If an installation with a fire pump includes feeds from two sources (i.e. a utility feed and an on site generator), how does the overcurrent protection for the feed from the generator need to be sized. There is a "commentary note" in the NEC Handbook that indicates this overcurrent should be sized like a normal motor feeder (not only for short circuit like the utility feeder), but I cannot find support for this in the NEC.
Fire Pump Feeder - Alternate Source Overcurrent Sizing
- Thread starter tcleis
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bonding jumper
Senior Member
Re: Fire Pump Feeder - Alternate Source Overcurrent Sizing
Sized greater than the LRA of the motor.
Sized greater than the LRA of the motor.
sceepe
Senior Member
Re: Fire Pump Feeder - Alternate Source Overcurrent Sizing
Ok here is what Generac Says...
First the circuit breaker should be std molded case; magnetic only(instant trip) breakers and non automatic molded case switches are not recommended.
You want to get short circuit protection but not motor overload protection. The breaker should be sized greater than 125% but less than 250% of the fire pump FLA.
Minimum rating must be greater than 125% FLA so that the device does not provide overload protection.
Max rating less than 250% is so that you get short circuit protection
Ok within the range of 125% to 250% FLA, choose the smallest overcurrent device that will allow locked rotor (LRA) in excess of 20 sec. (yep you gotta look at the curves) Reason for this is that a locked rotor protective device in the fire pump controller will trip within 20 sec at locked rotor current.
Good luck
Ok here is what Generac Says...
First the circuit breaker should be std molded case; magnetic only(instant trip) breakers and non automatic molded case switches are not recommended.
You want to get short circuit protection but not motor overload protection. The breaker should be sized greater than 125% but less than 250% of the fire pump FLA.
Minimum rating must be greater than 125% FLA so that the device does not provide overload protection.
Max rating less than 250% is so that you get short circuit protection
Ok within the range of 125% to 250% FLA, choose the smallest overcurrent device that will allow locked rotor (LRA) in excess of 20 sec. (yep you gotta look at the curves) Reason for this is that a locked rotor protective device in the fire pump controller will trip within 20 sec at locked rotor current.
Good luck
bonding jumper
Senior Member
Re: Fire Pump Feeder - Alternate Source Overcurrent Sizing
I don't trust Generac, they are not code experts, they are barely generator experts. I decided to use them for one job in particular because they had support that would actually call back, unlike Onan, and after giving them my loads they interpreted a 50hp fire pump as a standard 50hp pump. The alternator was had to be increased in size due to the allowable voltage drop of the generator being too great for a fire pump. They called to apologize after the project had already started and caused a change order, they nearly had to increase the generator size itself which would have been a problem cuz the already small room would have had to been enlarged. Just my rant about how I won't spec generac again.
[ January 21, 2005, 11:56 AM: Message edited by: bonding jumper ]
I don't trust Generac, they are not code experts, they are barely generator experts. I decided to use them for one job in particular because they had support that would actually call back, unlike Onan, and after giving them my loads they interpreted a 50hp fire pump as a standard 50hp pump. The alternator was had to be increased in size due to the allowable voltage drop of the generator being too great for a fire pump. They called to apologize after the project had already started and caused a change order, they nearly had to increase the generator size itself which would have been a problem cuz the already small room would have had to been enlarged. Just my rant about how I won't spec generac again.
[ January 21, 2005, 11:56 AM: Message edited by: bonding jumper ]
Re: Fire Pump Feeder - Alternate Source Overcurrent Sizing
Per 695.4(B) (last sentance before #1) any devices unique to the fire pump must comply with 695.4(B)(1). Sounds like only the main breaker on the generator (or a feeder breaker that is also for other equipment ) is allowed to be sized less than LR current.
Steve
Per 695.4(B) (last sentance before #1) any devices unique to the fire pump must comply with 695.4(B)(1). Sounds like only the main breaker on the generator (or a feeder breaker that is also for other equipment ) is allowed to be sized less than LR current.
Steve
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