695.4(B)(2)(b)

[m sleem]

I understand why we limit the fire pump's OCPD at the generator equals to 250%fla. I woulld think the main reason to maintain the continuity of the other emergency loads.
would we increase the gen capacity to carry-out the fire pump's LRA + other loads? To increase the system continuity during the fire case by using 600%fla OCPD.

 

[Bjenks]

You could put the generator breaker at 125%, but since we are assuming only the fire pump motor load and controls are on the generator we are also assuming the controls will have the overload protection in it. Thus the generator only needs to worry about short circuit protect. Per table 430.52 you have a maximum size of 250% to eliminate motor starting OCPD tripping.

The generator is an emergency power source and is not the primary. Only the primary source needs to handle the LRC continuously. The emergency source only needs to make sure the alternator can handle the start up of the motor without the controls seeing more than 15% VD (this is the controls and not the motor as they are worried a relay might drop during starting). Thus many fire pump starting circuits use some type of reduced voltage starting besides Across The Line.

When you say other loads, make sure that Jockey pumps (Yep Jockey Pump is not allowed on FP system) and other non fire pump equipment are either not on the generator or are on a separate generator breaker than the Fire Pump.

 

[m sleem]

The generator is an emergency power source and is not the primary. Only the primary source needs to handle the LRC continuously.

Why, i belive the main reason to allow the LRC to maintain the operation of fire pump up to damage in windings & that case could happen during emergency.

When you say other loads, make sure that Jockey pumps (Yep Jockey Pump is not allowed on FP system) and other non fire pump equipment are either not on the generator or are on a separate generator breaker than the Fire Pump.

Other loads mean emergency loads (ex. life safety & critical loads). Obviously we can connect those loads to the gen panel with separate breakers.

Basically my question was regarding increasing the gen size to allow 6xfla of fire pump in addition to other emergency loads.

 

[Bjenks]

You can oversize it 6 times or 20 times if you want, but the NEC or NFPA 20 doesn't require it. The fire pump on generator is only required to run 8 hours. You can also put the normal supply on the supply side of the service disconnect and not use a disconnect. When there is a fire they will cut the power, but the fire pump will still have power. You also are in a 2 hour rated room so for at least 2 hours you won't have any need for a generator and might not ever need the generator. Once the fire has gone to a point that you burned out the service then the generator will kick in and the pump has been working for at least 2 hours already. You shouldn't be concerned with a continuous lock rotor condition now unless the fire pump room has now caught on fire and the winding are melting. But now you pretty much are done.