I am confused with an issue I recently saw on a project. My question is in regards to a fire pump controller for a 200HP 460V pump motor fed from two sources, one is a utility source and the other is a standby generator source. The controller has an integral ATS, but I did not have the specifications for it. The utility source was tapped from a 2500kVA xfmr secondary with no overcurrent protection, which I agree with according to 695.6.D. However, the breaker in the generator paralleling switchgear that is the second source to the controller is has a 300AT rating. I thought 695.4.B.1 stated that overcurrent protection should allow for carrying locked rotor current indefinitely. But then I see in 695.4.B the statement "Overcurrent protective devices between an on-site standby generator and a fire pump controller shall be selected and sized in accordance to 430.62 to provide short circuit protection only." My question is, why wouldn't the breaker feeding the controller from the generator be required to carry the LR current? What if the pump has a LR when the generator is supplying power?
Fire Pump Controller
- Thread starter ryanee24
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rick hart
Senior Member
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- Dallas Texas
Re: Fire Pump Controller
Starting of a motor across the line is very unusual for this size of motor. Part winding start or soft start will get the motor turning enough to keep LRC manageable. When the firepump is connected to utility there is a large current reservoir that can force the motor to turn.That is why there are seemingly no restrictions on overcurrent sizing. When connected to a generator however, a large current draw will collapse the field of the generator, effecting all loads supplied. In this instance, the generator rating would need to be raised over 2.3 MW over present design size. Caterpiller would love this almost as much as the people that provide loadbanking services.
Therefore, you really don't want the unlimited current flowing to something even as critical as a fire pump, you want short circuit protection.
A fire pump that locks up, in a fire, while being supplied by the emergency standby system is not going to make much difference to a really bad situation as the getting the heck out of this building Life Safety loads would.
While you certainly want the firepump to turn, you don't want that at the expense of getting people out.
Starting of a motor across the line is very unusual for this size of motor. Part winding start or soft start will get the motor turning enough to keep LRC manageable. When the firepump is connected to utility there is a large current reservoir that can force the motor to turn.That is why there are seemingly no restrictions on overcurrent sizing. When connected to a generator however, a large current draw will collapse the field of the generator, effecting all loads supplied. In this instance, the generator rating would need to be raised over 2.3 MW over present design size. Caterpiller would love this almost as much as the people that provide loadbanking services.
Therefore, you really don't want the unlimited current flowing to something even as critical as a fire pump, you want short circuit protection.
A fire pump that locks up, in a fire, while being supplied by the emergency standby system is not going to make much difference to a really bad situation as the getting the heck out of this building Life Safety loads would.
While you certainly want the firepump to turn, you don't want that at the expense of getting people out.
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