fuse size for fire pump
- Thread starter defishu
- Start date
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kwired
Electron manager
- Location
- NE Nebraska
From what I can recall, the you are supposed be able to carry locked rotor current indefinitely.
that means 600%? This is too much..
kwired
Electron manager
- Location
- NE Nebraska
could be less then 600. There is a difference between peak inrush when first energizing and locked rotor - locked rotor already has established fields but draws a heavy current because there is no rotation.
Source and circuit impedance can impact both these figures also.
The concept with fire pumps is if the building is already on fire we don't care if the pump motor burns out - we want it to deliver whatever it can as long as it can.
It still needs short circuit and ground fault protection though.
gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
Per NFPA 20, the locked rotor current for a 50 Hp fire pump motor operating at 460 V is 362 amps.
It is ok If I make 400as/af for 50 hp fire pump?
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
If you are unsure, we are not the place to grant you permission, we can only suggest. The correct answer was given and is that the fuse must be sized to provide Locked Rotor Current indefinitely. We are not there to read the motor nameplate, but as a GENERALIZATION, the LRC will be 600% of FLA. So get the FLA of your motor, multiply it by 6 and call it a day.
When you say "This is too much!", you have to keep in mind the over-arching principal on all things related to Fire Pumps:
"Run To Destruction".
Interesting to cogitate on that one.
Could the motor withstand locked rotor current indefinitely?
Is it designed to meet that requirement?
Maybe a non-standard rotor could be used to limit the LRC to much less than 600%.
One other point. If it really is a locked rotor, the pump won't pump anything which would render it ineffective as a fire pump.
Just my idle thoughts over a glass of Merlot............:thumbsup:
gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
Over here you can't just slap a motor onto a pump and call the combination a fire pump. Motors for fire pumps must be listed to UL 1004-5, the pumps are listed to UL 448 (centrifugal), the controllers to UL 218, etc.
texie
Senior Member
- Location
- Fort Collins, Colorado
- Occupation
- Electrician, Contractor, Inspector
My question to you would be why are you putting an OCPD ahead of the fire pump controller at all. While it can be done, and in some install scenarios required, most garden variety installs of a fire pump this size the most common method would be to run the service conductors directly from the POCO to the controller.
695.4.B.2
> LRC of pump + LRC booster pump + FLC of all other connected devices
must be a listed device for a fire pump controller assy and carry FLC x 600% for 2 minutes
must withstand 24 x FLC upon a restart transient
carry 300% x FLC for 10 min
trip point non-filed adjustable
> LRC of pump + LRC booster pump + FLC of all other connected devices
must be a listed device for a fire pump controller assy and carry FLC x 600% for 2 minutes
must withstand 24 x FLC upon a restart transient
carry 300% x FLC for 10 min
trip point non-filed adjustable
I don't suppose you can here either.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Is this in NYC? If so you should check the NYC electrical code there are some significant changes from the NEC requirements.
I have seen so many 75 hp fire pumps only using 800amp breakers. I am just curious. We shall have about 211 amp FLA for 75 hp fire pump. In this case, we should use fuse size which over 1200 amp in accordance with the NEC code relate to locked-rotor current.
Correct me if i am wrong. i really appreciate it. I am still learning..
gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
You don't look to the NEC for the locked rotor current value. You go to NFPA 20.
yes, 208v/3 phase
kwired
Electron manager
- Location
- NE Nebraska
Use of listed fire pump controller will likely already have whatever is needed if it is properly matched to the fire pump. All you need to do is properly size the conductors to it. Make sure the supply side is service conductors.
gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
Then per NFPA 20-2013 9.5.1.5, multiply the previous value I gave of 362 by the ratio of 460/208 which gives you 800.58 amps. Your disconnect, fused or unfused, must be rated for this locked rotor current.
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