Sizing of Fire pump's Circuit Breaker according to NEC

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

Moderator
Staff member
The motor must have some specifications. What are they?

There is this

695.4(B)(2)(1) said:
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]
 
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
 

paulengr

Senior Member
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

First Chief Moderator
Staff member
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

Senior Member
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

Electron manager
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

Moderator
Staff member
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

Electron manager
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

First Chief Moderator
Staff member
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

Moderator
Staff member
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

New User
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

Senior Member
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.
 
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