Over Currrent Protection to Generator/Fire Pump ends

[vaaoc1425]

I am in the process of designing on a rush project for the first time that has to do with connecting two 100 HP Fire Pumps to an exisitng 600 KW Generator with only around 200 amps per phase on all three phases being used to date.

This is the first time I working with Fire Pumps and I am not sure that I understand the NEC 695 intermes of sizing the over curent protection at the connection to the Generator side in terms of Locker Roter Current and also at the other end which is a Fire Pump controller [100 HP] that comes with an ATS.

I will appreciate your assistance towards explaining this or direct me to a different reference than NEC. It will help if smaple sketchs and calculations available.

Thanking you in advance

 

[itsmoked]

You should get a copy of the NFPA document on wiring up fire pumps as they have very different requirements from the norm.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[dpc]

The basic concept is that the fire pump motor is not allowed to have overload protection and the short circuit protection must be set to allow full locked rotor current flow indefinitely. The reason is that this is a fire pump and it is more important to keep it running during a fire than to protect its motor against overheating. The fire pump motor is allowed to run to destruction.

But the devil is in the details, so you really need to review the NEC and NFPA requirements in detail.

 

[davidbeach]

The rules are also different for utility source vs. standby generation. Study the codes as mentioned.

 

[Noway2]

As everyone said above, Fire pumps have very different ratings that (normal) applications. Obtain a copy of NFPA-20, which will answer most of your questions. NFPA-20, though, is just a guideline and local jurisdictions (having the authority) can and do override its requirements.

Some basics to keep in mind, regarding fire pumps, typically if I remember correclty the breaker must be sized for 125% of the full load rating and the wiring needs to get sized to 110%.

As far as overload (locked rotor goes), it will depend on the motor. Historically, fire pump applications used Nema G motors, which had a locked rotor current of about 6X full load. There has been move in recent years to more efficient motors, which have higher locked rotor and in rush currents. The fire pump controller will be designed (and expect) to hold 6x full load current (locked rotor) for at least 12 to 20 seconds and 3x full load current indefinitely (per NFPA 20). Also, all fire pump applications MUST be capable of starting 'across the line', which will have the greatest demand on the generator and the generator must not stall on this condition - while the pump is operating at more than 100% rated load.

The generator system may be equipped with an auxiliary load shed panel which will cause it to dump non essential equipment from the generator when the fire pump controlelr calls for it.

Your original post didn't really have any specific questions so the information above, is more or less an overview. If you have a specific question, I may be able to answer it for you, or at least direct to a more knowledgeable source.

 

[vaaoc1425]

From what I understand from NEC 2002 695-4-B-1 “Over current protection devices shall be selected or set to carry indefinitely the sum of locker-rotor current of the fire pump motor and pressure maintenance and the full load current of the associated fire pump accessory equipment etc…

In this Case 100 hp Fire Pump + 3 HP Jockey Pump = 728 A + 32 A= 757 Amp, so you select 800 Circuit Breaker.

In my case I have two 100 HP Fire Pumps so I have two 800 Amp Over Current Protective Devices….

Again 696 B-C-1 says conductors can be sized at 125 % of the Full Load amps of the Pumps.

 

[dpc]

LOCKED rotor, not "locker" rotor.

So what's your question?

 

[Noway2]

One thing to keep in mind is that in an FPC application the breakers (or molded case switches) are NOT the primary overload protection means.

The FPC will have circuitry dedicated to this function that will be based upon the motor's full load current. This circuit will implement the requirements spelled out in NFPA-20, including locked rotor and fault conditions.

If in doubt, make the breaker bigger as tripping during a motor startup is a recipe for failure to pass startup and commissioning. After the pump runs for a while under load, the breaker (thermal/mag type?) will likely get warm and will be less tolerant of the high inrush currents associated with an across the line start under load.

 

[vaaoc1425]

FPC [Fire Pump Controller] comes as a enclosed unit with connection for normal and emergency power [ATS]. FPC is based on what ever the HP of the Fire Pump Motor so what ever OC protection inside of it is fixed per FLA of the motor.

However the main protection at the Generator[emergency]end or Normal Service end is depended on designer's choice.
Per NEC this OC protection is sized at Locked Roter current of the Fire Pump Motor.

For a 100 HP motor Locked Roter current is 724 Amps,
so the OC protecton size is 800 Amp breaker.

Some designers size them at 7 x FLA which is about 7X124 amps for 100 hp Fire Pump motor

 

[tommom]

NFPA 20 states the locked-rotor currents for 480 volt fire pumps - double check your number with that.

 

[RRaghunath]

I too find that NFPA doesn't cover fire pumps at medium voltage (say 6.6kV).
If the pumps are controlled through MV contactors (backed up by fuses), again should these not be "Latched" type!

In case the emergency generator is connected to supply the fire pump, should the generator overload / current protection also to be disabled (connected only for alarm)!