above with the example started above of the 100hp 460V fire pump motor, a 2hp jockey pump motor, and the pump motor controls (say 2A), the components would be something like this:
- 800A disconnecting means (thermal-mag breaker, disconnect, or disconnect with fuses) based on LRC of fire pump and jockey pump plus controls.
(124*6)+(3.4*6)+2=766.4A
- 1/0 THHN main conductor based on 125% of FLA of fire pump and jockey pump plus controls per NEC 695.6(B)(1) (assuming no temperature or VD adjustments required).
(124+3.4)*125%+2=161.25A
Am I understanding this correctly?

 

[gadfly56]

So to summarize what was said above with the example started above of the 100hp 460V fire pump motor, a 2hp jockey pump motor, and the pump motor controls (say 2A), the components would be something like this:
- 800A disconnecting means (thermal-mag breaker, disconnect, or disconnect with fuses) based on LRC of fire pump and jockey pump plus controls.
(124*6)+(3.4*6)+2=766.4A
- 1/0 THHN main conductor based on 125% of FLA of fire pump and jockey pump plus controls per NEC 695.6(B)(1) (assuming no temperature or VD adjustments required).
(124+3.4)*125%+2=161.25A
Am I understanding this correctly?

Pretty much, but you have to go to NFPA 20 for the LRC's of the fire and jockey pumps. Using 6x may or may not be adequate.

 

[Bindim]

Pretty much, but you have to go to NFPA 20 for the LRC's of the fire and jockey pumps. Using 6x may or may not be adequate.

What if I use a circuit breaker with only magnetic protection for a single fire pump (without a jockey pump).
Should the FRAME of the circuit breaker meet the rated current or just the current sensor of the circuit breaker?
Example:
Fire Pump 55kW (87.9A * 6.7) = 597.72A;
Circuit breaker (Compact NSX100 model with only MA Unit Trip - Schneider) = In 100A - Magnetic Unit Pickup (Im) = 9 to 14 * In
Setting Im = 9 * In = 9 * 100A = 900A

Im = 900A> 597.72A (LRC fire pump)

So would it be a circuit breaker with Frame 100A equipped with only magnetic sensor set to 900A or would the Frame also be for 900A?

I appreciate the help and apologize if I'm talking nonsense.

 

[don_resqcapt19]

What if I use a circuit breaker with only magnetic protection for a single fire pump (without a jockey pump).
Should the FRAME of the circuit breaker meet the rated current or just the current sensor of the circuit breaker?
Example:
Fire Pump 55kW (87.9A * 6.7) = 597.72A;
Circuit breaker (Compact NSX100 model with only MA Unit Trip - Schneider) = In 100A - Magnetic Unit Pickup (Im) = 9 to 14 * In
Setting Im = 9 * In = 9 * 100A = 900A

Im = 900A> 597.72A (LRC fire pump)

So would it be a circuit breaker with Frame 100A equipped with only magnetic sensor set to 900A or would the Frame also be for 900A?

I appreciate the help and apologize if I'm talking nonsense.

You can't use a magnetic only device. Those devices can only be used a part of a listed combination starter.

 

[Bindim]

You can't use a magnetic only device. Those devices can only be used a part of a listed combination starter.

Thank you for answear.

The starter set consists of an Isolation Switch, circuit breaker (model doubt) and motor contactor.
Including phase failure sensor and phase sequence sensor.
440Vac - 60Hz - 3P + PE
Induction motor, squirrel cage, 55kW.

 

[kwired]

Is the "starter set" a listed fire pump controller? Is it also the service disconnecting means for the fire pump?

Service disconnect for fire pump can not be located with other general service disconnecting means so I believe it is common to just run fire pump service directly to listed fire pump controller that is also listed for service equipment. If your arrangement of things doesn't allow for that it probably can get more complicated.

The whole idea is they don't want fire pump disconnected inadvertently when shutting off the general service switches and then if the pump is in demand the idea is to keep pumping until that motor burns out if something happens to it, so they are not concerned if it would happen to draw locked rotor current and want it to be able to do so indefinitely. Motor overload protection you would have on any other motor circuit is not needed or even wanted in this scheme of protection. Once motor fails you either have open circuit or develop a short circuit or ground fault which would raise current enough to go beyond instantaneous trip rating of your overcurrent device. Partial motor failure may not raise current enough to trip, but may also still allow to drive the pump to at least some extent, they want to get anything out of it they can when it is called upon.

 

[Bindim]

Thank you for answear.

The starter set consists of an Isolation Switch, circuit breaker (model doubt) and motor contactor.
Including phase failure sensor and phase sequence sensor.
440Vac - 60Hz - 3P + PE
Induction motor, squirrel cage, 55kW.

This is our design (see attached file).
The electrical system is only for the fire pump and jockey pumps.
All componnents will be installed in a exclusive fire pump house.

As NFPA 20:2019
10.4.3.3.1 The circuit breaker shall have the following electrical characteristics:
(1) A continuous current rating not less than 115 percent of the rated full-load current of the motor
(2) Overcurrent-sensing elements of the nonthermal type
(3) Instantaneous short-circuit overcurrent protection
(4)* An adequate interrupting rating to provide the suitability rating of the controller discussed in 10.1.2.2
(5) Capability of allowing normal and emergency starting and running of the motor without tripping (see 10.5.3.2)
(6) An instantaneous trip setting of not more than 20 times the full-load current

 

[kwired]

Where is the service disconnect? Is the 13.8kV the service voltage or are you showing some of POCO equipment in the drawing? Or is that 13.8 supply already on load side of service at the facility?

Generally fire pump needs to be supplied by separate service conductors from other "normal" loads, but can tie to same source ahead of service disconnecting means.

Will panel you build have a listing or does it need to be built entirely to applicable codes?